@article{voinov_smirnova_smirnov_2021, title={EPR Oximetry with Nitroxides: Effects of Molecular Structure, pH, and Electrolyte Concentration}, volume={11}, ISSN={["1613-7507"]}, url={https://doi.org/10.1007/s00723-021-01446-8}, DOI={10.1007/s00723-021-01446-8}, journal={APPLIED MAGNETIC RESONANCE}, author={Voinov, Maxim A. and Smirnova, Tatyana I and Smirnov, Alex I}, year={2021}, month={Nov} }
 @article{aggarwal_may-zhang_yermalitsky_dikalov_voynov_amarnath_kon_linton_vickers_davies_2021, title={Myeloperoxidase-induced modification of HDL by isolevuglandins inhibits paraoxonase-1 activity}, volume={297}, ISSN={["1083-351X"]}, url={https://doi.org/10.1016/j.jbc.2021.101019}, DOI={10.1016/j.jbc.2021.101019}, abstractNote={Reduced activity of paraoxonase 1 (PON1), a high-density lipoprotein (HDL)-associated enzyme, has been implicated in the development of atherosclerosis. Post-translational modifications of PON1 may represent important mechanisms leading to reduced PON1 activity. Under atherosclerotic conditions, myeloperoxidase (MPO) is known to associate with HDL. MPO generates the oxidants hypochlorous acid and nitrogen dioxide, which can lead to post-translational modification of PON1, including tyrosine modifications that inhibit PON1 activity. Nitrogen dioxide also drives lipid peroxidation, leading to the formation of reactive lipid dicarbonyls such as malondialdehyde and isolevuglandins, which modify HDL and could inhibit PON1 activity. Because isolevuglandins are more reactive than malondialdehyde, we used in vitro models containing HDL, PON1, and MPO to test the hypothesis that IsoLG formation by MPO and its subsequent modification of HDL contributes to MPO-mediated reductions in PON1 activity. Incubation of MPO with HDL led to modification of HDL proteins, including PON1, by IsoLG. Incubation of HDL with IsoLG reduced PON1 lactonase and antiperoxidation activities. IsoLG modification of recombinant PON1 markedly inhibited its activity, while irreversible IsoLG modification of HDL before adding recombinant PON1 only slightly inhibited the ability of HDL to enhance the catalytic activity of recombinant PON1. Together, these studies support the notion that association of MPO with HDL leads to lower PON1 activity in part via IsoLG-mediated modification of PON1, so that IsoLG modification of PON1 could contribute to increased risk for atherosclerosis, and blocking this modification might prove beneficial to reduce atherosclerosis.}, number={3}, journal={JOURNAL OF BIOLOGICAL CHEMISTRY}, publisher={Elsevier BV}, author={Aggarwal, Geetika and May-Zhang, Linda S. and Yermalitsky, Valery and Dikalov, Sergey and Voynov, Maxim A. and Amarnath, Venkataraman and Kon, Valentina and Linton, MacRae F. and Vickers, Kasey C. and Davies, Sean S.}, year={2021}, month={Sep} }
 @article{good_voinov_bolton_ward_sergeyev_caporini_scheffer_lo_rosay_marek_et al._2019, title={A biradical-tagged phospholipid as a polarizing agent for solid-state MAS Dynamic Nuclear Polarization NMR of membrane proteins}, volume={100}, ISSN={0926-2040}, url={http://dx.doi.org/10.1016/J.SSNMR.2019.04.003}, DOI={10.1016/J.SSNMR.2019.04.003}, abstractNote={A novel Dynamic Nuclear Polarization (DNP) NMR polarizing agent ToSMTSL-PTE representing a phospholipid with a biradical TOTAPOL tethered to the polar head group has been synthesized, characterized, and employed to enhance solid-state Nuclear Magnetic Resonance (SSNMR) signal of a lipid-reconstituted integral membrane protein proteorhodopsin (PR). A matrix-free PR formulation for DNP improved the absolute sensitivity of NMR signal by a factor of ca. 4 compared to a conventional preparation with TOTAPOL dispersed in a glassy glycerol/water matrix. DNP enhancements measured at 400 MHz/263 GHz and 600 MHz/395 GHz showed a strong field dependence but remained moderate at both fields, and comparable to those obtained for PR covalently modified with ToSMTSL. Additional continuous wave (CW) X-band electron paramagnetic resonance (EPR) experiments with ToSMTSL-PTE in solutions and in lipid bilayers revealed that an unfavorable conformational change of the linker connecting mononitroxides could be one of the reasons for moderate DNP enhancements. Further, differential scanning calorimetry (DSC) and CW EPR experiments indicated an inhomogeneous distribution and/or a possibility of a partial aggregation of ToSMTSL-PTE in DMPC:DMPA bilayers when the concentration of the polarizing agent was increased to 20 mol% to maximize the DNP enhancement. Thus, conformational changes and an inhomogeneous distribution of the lipid-based biradicals in lipid bilayers emerged as important factors to consider for further development of this matrix-free approach for DNP of membrane proteins.}, journal={Solid State Nuclear Magnetic Resonance}, publisher={Elsevier BV}, author={Good, Daryl B. and Voinov, Maxim A. and Bolton, David and Ward, Meaghan E. and Sergeyev, Ivan V. and Caporini, Marc and Scheffer, Peter and Lo, Andy and Rosay, Melanie and Marek, Antonin and et al.}, year={2019}, month={Aug}, pages={92–101} }
 @article{perelygin_voinov_marek_ou_krim_brenner_smirnova_smirnov_2019, title={Dielectric and Electrostatic Properties of the Silica Nanoparticle-Water Interface by EPR of pH-Sensitive Spin Probes}, volume={123}, ISSN={["1932-7455"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85076155729&partnerID=MN8TOARS}, DOI={10.1021/acs.jpcc.9b08007}, abstractNote={Interfacial electrostatic properties of monodisperse silica nanoparticles (SiNPs) in aqueous suspensions as a function of bulk pH were characterized by spin labeling EPR of two ionizable nitroxides: (1) IMTSL (S-(1-oxyl-2,2,3,5,5-pentamethylimidazolidin-4-yl)methyl methanesulfo-nothioate) and IKMTSL (S-4-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-1-oxyl-2,5-dihydro-1H-imidazol-2-yl). SiNPs of ca. 116 nm in diameter (by particle number) were synthesized using the Stober method and their surface was modified by silanization under harsh conditions to ensure robust attachment of the thiol-terminated ligands to the silica surface. These ligands were consequently modified with either IMTSL or IKMTSL to characterize the surface electrostatic potential of the nanoparticles from their EPR spectra. EPR titration data for these two pH-sensitive nitroxides allowed for differentiating the dielectric and electrostatic contributions to the interfacial properties of SiNPs. From such a titration at room temperature an effect...}, number={49}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, publisher={American Chemical Society (ACS)}, author={Perelygin, Vladislav and Voinov, Maxim A. and Marek, Antonin and Ou, Erkang and Krim, Jacqueline and Brenner, Donald and Smirnova, Tatyana I and Smirnov, Alex I}, year={2019}, month={Dec}, pages={29972–29985} }
 @article{ou_voinov_irving_smirnov_smirnova_2019, title={Effect of Silica Support on Electrostatics of Lipid Interfaces in Nano-Bio Hybrid Systems}, volume={116}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/J.BPJ.2018.11.479}, DOI={10.1016/J.BPJ.2018.11.479}, abstractNote={Design of new bio-nano hybrid systems calls for understanding and accounting for the influence of a nanostructured support and nanoconfinement on the structure and biophysical properties of lipid bilayer hybrid systems and membrane-protein interface. Here we report on spin-labeling EPR studies of pH-sensitive lipids and specifically labeled protein side chains to assess effects of solid inorganic interface, specifically, silica support in a form of monodispersed nanoparticles ranging from 20 to 300 nm in diameter on the surface electrostatic potential of lipid bilayers associated with the particles and effective pKa of the membrane-burred peptide ionisable sidechains. We have shown that bilayers formed from zwitterionic or mixed lipids on silica nanoparticle surfaces possess a higher negative electrostatic potential than the unsupported bilayers with the potential of mixed bilayers containing negatively charged lipids being significantly more sensitive to the silica support. Effect the silica nanoparticle size on the lipid bilayer surface electrostatic potential was also observed for particles smaller than 100 nm. pH-sensitive EPR probes were then employed to label model WALP peptide known to form an α-helix when integrated into a lipid bilayer. The silica support exerted pronounced effects on WALP dynamics and the effective pKa of the ionizable probe. It was demonstrated that the silica nanoparticles shift the effective pKa of the ionizable nitroxide probe in a membrane depth-dependent manner. Upon protonation of the membrane-burred model ionisable sidechain the silica support caused significant changes in the membrane association of WALP peptide that are not observed when WALP is integrated into unilamellar phospholipid vesicles of similar curvature. Supported by NSF 1508607.}, number={3}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Ou, Erkang and Voinov, Maxim and Irving, Alex and Smirnov, Alex and Smirnova, Tatyana I.}, year={2019}, month={Feb}, pages={81a} }
 @article{milikisiyants_voinov_marek_jafarabadi_liu_han_wang_smirnov_2019, title={Enhancing sensitivity of Double Electron-Electron Resonance (DEER) by using Relaxation-Optimized Acquisition Length Distribution (RELOAD) scheme}, volume={298}, ISSN={1090-7807}, url={http://dx.doi.org/10.1016/J.JMR.2018.12.004}, DOI={10.1016/J.JMR.2018.12.004}, abstractNote={Over the past decades pulsed electron-electron double resonance (PELDOR), often called double electron-electron resonance (DEER), became one of the major spectroscopic tools for measurements of nanometer-scale distances and distance distributions in non-crystalline biological and chemical systems. The method is based on detecting the amplitude of the primary (3-pulse DEER) or refocused (4-pulse DEER) spin echo for the so-called "observer" spins when the other spins coupled to the former by a dipolar interaction are flipped by a "pump" pulse at another EPR frequency. While the timing of the pump pulse is varied in steps, the positions of the observer pulses are typically fixed. For such a detection scheme the total length of the observer pulse train and the electron spin memory time determine the amplitude of the detected echo signal. Usually, the distance range considerations in DEER experiments dictate the total length of the observer pulse train to exceed the phase memory time by a factor of few and this leads to a dramatic loss of the signal-to-noise ratio (SNR). While the acquisition of the DEER signal seems to be irrational under such conditions, it is currently the preferred way to conduct DEER because of an effective filtering out of all other unwanted interactions. Here we propose a novel albeit simple approach to improve DEER sensitivity and decrease data acquisition time by introducing the signal acquisition scheme based on RELaxation Optimized Acquisition (Length) Distribution (DEER-RELOAD). In DEER-RELOAD the dipolar phase evolution signal is acquired in multiple segments in which the observer pulses are fixed at the positions to optimize SNR just for that specific segment. The length of the segment is chosen to maximize the signal acquisition efficiency according the phase relaxation properties of the spin system. The total DEER trace is then obtained by "stitching" the multiple segments into a one continuous trace. The utility of the DEER-RELOAD acquisition scheme has been demonstrated on an example of the standard 4-pulse DEER sequence applied to two membrane protein complexes labeled with nitroxides. While theoretical gains from the DEER-RELOAD scheme increase with the number of stitched segments, in practice, even dividing the acquisition of the DEER trace into two segments may improve SNR by a factor of >3, as it has been demonstrated for one of these two membrane proteins.}, journal={Journal of Magnetic Resonance}, publisher={Elsevier BV}, author={Milikisiyants, Sergey and Voinov, Maxim A. and Marek, Antonin and Jafarabadi, Morteza and Liu, Jing and Han, Rong and Wang, Shenlin and Smirnov, Alex I.}, year={2019}, month={Jan}, pages={115–126} }
 @article{ma_lin_kim_ko_kim_oh_sun_gorman_voinov_smirnov_et al._2019, title={Liquid Metal Nanoparticles as Initiators for Radical Polymerization of Vinyl Monomers}, volume={8}, ISSN={["2161-1653"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85074662246&partnerID=MN8TOARS}, DOI={10.1021/acsmacrolett.9b00783}, abstractNote={Sonication of gallium or gallium-based liquid metals in an aqueous solution of vinyl monomers leads to rapid free radical polymerization (FRP), without the need for conventional molecular initiators. Under ambient conditions, a passivating native oxide separates these metals from solution and renders the metal effectively inert. However, sonication generates liquid metal nanoparticles (LMNPs) of ∼100 nm diameter and thereby increases the surface area of the metal. The exposed metal initiates polymerization, which proceeds via a FRP mechanism and yields high molecular weight polymers that can form physical gels. Spin trapping EPR reveals the generation of free radicals. Time-of-flight secondary ion mass spectrometry measurements confirm direct polymer bonding to gallium, verifying the formation of surface-anchored polymer grafts. The grafted polymers can modify the interfacial properties, that is, the preference of the metal particles to disperse in aqueous versus organic phases. The polymer can also be degrafted and isolated from the particles using strong acid or base. The concept of physically disrupting passivated metal surfaces offers new routes for surface-initiated polymerization and has implications for surface modification, reduction reactions, and fabrication of mechanically responsive materials.}, number={11}, journal={ACS MACRO LETTERS}, publisher={American Chemical Society (ACS)}, author={Ma, Jinwoo and Lin, Yiliang and Kim, Yong-Woo and Ko, Yeongun and Kim, Jongbeom and Oh, Kyu Hwan and Sun, Jeong-Yun and Gorman, Christopher B. and Voinov, Maxim A. and Smirnov, Alex I. and et al.}, year={2019}, month={Nov}, pages={1522–1527} }
 @article{milikisiyants_voinov_smirnov_2018, title={Refocused  Out-Of-Phase (ROOPh) DEER: A pulse scheme for suppressing an unmodulated background in double electron-electron resonance experiments}, volume={293}, ISSN={1090-7807}, url={http://dx.doi.org/10.1016/J.JMR.2018.05.007}, DOI={10.1016/J.JMR.2018.05.007}, abstractNote={EPR pulsed dipolar spectroscopy (PDS) is indispensable for measurements of nm-scale distances between electronic spins in biological and other systems. While several useful modifications and pulse sequences for PDS have been developed in recent years, DEER experiments utilizing pump and observer pulses at two different frequencies remain the most popular for practical applications. One of the major drawbacks of all the available DEER approaches is the presence of a significant unmodulated fraction in the detected signal that arises from an incomplete inversion of the coupled spins by the pump pulse. The latter fraction is perceived as one of the major sources of error for the reconstructed distance distributions. We describe an alternative detection scheme – a Refocused Out-Of-Phase DEER (ROOPh-DEER) – to acquire only the modulated fraction of the dipolar DEER signal. When Zeeman splitting is small compared to the temperature, the out-of-phase magnetization components cancel each other and are not observed in 4-pulse DEER experiment. In ROOPh-DEER these components are refocused by an additional pump pulse while the in-phase component containing an unmodulated background is filtered out by a pulse at the observed frequency applied right at the position of the refocused echo. Experimental implementation of the ROOPh-DEER detection scheme requires at least three additional pulses as was demonstrated on an example of a 7-pulse sequence. The application of 7-pulse ROOPh-DEER sequence to a model biradical yielded the interspin distance of 1.94 ± 0.07 nm identical to the one obtained with the conventional 4-pulse DEER, however, without the unmodulated background present as a dominant fraction in the latter signal.}, journal={Journal of Magnetic Resonance}, publisher={Elsevier BV}, author={Milikisiyants, Sergey and Voinov, Maxim A. and Smirnov, Alex I.}, year={2018}, month={Aug}, pages={9–18} }
 @article{ou_voinov_smirnov_smirnova_2018, title={Silica-Supported Lipid Bilayers: Electrostatic Effects at Lipid Interfaces as Reported by Spin-Labeling EPR}, volume={114}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/J.BPJ.2017.11.565}, DOI={10.1016/J.BPJ.2017.11.565}, abstractNote={Interfacing biological and artificial systems at the nano-scale level is essential for developing novel living-nonliving biotechnology platforms including biosensors based on biological systems. Despite an impressive progress, the needs remain high to understand the influence of a nanostructured support and nanoconfinement on structure and properties of the membrane-protein interface. We have utilized novel pH-sensitive lipids IMTSL-PE and IKMTSL-PE to assess the phospholipid membrane surface potential. It was shown that bilayers formed from zwitterionic or mixed lipids on silica nanoparticle surfaces possess a higher negative electrostatic potential than the unsupported bilayers with the potential of mixed bilayers containing negatively charged lipids being significantly more sensitive to the silica support. pH-sensitive EPR probes were then employed to label model WALP peptide known to form α-helix when integrated into a lipid bilayer. The effects of silica support on the peptide dynamics and the effective pKa of the ionizable probe have been investigated. It was demonstrated that the silica nanoparticles affect the peptide dynamics and shift the effective pKa of the ionizable nitroxide probe in a membrane depth-dependent manner. Our data also show that upon protonation of the membrane-burred model ionisable sidechain the silica support causes changes in the membrane association of WALP peptide that are not observed when WAPL is integrated into unilamellar phospholipid vesicles. Supported by NSF 1508607 to TIS.}, number={3}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Ou, Erkang and Voinov, Maxim A. and Smirnov, Alex I. and Smirnova, Tatyana I.}, year={2018}, month={Feb}, pages={96a} }
 @article{chestnut_milikisiyants_koolivand_voynov_smirnova_smirnov_2018, title={Using Hyscore Spectroscopy of Nitroxides to Profile Water Content of Lipid Bilayers with 2 Å Spatial Resolution}, volume={114}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/J.BPJ.2017.11.130}, DOI={10.1016/J.BPJ.2017.11.130}, abstractNote={Mapping water molecules across lipid bilayers at high spatial resolution is important for both understanding membrane biophysics and membrane protein biological function. Current biophysical methods to determine water concentration in biological systems have several limitations, the main one being insufficient spatial resolution. Water penetration profiles of lipid bilayers have been measured using electron spin echo envelope modulation (ESEEM) of nitroxide spin labels based on magnetic interactions of nitroxides with the matrix water molecules. However, the matrix ESEEM effect is caused by through-space dipolar interactions, restricting spatial sensitivity to ∼10 Å, a distance which spans a significant portion of the lipid bilayer. Overhauser DNP is a less direct method to determine local water concentration, via measuring bulk water polarization induced by microwave irradiation of a spin label, and depends on complex spin dynamics inside the lipid bilayer. Hyperfine sublevel correlation (HYSCORE) spectroscopy is a sensitive technique to detect hydrogen bonds formed with paramagnetic centers. Here we demonstrate the use of HYSCORE spectroscopy to directly and accurately measure the fraction of water molecules hydrogen bonded to the nitroxide oxygen atom. To convert HYSCORE data into local water concentration we employed a normalization factor for the H-bonded deuteron signal that was taken as intensity of the ESEEM signal measured under identical experimental conditions and spectrometer tuning parameters. We demonstrate that the water molecules hydrogen-bonded to transmembrane WALP peptides, which were spin-labeled at various positions with MTSL, can be accurately determined even in the hydrophobic region of the lipid membrane. A correlation between the observed H-bonded signals and local water concentration has been established using model systems containing mixtures of diglyme and CH3OD, as well as diglyme and deuterated water, with Tempol as the spin probe.}, number={3}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Chestnut, Melanie and Milikisiyants, Sergey and Koolivand, Amir and Voynov, Maxim A. and Smirnova, Tatyana I. and Smirnov, Alex I.}, year={2018}, month={Feb}, pages={16a} }
 @article{margita_voinov_smirnov_2017, title={Effect of Solution Ionic Strength on the pKa of the Nitroxide pH EPR Probe 2,2,3,4,5,5-Hexamethylimidazolidin-1-oxyl}, volume={75}, ISSN={1085-9195 1559-0283}, url={http://dx.doi.org/10.1007/S12013-017-0780-Y}, DOI={10.1007/S12013-017-0780-Y}, abstractNote={Spin probe and spin labeling Electron Paramagnetic Resonance methods are indispensable research tools for solving a wide range of bioanalytical problems-from measuring microviscosity and polarity of phase-separated liquids to oxygen concentrations in tissues. One of the emerging uses of spin probes are the studies of proton transfer-related and surface electrostatic phenomena. The latter Electron Paramagnetic Resonance methods rely on molecular probes containing an additional functionality capable of reversible ionization (protonation, in particular) in the immediate proximity to an Electron Paramagnetic Resonance-active reporter group, such as (N-O • ) for nitroxides. The consequent formation of protonated and nonprotonated nitroxide species with different magnetic parameters (A  iso , g  iso ) could be readily distinguished by Electron Paramagnetic Resonance. Bioanalytical Electron Paramagnetic Resonance studies employing pH-sensitive paramagnetic probes typically involve determination of the equilibrium constant (pK  a ) between the protonated and nonprotonated forms of the nitroxide. However, any chemical equilibrium involving charged species, such as ionization of acids and bases, and so the reversible protonation of the nitroxide, is known to be affected by an ionic strength of the solution. Currently, only scarce data for the effect of the solution ionic strength on the experimental pK  a 's of the ionizable nitroxides can be found in the literature. Here we have carried out a series of Electron Paramagnetic Resonance titration experiments for aqueous solutions of 2,2,3,4,5,5-hexamethylimidazolidin-1-oxyl (HMI) nitroxide known for one of the largest differences in the isotropic nitrogen hyperfine coupling constant A  iso  between the protonated and nonprotonated forms. Electrolyte concentration was varied over an exceptionally large range (i.e., from 0.05 to 5.0 M) to elucidate the effect of ionic strength on the ionization constant of this pH-sensitive Electron Paramagnetic Resonance probe and the data were compared to the Debye-Hückel limiting law. Effects of the ionic strength on the magnetic parameters of the ionizable nitroxides are also discussed.}, number={2}, journal={Cell Biochemistry and Biophysics}, publisher={Springer Nature}, author={Margita, Kaleigh and Voinov, Maxim A. and Smirnov, Alex I.}, year={2017}, month={Feb}, pages={185–193} }
 @article{ou_voinov_smirnov_smirnova_2017, title={Effects of Silica Support on Dynamics of Transmembrane Peptides and Effective p K a of Ionisable Sidechains}, volume={112}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/J.BPJ.2016.11.967}, DOI={10.1016/J.BPJ.2016.11.967}, abstractNote={Hybrid nanostructures composed of lipid bilayers and nanoparticles provide new avenues for biophysical studies as well as development of biotechnological platforms to interface manmade non-living systems with living organisms. At present, however, little is known about the influence of the nanostructured support and confinement on electrostatic properties of the membrane-protein interface. Here we report on spin-labeling EPR studies to 1) evaluate the effect of anionic lipid surface charge density on the effective pKa of membrane-burred ionisable sidechains and 2) assess effects of the solid inorganic interface, specifically, silica support, on heterogeneous dielectric environment along the α-helix of a WALP peptide integrated in a lipid bilayer. The change in the protonation state of the pH-sensitive ionisable nitroxide label was directly observed by CW EPR. We have shown that the effective pKa of the probe increases by 2.1 to 2.3 pH units (depending on the depth of the probe) upon replacing zwitterionic PC with anionic PG lipids, with almost 80% of that pKa shift observed upon replacing only half of the PC with PG lipids. We have also shown that placing a lipid bilayer with integrated transmembrane α-helical WALP peptide on the surface of silica nanoparticles affects the peptide dynamics and shifts the effective pKa of the probe in a membrane depth-dependent manner. The latter effect was attributed to the negative charge of the silica surface. Supported by NSF 1508607 to TIS.}, number={3}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Ou, Erkang and Voinov, Maxim and Smirnov, Alex I. and Smirnova, Tatyana I.}, year={2017}, month={Feb}, pages={175a} }
 @article{mitra_gao_zheng_wu_voinov_smirnov_smirnova_wang_chavala_han_2017, title={Glycol Chitosan Engineered Autoregenerative Antioxidant Significantly Attenuates Pathological Damages in Models of Age-Related Macular Degeneration}, volume={11}, ISSN={1936-0851 1936-086X}, url={http://dx.doi.org/10.1021/ACSNANO.7B00429}, DOI={10.1021/ACSNANO.7B00429}, abstractNote={Age-related macular degeneration (AMD) is the foremost cause of irreversible blindness in people over the age of 65 especially in developing countries. Therefore, an exploration of effective and alternative therapeutic interventions is an unmet medical need. It has been established that oxidative stress plays a key role in the pathogenesis of AMD, and hence, neutralizing oxidative stress is an effective therapeutic strategy for treatment of this serious disorder. Owing to autoregenerative properties, nanoceria has been widely used as a nonenzymatic antioxidant in the treatment of oxidative stress related disorders. Yet, its potential clinical implementation has been greatly hampered by its poor water solubility and lack of reliable tracking methodologies/processes and hence poor absorption, distribution, and targeted delivery. The water solubility and surface engineering of a drug with biocompatible motifs are fundamental to pharmaceutical products and precision medicine. Here, we report an engineered water-soluble, biocompatible, trackable nanoceria with enriched antioxidant activity to scavenge intracellular reactive oxygen species (ROS). Experimental studies with in vitro and in vivo models demonstrated that this antioxidant is autoregenerative and more active in inhibiting laser-induced choroidal neovascularization by decreasing ROS-induced pro-angiogenic vascular endothelial growth factor (VEGF) expression, cumulative oxidative damage, and recruitment of endothelial precursor cells without exhibiting any toxicity. This advanced formulation may offer a superior therapeutic effect to deal with oxidative stress induced pathogeneses, such as AMD.}, number={5}, journal={ACS Nano}, publisher={American Chemical Society (ACS)}, author={Mitra, Rajendra N. and Gao, Ruijuan and Zheng, Min and Wu, Ming-Jing and Voinov, Maxim A. and Smirnov, Alex I. and Smirnova, Tatyana I. and Wang, Kai and Chavala, Sai and Han, Zongchao}, year={2017}, month={May}, pages={4669–4685} }
 @article{voinov_scheid_kirilyuk_trofimov_smirnov_2017, title={IKMTSL-PTE, a Phospholipid-Based EPR Probe for Surface Electrostatic Potential of Biological Interfaces at Neutral pH: Effects of Temperature and Effective Dielectric Constant of the Solvent}, volume={121}, ISSN={1520-6106 1520-5207}, url={http://dx.doi.org/10.1021/ACS.JPCB.7B00592}, DOI={10.1021/ACS.JPCB.7B00592}, abstractNote={The synthesis and characterization of a lipidlike electrostatic spin probe, (S)-2,3-bis(palmitoyloxy)propyl 2-((4-(4-(dimethylamino)-2-ethyl-1-oxyl-5,5-dimethyl-2,5-dihydro-1H-imidazol-2-yl)benzyl)disulfanyl)ethyl phosphate (IKMTSL-PTE), are being reported. The intrinsic pKa0 of IKMTSL-PTE was determined by X-band (9.5 GHz) electron paramagnetic resonance (EPR) titration of a water-soluble model compound, 4-(dimethylamino)-2-ethyl-2-(4-(((2-hydroxyethyl)disulfanyl)methyl)phenyl)-5,5-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl (IKMTSL-ME), an adduct of methanethiosulfonate spin label IKMTSL and 2-mercaptoethanol. The pKa0 of IKMTSL-ME in bulk aqueous solutions was found to be significantly higher than that of 4-(((2-hydroxyethyl)disulfanyl)methyl)-2,2,3,5,5-pentamethylimidazolidin-1-oxyl (IMTSL-ME), an adduct of the corresponding methanethiosulfonate spin label IMTSL and 2-mercaptoethanol (17 °C, pKa0 = 6.16 ± 0.03 vs 20 °C, pKa0 = 3.33 ± 0.03, respectively). A series of EPR titration experiments with IKMTSL-ME in aqueous solutions containing 0-60% v/v isopropanol have been carried out at 17 and 48 °C to determine the effects of temperature and bulk dielectric permittivity constant, ε, on the probe pKa. A linear relationship between the probe pKa and ε has been established and found to be essentially the same at 17 and 48 °C. The polarity term contributing to the pKa of IKMTSL-PTE at an uncharged lipidlike interface was determined by incorporating the probe into electrically neutral micelles formed from nonionic detergent Triton X-100, and it was found, similar to IMTSL-PTE, to be negative. In negatively charged DMPG lipid bilayers, IKMTSL-PTE exhibits ionization transitions with significantly higher pKa values than those previously reported for IMTSL-PTE (e.g., at 17 °C, pKai = 7.80 ± 0.03 vs pKa0 = 5.70 ± 0.05). The surface electrostatic potentials of DMPG lipid bilayers calculated using IKMTSL-PTE titration data were found to be somewhat lower than those calculated using IMTSL-PTE. The lower values measured by IKMTSL-PTE are the likely consequences of the structure of the linker that positions the reporter nitroxide further away from the bilayer plane into aqueous phase. Overall, the ionization transitions of IKMTSL-PTE with pKa values close to the neutral pH range make this lipidlike molecule a valuable spectroscopic EPR probe for studying the electrostatic phenomena at biological interfaces, including lipid bilayer/membrane protein systems, that could be unstable in the acidic pH range accessible by the previously available probes.}, number={11}, journal={The Journal of Physical Chemistry B}, publisher={American Chemical Society (ACS)}, author={Voinov, Maxim A. and Scheid, Christina T. and Kirilyuk, Igor A. and Trofimov, Dmitrii G. and Smirnov, Alex I.}, year={2017}, month={Mar}, pages={2443–2453} }
 @article{marek_voinov_smirnov_2017, title={Spin Probe Multi-Frequency EPR Study of Unprocessed Cotton Fibers}, volume={75}, ISSN={1085-9195 1559-0283}, url={http://dx.doi.org/10.1007/S12013-017-0787-4}, DOI={10.1007/S12013-017-0787-4}, abstractNote={Known since the ancient times, cotton continues to be one of the essential materials for the human civilization. Cotton fibers are almost pure cellulose and contain both crystalline and amorphous nanodomains with different physicochemical properties. While understanding of interactions between the individual cellulose chains within the crystalline phase is important from a perspective of mechanical properties, studies of the amorphous phase lead to characterization of the essential transport parameters, such as solvent diffusion, dyeing, drug release, and toxin absorption, as well as more complex processes of enzymatic degradation. Here, we describe the use of spin probe electron paramagnetic resonance methods to study local polarity and heterogeneous viscosity of two types of unprocessed cotton fibers, G. hirsutum and G. barbadense, harvested in the State of North Carolina, USA. These fibers were loaded with two small molecule nitroxide probes that differ in polarity-Tempo and its more hydrophilic derivative Tempol-using a series of polar and non-polar solvents. The electron paramagnetic resonance spectra of the nitroxide-loaded cotton fibers were analyzed both semi-empirically and by least-squares simulations using a rigorous stochastic theory of electron paramagnetic resonance spectra developed by Freed and coworkers. A software package and least-squares fitting protocols were developed to carry out automatic simulations of multi-component electron paramagnetic resonance spectra in both first-derivative and the absorption forms at multiple resonance frequencies such as X-band (9.5 GHz) and W-band (94.3 GHz). The results are compared with the preceding electron paramagnetic resonance spin probe studies of a commercial bleached cotton sheeting carried out by Batchelor and coworkers. One of the results of this study is a demonstration of a co-existence of cellulose nanodomains with different physicochemical properties such as polarity and microviscosity that are affected by solvents and temperature. Spin labeling studies also revealed a macroscopic heterogeneity in the domain distribution along the cotton fibers and a critical role the cuticular layer is playing as a barrier for spin probe penetration. Finally but not lastly, the simultaneous multi-component least-squares simulation method of electron paramagnetic resonance spectra acquired at different resonant frequencies and the display forms (e.g., absorption and first-derivative displays) and the strategy of spectral parameter sharing could be potentially applicable to other heterogeneous biological systems in addition to the cotton fibers studies here.}, number={2}, journal={Cell Biochemistry and Biophysics}, publisher={Springer Nature}, author={Marek, Antonin and Voinov, Maxim A. and Smirnov, Alex I.}, year={2017}, month={Mar}, pages={211–226} }
 @article{roberts_voinov_schmidt_smirnova_sombers_2016, title={The Hydroxyl Radical is a Critical Intermediate in the Voltammetric Detection of Hydrogen Peroxide}, volume={138}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/JACS.5B13376}, DOI={10.1021/JACS.5B13376}, abstractNote={Cyclic voltammetry is a widely used and powerful tool for sensitively and selectively measuring hydrogen peroxide (H2O2). Herein, voltammetry was combined with electron paramagnetic resonance spectroscopy to identify and define the role of an oxygen-centered radical liberated during the oxidation of H2O2. The spin-trap reagents, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and 2-ethoxycarbonyl-2-methyl-3,4-dihydro-2H-pyrrole-1-oxide (EMPO), were employed. Spectra exhibit distinct hyperfine patterns that clearly identify the DMPO(•)-OH and EMPO(•)-OH adducts. Multiple linear regression analysis of voltammograms demonstrated that the hydroxyl radical is a principal contributor to the voltammetry of H2O2, as signal is attenuated when this species is trapped. These data incorporate a missing, fundamental element to our knowledge of the mechanisms that underlie H2O2 electrochemistry.}, number={8}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Roberts, James G. and Voinov, Maxim A. and Schmidt, Andreas C. and Smirnova, Tatyana I. and Sombers, Leslie A.}, year={2016}, month={Feb}, pages={2516–2519} }
 @article{voinov_good_ward_milikisiyants_marek_caporini_rosay_munro_ljumovic_brown_et al._2015, title={Cysteine-Specific Labeling of Proteins with a Nitroxide Biradical for Dynamic Nuclear Polarization NMR}, volume={119}, ISSN={1520-6106 1520-5207}, url={http://dx.doi.org/10.1021/ACS.JPCB.5B05230}, DOI={10.1021/ACS.JPCB.5B05230}, abstractNote={Dynamic nuclear polarization (DNP) enhances the signal in solid-state NMR of proteins by transferring polarization from electronic spins to the nuclear spins of interest. Typically, both the protein and an exogenous source of electronic spins, such as a biradical, are either codissolved or suspended and then frozen in a glycerol/water glassy matrix to achieve a homogeneous distribution. While the use of such a matrix protects the protein upon freezing, it also reduces the available sample volume (by ca. a factor of 4 in our experiments) and causes proportional NMR signal loss. Here we demonstrate an alternative approach that does not rely on dispersing the DNP agent in a glassy matrix. We synthesize a new biradical, ToSMTSL, which is based on the known DNP agent TOTAPOL, but also contains a thiol-specific methanethiosulfonate group to allow for incorporating this biradical into a protein in a site-directed manner. ToSMTSL was characterized by EPR and tested for DNP of a heptahelical transmembrane protein, Anabaena sensory rhodopsin (ASR), by covalent modification of solvent-exposed cysteine residues in two (15)N-labeled ASR mutants. DNP enhancements were measured at 400 MHz/263 GHz NMR/EPR frequencies for a series of samples prepared in deuterated and protonated buffers and with varied biradical/protein ratios. While the maximum DNP enhancement of 15 obtained in these samples is comparable to that observed for an ASR sample cosuspended with ~17 mM TOTAPOL in a glycerol-d8/D2O/H2O matrix, the achievable sensitivity would be 4-fold greater due to the gain in the filling factor. We anticipate that the DNP enhancements could be further improved by optimizing the biradical structure. The use of covalently attached biradicals would broaden the applicability of DNP NMR to structural studies of proteins.}, number={32}, journal={The Journal of Physical Chemistry B}, publisher={American Chemical Society (ACS)}, author={Voinov, Maxim A. and Good, Daryl B. and Ward, Meaghan E. and Milikisiyants, Sergey and Marek, Antonin and Caporini, Marc A. and Rosay, Melanie and Munro, Rachel A. and Ljumovic, Milena and Brown, Leonid S. and et al.}, year={2015}, month={Jul}, pages={10180–10190} }
 @inbook{voinov_smirnov_2015, title={Ionizable Nitroxides for Studying Local Electrostatic Properties of Lipid Bilayers and Protein Systems by EPR}, volume={564}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84944354832&partnerID=MN8TOARS}, DOI={10.1016/bs.mie.2015.08.007}, abstractNote={Electrostatic interactions are known to play a major role in the myriad of biochemical and biophysical processes. Here, we describe biophysical methods to probe local electrostatic potentials of proteins and lipid bilayer systems that are based on an observation of reversible protonation of nitroxides by electron paramagnetic resonance (EPR). Two types of probes are described: (1) methanethiosulfonate derivatives of protonatable nitroxides for highly specific covalent modification of the cysteine's sulfhydryl groups and (2) spin-labeled phospholipids with a protonatable nitroxide tethered to the polar head group. The probes of both types report on their ionization state through changes in magnetic parameters and degree of rotational averaging, thus, allowing the electrostatic contribution to the interfacial pKa of the nitroxide, and, therefore, the local electrostatic potential to be determined. Due to their small molecular volume, these probes cause a minimal perturbation to the protein or lipid system. Covalent attachment secures the position of the reporter nitroxides. Experimental procedures to characterize and calibrate these probes by EPR, and also the methods to analyze the EPR spectra by simulations are outlined. The ionizable nitroxide labels and the nitroxide-labeled phospholipids described so far cover an exceptionally wide range of ca. 2.5-7.0 pH units, making them suitable to study a broad range of biophysical phenomena, especially at the negatively charged lipid bilayer surfaces. The rationale for selecting proper electrostatically neutral interface for probe calibration, and examples of lipid bilayer surface potential studies, are also described.}, booktitle={Methods in Enzymology}, publisher={Elsevier BV}, author={Voinov, Maxim A. and Smirnov, Alex I.}, year={2015}, pages={191–217} }
 @article{donohue_voynov_milikisiyants_smirnov_smirnova_2015, title={“Snorkeling” of the Charged Sidechain of a Transmembrane Peptide as Directly Observed by Double Electron-Electron Resonance Experiment}, volume={108}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/J.BPJ.2014.11.1120}, DOI={10.1016/J.BPJ.2014.11.1120}, abstractNote={While hydrophobic amino acids constitute the bulk of transmembrane protein domains, polar and even charged amino acids are not uncommon and often play significant roles in membrane protein function. Positioning a polar residue within the bilayer core is highly unfavorable thermodynamically; however, the free energy penalty could be minimized by “stretching” the side chain of the amino acid to bring the charged moiety closer to the bilayer surface while keeping the rest of the side chain inside the hydrophobic core. This biophysical phenomenon is known as “snorkeling”. Here we report experimental observations of “snorkeling” for nitroxide-modified side-chains upon protonation, its dependence upon the location along the transmembrane peptide helix, and how this snorkeling is affected by the membrane electrostatic surface potential. pH sensitive spin labels, either IMTSL or IKMTSL (JPCB 2009, 113, 3453) were attached to two cysteine residues positioned equidistant from the center of the WALP peptide so that the primary sequence of each peptide is palindromic, thus, ensuring symmetric location of the labels with respect to the bilayer. The change in protonation states of the nitroxide was directly observed from EPR spectra. The distance between two nitroxide moieties was measured by Q-band double electron-electron resonance (DEER) experiment. Upon protonation, the distance between the two IMTSL probes increased compared to that of the neutral forms, by approximately 3 Å indicating displacements of the charged nitroxide sidechain towards the polar head region. The “snorkeling” of the label was observed to be depth dependent - no changes in the positioning of the sidechain upon protonation was observed for labels located within 10-8 Å from the center of the bilayer. Supported by NSF-0843632 to TIS.}, number={2}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Donohue, Matthew and Voynov, Maxim and Milikisiyants, Sergey and Smirnov, Alex I. and Smirnova, Tatyana I.}, year={2015}, month={Jan}, pages={203a} }
 @article{marek_koolivand_song_voinov_smirnov_2014, title={Curved Lipid Bilayers: Structure, Dynamics, Phase Properties and Surface Electrostatics}, volume={106}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/J.BPJ.2013.11.1296}, DOI={10.1016/J.BPJ.2013.11.1296}, abstractNote={A growing body of experimental data suggests that at least some of membrane-anchored and membrane-associated proteins are capable of sensing the membrane curvature. Further, highly curved lipid bilayers and small vesicles are involved in such important cellular processes as membrane fusion, endo- and exocytosis, and tubules' formation. Finally, Golgi apparatus represents an example of highly curved lipid structure. While significance of membrane curvature in cellular regulatory processes is emerging, limited data exist on biophysical properties of highly curved lipid bilayers. Here we summarize results of differential scanning calorimetry and spin labeling EPR studies of unilamellar vesicles (SUV) with average diameter ranging from 200 to 30 nm. Analysis of DSC data at multiple scan rates revealed broadening and shifts of the main phase transition of DMPC from ca. 22.9 to 23.6 oC. This observation is consistent with bilayer compression and an increase in local order parameter reveled by EPR and oxygen accessibility measurements. To assess the surface electrostatics of lipid vesicles we employed EPR of a recently introduced phospholipid (IMTSL-PTE) bearing a pH-sensitive nitroxide covalently attached to the lipid head group (Biophys. J. 2013, 104: 106). The magnitude of the negative surface electrostatic potential, Ψ, for POPG increased from −137 to −167 mV upon decrease in the vesicle diameter from 107 to 31 nm even though zeta-potentials were identical. This effect could be again rationalized by increase in lipid packing upon increase in curvature for the bilayer in fluid phase. However, the effect vanished for the gel phase. We conclude that biologically relevant fluid bilayer phase allows for a larger variability in the lipid packing density in the lipid polar head group region than a more ordered gel phase. Supported by U.S. DOE Contract DE-FG02-02ER15354.}, number={2}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Marek, Antonin and Koolivand, Amir and Song, David and Voinov, Maxim A. and Smirnov, Alex I.}, year={2014}, month={Jan}, pages={221a} }
 @article{donohue_voynov_smirnov_smirnova_2014, title={Profiling the Dielectric Constant at the Membrane-Peptide Interface using Ionizable EPR Probes}, volume={106}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/J.BPJ.2013.11.2841}, DOI={10.1016/J.BPJ.2013.11.2841}, abstractNote={Polarity, electric potentials, and hydration are the major physico-chemical characteristics of lipid membranes that govern membrane-protein and protein-protein interactions as well as small molecules transport. Insertion of transmembrane proteins perturbs membrane structure altering local dielectric environment and hydration at the membrane-protein interface. The significance of distorting local membrane structure at the lipid-protein interface for modulating protein-protein interactions should not be overlooked. In this work we report on employing pH-sensitive ionizable EPR labels to profile a heterogeneous dielectric environment along the α-helix of a WALP peptide integrated in a lipid bilayer. Labels were attached to two cysteine residues positioned equidistant from the center of the peptide so that the primary sequence of each peptide is palindromic, thus insuring symmetric location of the labels with respect to the bilayer center. The change in protonation state of the nitroxide was directly observed by EPR. Q-band double electron-electron resonance (DEER) experiments were carried out to determine the distance between spin labels when imbedded in lipid bilayers to provide information about the label location. Thus, for the first time measurements of local electrostatics at peptide-bilayer interface were based on direct distance measurements rather than on assumptions on the probe location. Two pH sensitive spin labels, methanethiosulfonic acid S-(1-oxyl-2,2,3,5,5-pentamethyl-imidazolidin-4-ylmethyl) ester (IMTSL) and S-4-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-1-oxyl-2,5-dihydro-1H-imidazol-2-yl) benzylmethanethiosulfonate (IKMTSL), with intrinsic pKa's differing by approximately 2 pH units were used to expand the pH range of the titration experiments. This provided the opportunity to vary the lipid composition in order to investigate effect of the surface charge on dielectric profile at peptide-membrane interface. Water penetration at the peptide-membrane interface was assessed by hyperfine sublevel correlation spectroscopy (HYSCORE) experiment in which the hyperfine coupling between the nitroxide and hydrogen/deuterium atom of water is measured. Supported by NSF-0843632 to TIS.}, number={2}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Donohue, Matthew and Voynov, Maxim A. and Smirnov, Alex I. and Smirnova, Tatyana I.}, year={2014}, month={Jan}, pages={508a} }
 @article{donohue_voynov_smirnov_smirnova_2013, title={Molecular pH Probes at a Protein-Lipid Interface: Assessment of Local Dielectric Environment for Transmembrane Peptide}, volume={104}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/j.bpj.2012.11.2073}, DOI={10.1016/j.bpj.2012.11.2073}, abstractNote={Spin-labeling EPR spectroscopy has found many applications in studying structure and dynamics of proteins and biological membranes. Recently, there has been substantial interest in utilizing EPR to characterize local effects of polarity and hydrogen bonding in these systems. Here we report on employing an pH-sensitive EPR probe IMSTL (S-(1-oxyl-2,2,3,5,5-pentamethylimidazolidin-4-ylmethyl) ester) to profile heterogeneous dielectric environments along the α-helix of a WALP peptide integrated in a lipid bilayer. A series of symmetrically positioned double cysteine mutants were labeled with a pH-sensitive nitroxide and the protonation state of IMTSL was directly observed by EPR. Q-band DEER experiments with double-labeled WALPs were employed to derive nitroxide-nitroxide distances of nitroxides before and after the protonation and, therefore, the positions of pH probes with respect to lipid bialyer. Thus, for the first time measurements of local electrostatics at peptide-bilayer interface were based on direct distance measurements rather than on assumptions on the probe location. For double-labeled WALP consecutive protonation of symmetrically positioned nitroxide tags was observed. The difference in observable pKa values was interpreted in terms of electrostatic interaction energy between titratable probes allowing us to estimate effective dielectric constant. Supported by NSF-0843632 to TIS.}, number={2}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Donohue, Matthew and Voynov, Maxim A. and Smirnov, Alex I. and Smirnova, Tatyana I.}, year={2013}, month={Jan}, pages={373a} }
 @article{voinov_marek_kett_smirnov_2013, title={Surface Electrostatics and Peptide Binding to Lipid Bilayer of Defined Curvature}, volume={104}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/j.bpj.2012.11.579}, DOI={10.1016/j.bpj.2012.11.579}, abstractNote={Many proteins are known to have strong binding preferences for curved lipid bilayer surfaces. This property of proteins to sense membrane curvature has a number of implications in cellular processes including endocytosis, exocytosis, and vesicle trafficking as well as pathogenic processes associated with viral infections or protein aggregation disorders. Here we employ small unilamellar vesicles (SUVs) that are sized by extrusion through nanoporous filters as a model of convex membranes with accessible outer surface. For concave structures we developed nanopore-confined tubular lipid bilayers with tunable curvature radii ranging from ca. 40 to 250 nm. The nanotubular bilayers are formed by lipid self-assembly inside cylindrical nanopores of anodic aluminum oxide (AAO). These systems impose essentially no restrictions on the lipid composition and have the inner surface of negative curvature accessible for solute molecules and peptide binding. Using these model structures we show that the surface electrostatic potential of lipid bilayers is affected by the curvature to a rather large degree. For example, for SUVs composed of negatively charged lipids the magnitude of the surface potential increased with bilayer bending from ca. −106 mV for 100 nm SUV to −166 mV for 30 nm SUVs. These measurements were carried out by spin probe EPR method using recently synthesized lipids having pH-reporting nitroxides covalently tethered to the lipid polar head. EPR titration experiments were followed by the measurement of the lipid vesicle electrophoretic mobility and peptide binding. Overall, the data indicate that the bilayer bending affects the local electrostatic potential in a rather large degree providing a likely biophysical mechanism for affecting protein binding to lipid membranes without adjusting lipid composition. Supported by U.S. DOE Contract DE-FG02-02ER15354.}, number={2}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Voinov, Maxim A. and Marek, Antonin and Kett, Peter and Smirnov, Alex I.}, year={2013}, month={Jan}, pages={98a} }
 @article{voinov_rivera-rivera_smirnov_2013, title={Surface Electrostatics of Lipid Bilayers by EPR of a pH-Sensitive Spin-Labeled Lipid}, volume={104}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/j.bpj.2012.11.3806}, DOI={10.1016/j.bpj.2012.11.3806}, abstractNote={Many biophysical processes such as insertion of proteins into membranes and membrane fusion are governed by bilayer electrostatic potential. At the time of this writing, the arsenal of biophysical methods for such measurements is limited to a few techniques. Here we describe a, to our knowledge, new spin-probe electron paramagnetic resonance (EPR) approach for assessing the electrostatic surface potential of lipid bilayers that is based on a recently synthesized EPR probe (IMTSL-PTE) containing a reversibly ionizable nitroxide tag attached to the lipids' polar headgroup. EPR spectra of the probe directly report on its ionization state and, therefore, on electrostatic potential through changes in nitroxide magnetic parameters and the degree of rotational averaging. Further, the lipid nature of the probe provides its full integration into lipid bilayers. Tethering the nitroxide moiety directly to the lipid polar headgroup defines the location of the measured potential with respect to the lipid bilayer interface. Electrostatic surface potentials measured by EPR of IMTSL-PTE show a remarkable (within ±2%) agreement with the Gouy-Chapman theory for anionic DMPG bilayers in fluid (48°C) phase at low electrolyte concentration (50 mM) and in gel (17°C) phase at 150-mM electrolyte concentration. This agreement begins to diminish for DMPG vesicles in gel phase (17°C) upon varying electrolyte concentration and fluid phase bilayers formed from DMPG/DMPC and POPG/POPC mixtures. Possible reasons for such deviations, as well as the proper choice of an electrostatically neutral reference interface, have been discussed. Described EPR method is expected to be fully applicable to more-complex models of cellular membranes.}, number={1}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Voinov, Maxim A. and Rivera-Rivera, Izarys and Smirnov, Alex I.}, year={2013}, month={Jan}, pages={106–116} }
 @article{li_voinov_smirnov_2012, title={Chaperon and Lipid Composition Requirements for Transmembrane Insertion of CesA Helices 4 and 5}, volume={102}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/j.bpj.2011.11.2411}, DOI={10.1016/j.bpj.2011.11.2411}, abstractNote={Folding of proteins with large transmembrane domains such as cellulose synthase CesA occurs in a highly heterogeneous environment of cellular membranes. Currently, detailed biophysical understanding of these processes is missing with only few examples of membrane proteins allowing for reversible folding found in the literature. Recently, in the course of ongoing investigation of the transmembrane structure of selected CesA helices, we have determined that both transmembrane helices (TMH) 4 and 5 would not insert properly into lipid bilayers and form aggregates unless specific requirements for lipid composition and/or chaperones are satisfied. In cellular membranes the roles of folding chaperons could be played by other proteins or even other domains of the same CesA. Here we have shown that an alanine-rich and generally non-aggregating membrane-spanning α-helical WALP peptide could also play such a role: when co-inserted with CesA TMHs, WALP promotes formation of transmembrane α-helices and prevents TMHs aggregation. Further, from screening of various lipids we determined that a short lipid DLPC (1,2-dilauroyl-sn-glycero- 3-phosphocholine) is the most suitable for formation of unaggregated CesA TMHs. Interestingly, a longer DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine) lipid, when doped with 20 mol% of DLPC, showed even lower fraction of unaggregated peptide than in pure DLPC. Additional data on the hydrophobic matching of TMH4 and 5 to the bilayer thickness were provided by spin-labeling EPR studies. EPR data also showed that TMH4 and 5 are correctly assembled together in lipid bilayers even in the absence of the extracellular loop. Overall, the presence of various lipids in cellular membranes appears to be an important requirement for proper folding of complex integral membrane proteins. Supported as a part of the Center for LignoCellulose Structure and Formation under DOE Award DE-SC0001090.}, number={3}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Li, Le and Voinov, Maxim A. and Smirnov, Alex I.}, year={2012}, month={Jan}, pages={440a} }
 @article{kirilyuk_svyatchenko_morozov_kazachinskaya_kiselev_bakunova_voinov_loktev_grigoryev_2012, title={In vitro cytotoxicity of nitroxyl radicals with respect: To tumor and diploid human cells and estimation of their antiviral activity}, volume={57}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84874999709&partnerID=MN8TOARS}, number={1-2}, journal={Antibiotiki i Khimioterapiya}, author={Kirilyuk, I.A. and Svyatchenko, V.A. and Morozov, D.A. and Kazachinskaya, E.I. and Kiselev, N.N. and Bakunova, S.M. and Voinov, M.A. and Loktev, V.B. and Grigoryev, I.A.}, year={2012}, pages={3–12} }
 @article{smirnova_voynov_poluektov_smirnov_2012, title={Probing Dielectric and Hydrogen Bonding Gradients in Biological Membranes}, volume={102}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/j.bpj.2011.11.2262}, DOI={10.1016/j.bpj.2011.11.2262}, abstractNote={Nitroxide spin-labeling in combination with EPR spectroscopy has found many applications in studying structure and dynamics of proteins and biological membranes. Recently, there has been a substantial interest in utilizing EPR to characterize local effects of polarity and hydrogen bonding in proteins and biological membrane systems. Here we report on employing an arsenal of advanced spin-labeling EPR methods to profile heterogeneous dielectric and hydrogen bonding environment along the α-helical chain of an alanine-rich WALP peptide that is anchored in a lipid bilayer in a transmembrane orientation. A series of WALP cysteine mutants was labeled with a pH-sensitive nitroxide IMSTL (S-(1-oxyl-2,2,3,5,5-pentamethylimidazolidin-4-ylmethyl) ester) that is similar in molecular volume to phenylalanine. The protonation state of this nitroxide could be directly observed by EPR allowing us to follow proton gradient across the membrane in the vicinity of the WALP α-helix, and, thus, to reconstruct the gradient in the effective dielectric constant. These experiments were complemented by assessing local polarity from characteristic changes in EPR spectra that were enhanced by the use of perdeuterated and 15N-substituted nitroxides and high field EPR at 130 GHz (D-band). Formation of hydrogen bonds between the nitroxides and membrane-penetrating water molecules was observed directly in HYSCORE X-band experiments. Such measurements allowed us to derive experimental profiles of heterogeneous dielectric and hydrogen bonding environment along a typical transmembrane α-helix. Supported by: NSF-0843632 to TIS and NIH 1R01GM072897 to AIS.}, number={3}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Smirnova, Tatyana I. and Voynov, Maxim A. and Poluektov, Oleg G. and Smirnov, Alex I.}, year={2012}, month={Jan}, pages={414a} }
 @article{bobko_efimova_voinov_khramtsov_2012, title={Unique oxidation of imidazolidine nitroxides by potassium ferricyanide: Strategy for designing paramagnetic probes with enhanced sensitivity to oxidative stress}, volume={46}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84864674603&partnerID=MN8TOARS}, DOI={10.3109/10715762.2012.692785}, abstractNote={Abstract Potassium ferricyanide (PF), routinely employed for the oxidation of sterically-hindered hydroxylamines to nitroxides, is considered to be chemically inert towards the latter. In the present study, we report on an unexpected oxidative fragmentation of the imidazolidine nitroxides containing hydrogen atom in the 4-position of the heterocycle (HIMD) by PF resulting in the loss of the EPR signal. The mechanistic EPR, spectrophotometric, electrochemical and HPLC–MS studies support the assumption that the HIMD fragmentation is facilitated by the proton abstraction from the 4-position of the oxoammonium cation formed as a result of the initial one-electron HIMD oxidation. Increase in steric hindrance around the radical fragment by introducing ethyl substituents decreased the rate of ascorbate-induced HIMD reduction by more than 20 times, but did not affect the rate of ferricyanide-induced HIMD oxidation. This preferential sensitivity of HIMDs to oxidative processes has been used to detect peroxyl radicals in the presence of high concentration of the reducing agent, ascorbate. HIMD-based EPR probes capable to discriminate oxidative and reductive processes might find application in biomedicine and related fields for monitoring the oxidative stress and reactive radical species in biological systems.}, number={9}, journal={Free Radical Research}, author={Bobko, A.A. and Efimova, O.V. and Voinov, M.A. and Khramtsov, V.V.}, year={2012}, pages={1115–1122} }
 @article{dikalov_kirilyuk_voinov_grigor’ev_2011, title={EPR detection of cellular and mitochondrial superoxide using cyclic hydroxylamines}, volume={45}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79952406501&partnerID=MN8TOARS}, DOI={10.3109/10715762.2010.540242}, abstractNote={Abstract Superoxide (O2•−) has been implicated in the pathogenesis of many human diseases, but detection of the O2•− radicals in biological systems is limited due to inefficiency of O2•− spin trapping and lack of site-specific information. This work studied production of extracellular, intracellular and mitochondrial O2•− in neutrophils, cultured endothelial cells and isolated mitochondria using a new set of cationic, anionic and neutral hydroxylamine spin probes with various lipophilicity and cell permeability. Cyclic hydroxylamines rapidly react with O2•−, producing stable nitroxides and allowing site-specific O2•− detection in intracellular, extracellular and mitochondrial compartments. Negatively charged 1-hydroxy-4-phosphono-oxy-2,2,6,6-tetramethylpiperidine (PP-H) and positively charged 1-hydroxy-2,2,6,6-tetramethylpiperidin-4-yl-trimethylammonium (CAT1-H) detected only extramitochondrial O2•−. Inhibition of EPR signal by SOD2 over-expression showed that mitochondria targeted mitoTEMPO-H detected intramitochondrial O2•− both in isolated mitochondria and intact cells. Both 1-hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine (CP-H) and 1-hydroxy-3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine (CM-H) detected an increase in cytoplasm O2•− stimulated by PMA, but only CM-H and mitoTEMPO-H showed an increase in rotenone-induced mitochondrial O2•−. These data show that a new set of hydroxylamine spin probes provide unique information about site-specific production of the O2•− radical in extracellular or intracellular compartments, cytoplasm or mitochondria.}, number={4}, journal={Free Radical Research}, author={Dikalov, S.I. and Kirilyuk, I.A. and Voinov, M. and Grigor’Ev, I.A.}, year={2011}, pages={417–430} }
 @book{voinov_smirnov_2011, title={Spin labels and spin probes for measurements of local pH and electrostatics by EPR}, volume={22}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84876733821&partnerID=MN8TOARS}, DOI={10.1039/9781849730877-00071}, abstractNote={We dedicate this review to the memory of Prof. Leonid Volodarsky – the true pioneer of the synthesis and applications of pH-sensitive nitroxides.}, number={1}, journal={Electron Paramagnetic Resonance}, publisher={Royal Society of Chemistry (RSC)}, author={Voinov, Maxim A. and Smirnov, Alex I.}, year={2011}, pages={71–106} }
 @article{voinov_marek_li_smirnov_2011, title={Surface Electrostatics Associated with Lipid Bilayer Curvature}, volume={100}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/j.bpj.2010.12.2956}, DOI={10.1016/j.bpj.2010.12.2956}, abstractNote={Spontaneous lipid bilayer bending and related curvature are being recognized as an essential mechanism associated with many cellular functions. Experimental observations indicate that tubular vesicles of just 50 to 100 nm in diameter are commonly formed within the Golgi body and also between the endoplasmic reticulum and the Golgi. While changes in various conditions may promote the tubulation of the Golgi membrane, detailed understanding of basic biophysical processes beyond the bilayer bending are missing in the literature and so are convenient models of highly curved lipid tubules. Here we report a systematic study of a surface potential for both small unilamellar monodisperse lipid vesicles (SUMV) with diameters ranging from 30 to 100 nm and also lipid tubules that are stabilized by confining these structures within rigid homogeneous nanopores of similar diameters. For example, for SUMVs composed of negatively charged lipids the magnitude of the surface potential increased with bilayer bending from ca. −106 mV for 100 nm SUMV to −166 mV for 30 nm SUMV. These measurements were carried out by spin probe EPR method using recently synthesized lipids having pH-reporting nitroxides covalently tethered to the lipid polar head. Parallel differential scanning calorimetry experiments indicated the presence of at least two components within the lipid phase. These phase components were characterized by measurably different phase transition temperatures and correlation times of the lipid thermal relaxation. Overall, the data indicate that the bilayer bending affects the local electrostatic potential and lipid fluctuation properties in a rather large degree and is likely associated with a mechanism for cellular machinery function. Supported by U.S. DOE Contract DE-FG02-02ER15354.}, number={3}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Voinov, Maxim A. and Marek, Antonin and Li, Le and Smirnov, Alex I.}, year={2011}, month={Feb}, pages={505a} }
 @article{voinov_pagán jason o. sosa_morrison_smirnova_smirnov_2011, title={Surface-Mediated Production of Hydroxyl Radicals as a Mechanism of Iron Oxide Nanoparticle Biotoxicity}, volume={133}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/ja104683w}, DOI={10.1021/ja104683w}, abstractNote={Emerging applications of nanosized iron oxides in nanotechnology introduce vast quantities of nanomaterials into the human environment, thus raising some concerns. Here we report that the surface of γ-Fe(2)O(3) nanoparticles 20-40 nm in diameter mediates production of highly reactive hydroxyl radicals (OH(•)) under conditions of the biologically relevant superoxide-driven Fenton reaction. By conducting comparative spin-trapping EPR experiments, we show that the free radical production is attributed primarily to the catalytic reactions at the nanoparticles' surface rather than being caused by the dissolved metal ions released by the nanoparticles as previously thought. Moreover, the catalytic centers on the nanoparticle surface were found to be at least 50-fold more effective in OH(•) radical production than the dissolved Fe(3+) ions. Conventional surface modification methods such as passivating the nanoparticles' surface with up to 935 molecules of oleate or up to 18 molecules of bovine serum albumin per iron oxide core were found to be rather ineffective in suppressing production of the hydroxyl radicals. The experimental protocols developed in this study could be used as one of the approaches for developing analytical assays for assessing the free radical generating activity of a variety of nanomaterials that is potentially related to their biotoxicity.}, number={1}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Voinov, Maxim A. and Pagán Jason O. Sosa and Morrison, Erin and Smirnova, Tatyana I. and Smirnov, Alex I.}, year={2011}, month={Jan}, pages={35–41} }
 @article{smirnova_voynov_poluektov_smirnov_2010, title={Heterogeneous Dielectric and Hydrogen Bonding Environment of Transmembrane Peptides}, volume={98}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/j.bpj.2009.12.494}, DOI={10.1016/j.bpj.2009.12.494}, abstractNote={A complex set of hydrogen bonding and hydrophobic interactions between the protein side chains and cellular membrane components is primary responsible for such important biophysical processes as initial protein binding/docking to cellular membranes, membrane insertion, folding, and the final adaptation of the correct transmembrane position. Although general concepts of membrane protein folding and thermodynamic stability are beginning to emerge, the experimental data on the transmembrane profile of the effective dielectric constant and the local hydrogen bond network formed by membrane protein side chains remain severely limited. Here we describe the use of an arsenal of modern spin-labeling EPR methods to profile heterogeneous dielectric and hydrogen bonding environment along a series of the alpha-helical chain of the alanine-rich WALP peptide that adopts a transmembrane orientation. Firstly, we have employed a recently described pH-sensitive cysteine-specific spin-label IMSTL (methanethiosulfonic acid S-(1-oxyl-2,2,3,5,5-pentamethylimidazolidin-4-ylmethyl) ester) to label a series of WALP cysteine mutants. EPR titrations of such peptides reconstituted into anionic lipid bilayers yield the magnitude of relative changes in the effective dielectric constant across the bilayer in the vicinity of the peptide alpha-helix. Secondly, perdeuterated and 15N-substituted nitroxides in combination with High Field EPR at 130 GHz (D-band) were used to assess local polarity and formation of hydrogen bonds for the same series of spin-labeled WALP mutants. Finally, the nature of the hydrogen bonds observed by EPR was ascertained by a series of HYSCORE X-band measurements. It was concluded that such combination of EPR techniques significantly expands the capabilities of spin-labeling methods in studies of membrane proteins as demonstrated by deriving profiles of heterogeneous dielectric and hydrogen bonding environment along a typical transmembrane alpha-helix. Supported by NSF-0843632 to TIS and NIH 1R01GM072897 to AIS.}, number={3}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Smirnova, Tatyana I. and Voynov, Maxim A. and Poluektov, Oleg G. and Smirnov, Alex I.}, year={2010}, month={Jan}, pages={87a} }
 @article{marek_voinov_smirnov_2010, title={Surface Electrostatics and Lipid-Substrate Interactions of Nanopore-Confined Lipid Bilayers}, volume={98}, ISSN={0006-3495}, url={http://dx.doi.org/10.1016/j.bpj.2009.12.2165}, DOI={10.1016/j.bpj.2009.12.2165}, abstractNote={Substrate-supported lipid bilayers serve many purposes: from acting as versatile models of cellular membranes to biotechnological applications including substrate functionalization and stabilizing membrane proteins in functional conformations. While adsorption and subsequent reorganization of phospholipid vesicles on solid substrates were studied in the past, the exact nature of physicochemical interactions between the lipids and substrate surfaces remain largely unknown. Here we employed recently synthesized pH-sensitive spin-labeled phospholipids - derivatives of 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (PTE) with pH-reporting nitroxides that are covalently attached to the lipid's headgroup - to investigate surface electrostatics of nanotubular lipid bilayers confined in cylindrical nanopores. The lipid nanotubes were formed by self-assembling phospholipids inside ordered nanochannels of anodic aluminum oxide with pore diameters from 60 to 170 nm and diameter-to-pore length ratio of up to 1:1000. 31P NMR confirmed formation of macroscopically aligned lipid nanotubes with just 1-2° mosaic spread from zwitterionic DMPC, anionic DMPG, and their mixtures. Interfacial potentials were measured by carrying out titration experiments and observing the protonation state of the nitroxide tag by EPR. For nanopore-confined DMPC:DMPG (1:1) bilayers the protonation equilibrium was shifted to more acidic values: when the single lipid bilayer was deposited per nanopore the pKa of the nitroxide probe was shifted by (−0.91±0.05) pH units but only by (−0.34±0.05) when three bilayers per nanopore were present. Notably, the nitrogen hyperfine coupling constant for non-protonated nitroxides remained the same in all the samples indicating essentially the same interfacial dielectric environment. Thus, these shifts in pKa must come from changes in the lipid bialyer surface potential that was estimated to increase by 52±3 mV. EPR data on the lipid-substrate interface were combined with differential scanning calorimetry to elucidate effects of pore curvature, surface modification, and binding of antibacterial peptides on lipid-substrate interactions. Supported by DE-FG02-02ER15354.}, number={3}, journal={Biophysical Journal}, publisher={Elsevier BV}, author={Marek, Antonin and Voinov, Maxim A. and Smirnov, Alex I.}, year={2010}, month={Jan}, pages={402a} }
 @article{woldman_semenov_bobko_kirilyuk_polienko_voinov_bagryanskaya_khramtsov_2009, title={Design of liposome-based pH sensitive nanoSPIN probes: nano-sized particles with incorporated nitroxides}, volume={134}, ISSN={0003-2654 1364-5528}, url={http://dx.doi.org/10.1039/b818184e}, DOI={10.1039/b818184e}, abstractNote={Liposome-based nanoSized Particles with Incorporated Nitroxides, or nanoSPINs, were designed for EPR applications as pH probes in biological systems. Phospholipid membrane of the liposomes with incorporated gramicidin A showed selective permeability to a small analyte, H(+), while protecting entrapped sensing nitroxide from biological reductants. An application of the pH-sensitive nanoSPIN in an ischemia model in rat heart homogenate allows for monitoring ischemia-induced acidosis while protecting encapsulated nitroxide against bioreduction.}, number={5}, journal={The Analyst}, publisher={Royal Society of Chemistry (RSC)}, author={Woldman, Yakov Y. and Semenov, Sergey V. and Bobko, Andrey A. and Kirilyuk, Igor A. and Polienko, Julya F. and Voinov, Maxim A. and Bagryanskaya, Elena G. and Khramtsov, Valery V.}, year={2009}, pages={904} }
 @article{voinov_kirilyuk_smirnov_2009, title={Spin-Labeled pH-Sensitive Phospholipids for Interfacial pKa Determination: Synthesis and Characterization in Aqueous and Micellar Solutions}, volume={113}, ISSN={1520-6106 1520-5207}, url={http://dx.doi.org/10.1021/jp810993s}, DOI={10.1021/jp810993s}, abstractNote={The synthesis and characterization of spin-labeled phospholipids (SLP)--derivatives of 1,2-dipalmitoyl-sn-glycero-3-phosphothioethanol (PTE)--with pH-reporting nitroxides that are covalently attached to the lipid's polar headgroup are being reported. Two lipids were synthesized by reactions of PTE with thiol-specific, pH-sensitive methanethiosulfonate spin labels methanethiosulfonic acid S-(1-oxyl-2,2,3,5,5-pentamethylimidazolidin-4-ylmethyl) ester (IMTSL) and S-4-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-1-oxyl-2,5-dihydro-1H-imidazol-2-yl)benzyl methanethiosulfonate (IKMTSL). The pKa values of the IMTSL-PTE lipid measured by EPR titration in aqueous buffer/isopropyl alcohol solutions of various compositions were found to be essentially the same (pKa approximately 2.35), indicating that in mixed aqueous/organic solvents, the amphiphilic lipid molecules could be shielded from changing bulk conditions by a local shell of solvent molecules. To overcome this problem, the spin-labeled lipids were modeled by synthesizing IMTSL- and IKMTSL-2-mercaptoethanol adducts. These model compounds yielded the intrinsic pKa0's for IMTSL-PTE and IKMTSL-PTE in aqueous buffers as 3.33 +/- 0.03 and 5.98 +/- 0.03, respectively. A series of EPR titrations of IMTSL-PTE in mixed water/isopropyl alcohol solution allowed for calibrating the polarity-induced pKa shifts, deltapKapol, vs bulk solvent dielectric permittivity. These calibration data allowed for estimating the local dielectric constant, epsilon(eff), experienced by the reporter nitroxide of the IMTSL-PTE lipid incorporated into the nonionic Triton X-100 micelles as 60 +/- 5 and 57 +/- 5 at 23 and 48 degrees C, respectively. For micelles formed from an anionic surfactant sodium dodecyl sulfate (SDS) the electrostatic-induced pKa shift, deltapKael = 2.06 +/- 0.04 units of pH, was obtained by subtracting the polarity-induced contribution. This shift yields psi = -121 mV electric potential of the SDS micelle surface.}, number={11}, journal={The Journal of Physical Chemistry B}, publisher={American Chemical Society (ACS)}, author={Voinov, Maxim A. and Kirilyuk, Igor A. and Smirnov, Alex I.}, year={2009}, month={Mar}, pages={3453–3460} }
 @article{polienko_grigor’yev_voinov_2009, title={Synthesis of N'-substituted amidines through the cleavage an oxadiazolone heterocycle by weakly basic nucleophiles. Effect of the nature of the nucleophile and of the nucleophile/substrate molar ratio}, volume={45}, ISSN={0009-3122 1573-8353}, url={http://dx.doi.org/10.1007/S10593-009-0226-6}, DOI={10.1007/S10593-009-0226-6}, number={1}, journal={Chemistry of Heterocyclic Compounds}, publisher={Springer Science and Business Media LLC}, author={Polienko, Yu. F. and Grigor’yev, I. A. and Voinov, M. A.}, year={2009}, month={Jan}, pages={59–65} }
 @article{khlestkin_polienko_voinov_smirnov_chechik_2008, title={Interfacial Surface Properties of Thiol-Protected Gold Nanoparticles:  A Molecular Probe EPR Approach}, volume={24}, ISSN={0743-7463 1520-5827}, url={http://dx.doi.org/10.1021/la702823n}, DOI={10.1021/la702823n}, abstractNote={We present a molecular probe technique for accessing interfacial surface electrostatics of ligand-protected gold nanoparticles. A series of ligands with variable length of the hydrocarbon bridge between the anchoring sulfur and the reporting pH-sensitive nitroxide is described. The protonation state of this probe is directly observed by EPR spectroscopy. For tiopronin-protected Au nanoparticles, we observed an increase in pKa of up to ca. 1.1 pH units that was affected by the position of the reporter moiety with respect to the monolayer interface.}, number={3}, journal={Langmuir}, publisher={American Chemical Society (ACS)}, author={Khlestkin, Vadim K. and Polienko, Julya F. and Voinov, Maxim A. and Smirnov, Alex I. and Chechik, Victor}, year={2008}, month={Feb}, pages={609–612} }
 @article{voinov_ruuge_reznikov_grigor’ev_smirnov_2008, title={Mapping Local Protein Electrostatics by EPR of pH-Sensitive Thiol-Specific Nitroxide}, volume={47}, ISSN={0006-2960 1520-4995}, url={http://dx.doi.org/10.1021/bi800272f}, DOI={10.1021/bi800272f}, abstractNote={A first thiol-specific pH-sensitive nitroxide spin-label of the imidazolidine series, methanethiosulfonic acid S-(1-oxyl-2,2,3,5,5-pentamethylimidazolidin-4-ylmethyl) ester (IMTSL), has been synthesized and characterized. X-Band (9 GHz) and W-band (94 GHz) EPR spectral parameters of the new spin-label in its free form and covalently attached to an amino acid cysteine and a tripeptide glutathione were studied as a function of pH and solvent polarity. The pKa value of the protonatable tertiary amino group of the spin-label was found to be unaffected by other ionizable groups present in side chains of unstructured small peptides. The W-band EPR spectra were shown to allow for pKa determination from precise g-factor measurements. Is has been demonstrated that the high accuracy of pKa determination for pH-sensitive nitroxides could be achieved regardless of the frequency of measurements or the regime of spin exchange: fast at X-band and slow at W-band. IMTSL was found to react specifically with a model protein, iso-1-cytochrome c from the yeast Saccharomyces cerevisiae, giving EPR spectra very similar to those of the most commonly employed cysteine-specific label MTSL. CD data indicated no perturbations to the overall protein structure upon IMTSL labeling. It was found that for IMTSL, g iso correlates linearly with A iso, but the slopes are different for the neutral and charged forms of the nitroxide. This finding was attributed to the solvent effects on the spin density at the oxygen atom of the NO group and on the excitation energy of the oxygen lone-pair orbital.}, number={20}, journal={Biochemistry}, publisher={American Chemical Society (ACS)}, author={Voinov, Maxim A. and Ruuge, Andres and Reznikov, Vladimir A. and Grigor’ev, Igor A. and Smirnov, Alex I.}, year={2008}, month={May}, pages={5626–5637} }
 @article{polienko_schanding_gatilov_grigor'ev_voinov_2008, title={Studies toward the Synthesis of 4-(2-R-ethyl)amino-2,2,5,5-tetramethyl-3-imidazoline 1-Oxyls. Nucleophilic Substitution of Bromide in the N-Alkyl Chain of the 1,2,4-Oxadiazol-2-one Precursor}, volume={73}, ISSN={0022-3263 1520-6904}, url={http://dx.doi.org/10.1021/jo701803a}, DOI={10.1021/jo701803a}, abstractNote={A synthetic approach to access the new nitroxides of the amidine type exhibiting pH-dependent EPR spectra through substitution of a halide in the exo-N-halogenoalkyl chain of 1-(2-bromoethyl)-6-oxyl-5,5,7,7-tetramethyltetrahydroimidazo[1,5-b][1,2,4]oxadiazol-2-one is reported. In this approach, an oxycarbonyl moiety of the oxadiazolone heterocycle plays the role of a "protecting group" for the amidine functionality. A nucleophilic cleavage of the oxadiazolone heterocycle under mild nonbasic conditions, applicable to substrates bearing substituents vulnerable to attack by strong basic nucleophiles, is elaborated. The approach allows for the new amidine nitroxides bearing various functional groups (e.g., such as CN, N3, NH2, COOEt) to be synthesized. A series of nitroxides obtained through the Staudinger/intermolecular aza-Wittig reaction of the azido derivative is also described. The nitroxides synthesized here were found to have pH-dependent two-component EPR spectra indicative of a slow, on the EPR time scale, R* left arrow over right arrow R*H+ chemical exchange, and pKa values ranging from 2.8 to 12.5 units. The guanidine derivatives synthesized in this work show the highest pKa values (pKa = 10.2 and 12.5, respectively) ever reported for the nitroxide pH-probes of a "basic type".}, number={2}, journal={The Journal of Organic Chemistry}, publisher={American Chemical Society (ACS)}, author={Polienko, Julya F. and Schanding, Thomas and Gatilov, Yury V. and Grigor'ev, Igor A. and Voinov, Maxim A.}, year={2008}, month={Jan}, pages={502–510} }
 @article{smirnova_chadwick_voinov_poluektov_tol_ozarowski_schaaf_ryan_bankaitis_2007, title={Local polarity and hydrogen bonding inside the Sec14p phospholipid-binding cavity: High-field multi-frequency electron paramagnetic resonance studies}, volume={92}, ISSN={["1542-0086"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34247863841&partnerID=MN8TOARS}, DOI={10.1529/biophysj.106.097899}, abstractNote={Sec14p promotes the energy-independent transfer of either phosphatidylinositol (PtdIns) or phosphatidylcholine (PtdCho) between lipid bilayers in vitro and represents the major PtdIns/PtdCho transfer protein in the budding yeast Saccharomyces cerevisiae. Herein, we employ multi-frequency high-field electron paramagnetic resonance (EPR) to analyze the electrostatic and hydrogen-bonding microenvironments for series of doxyl-labeled PtdCho molecules bound by Sec14p in a soluble protein-PtdCho complex. A structurally similar compound, 5-doxyl stearic acid dissolved in a series of solvents, was used for experimental calibration. The experiments yielded two-component rigid limit 130- and 220-GHz EPR spectra with excellent resolution in the gx region. Those components were assigned to hydrogen-bonded and nonhydrogen-bonded nitroxide species. Partially resolved 130-GHz EPR spectra from n-doxyl-PtdCho bound to Sec14p were analyzed using this two-component model and allowed quantification of two parameters. First, the fraction of hydrogen-bonded nitroxide species for each n-doxyl-PtdCho was calculated. Second, the proticity profile along the phospholipid-binding cavity of Sec14p was characterized. The data suggest the polarity gradient inside the Sec14p cavity is a significant contributor to the driving molecular forces for extracting a phospholipid from the bilayer. Finally, the enhanced g-factor resolution of EPR at 130 and 220 GHz provides researchers with a spectroscopic tool to deconvolute two major contributions to the x-component of the nitroxide g-matrix: hydrogen-bond formation and local electrostatic effects.}, number={10}, journal={BIOPHYSICAL JOURNAL}, author={Smirnova, Tatyana I. and Chadwick, Thomas G. and Voinov, Maxim A. and Poluektov, Oleg and Tol, Johan and Ozarowski, Andrzej and Schaaf, Gabriel and Ryan, Margaret M. and Bankaitis, Vytas A.}, year={2007}, month={May}, pages={3686–3695} }
 @article{voinov_shevelev_rybalova_gatilov_pervukhina_burdukov_grigor'ev_2007, title={α-Organoelement Nitrones:  Synthesis, Properties, and IR and 13C NMR Spectral and X-ray Structural Characterization}, volume={26}, ISSN={0276-7333 1520-6041}, url={http://dx.doi.org/10.1021/om060883o}, DOI={10.1021/om060883o}, abstractNote={α-Organoelement-substituted nitrones have been synthesized for the first time through the reaction of the α-lithiated cyclic aldonitrones of 3-imidazoline 3-oxide, pyrroline 1-oxide, 2H-imidazole 1-oxide, and 3,4-dihydroisoquinoline 2-oxide series with electrophilic reagents such as HgCl2, (CH3)3SiCl, (C2H5)3GeCl, (n-C4H9)3SnBr, Ph2P(O)Cl, Ph2PCl, PhSSPh, PhSeSePh, TsCl, and TsF. Aldonitrones of the 3-imidazoline 3-oxide and pyrroline 1-oxide series were shown to readily afford the products of the lithiation−electrophilic substitution reaction. In contrast, aldonitrones of the 2H-imidazole 1-oxide and 3,4-dihydroisoquinoline 2-oxide series react smoothly only with halogen-free electrophiles. It was found that an aldonitrone group could be lithiated and selectively reacted with electrophiles even when kinetically more acidic methylene and amino groups are present in the molecule. Characteristic features of IR and 13C NMR spectra of the compounds synthesized are discussed. Selected α-organoelement nitrones ...}, number={7}, journal={Organometallics}, publisher={American Chemical Society (ACS)}, author={Voinov, Maxim A. and Shevelev, Tikhon G. and Rybalova, Tatyana V. and Gatilov, Yury V. and Pervukhina, Natalie V. and Burdukov, Aleksei B. and Grigor'ev, Igor A.}, year={2007}, month={Mar}, pages={1607–1615} }
 @article{polienko_schanding_voinov_grigor'ev_2006, title={Improved Synthesis of 1‐Hydroxy‐2,2,5,5‐tetramethyl‐3‐imidazoline 3‐Oxide (HTIO)}, volume={36}, ISSN={0039-7911 1532-2432}, url={http://dx.doi.org/10.1080/00397910600767272}, DOI={10.1080/00397910600767272}, abstractNote={Abstract A simple, efficient, mild, and reproducible method for the synthesis of 1‐hydroxy‐2,2,5,5‐tetramethyl‐3‐imidazoline 3‐oxide is described. The method is based on the condensation of 2‐hydroxyamino‐2‐methylpropanal oxime with 2,2‐diethoxypropane in the presence of an equimolar quantity of acetic acid. Cost‐effectiveness of the condensation procedure could be also achieved by replacing 2,2‐diethoxypropane with less expensive 2,2‐dimethoxypropane.}, number={19}, journal={Synthetic Communications}, publisher={Informa UK Limited}, author={Polienko, Julya F. and Schanding, Thomas and Voinov, Maxim A. and Grigor'ev, Igor A.}, year={2006}, month={Sep}, pages={2763–2768} }
 @article{grigor'ev_voinov_fedotov_2005, title={NMR Spectra of Cyclic Nitrones. 7. The Influence of Substituents and a Hydrogen Bond on 14N and 17O Chemical Shifts in Derivatives of 3-Imidoazoline 3-Oxide}, volume={41}, ISSN={0009-3122 1573-8353}, url={http://dx.doi.org/10.1007/S10593-005-0292-3}, DOI={10.1007/S10593-005-0292-3}, number={9}, journal={Chemistry of Heterocyclic Compounds}, publisher={Springer Science and Business Media LLC}, author={Grigor'ev, I. A. and Voinov, M. A. and Fedotov, M. A.}, year={2005}, month={Sep}, pages={1134–1138} }
 @article{voinov_polienko_schanding_bobko_khramtsov_gatilov_rybalova_smirnov_grigor'ev_2005, title={Synthesis, Structure, and X-Band (9.5 GHz) EPR Characterization of the New Series of pH-Sensitive Spin Probes: N,N-Disubstituted 4-Amino-2,2,5,5-tetramethyl-3-imidazoline 1-Oxyls}, volume={70}, ISSN={0022-3263 1520-6904}, url={http://dx.doi.org/10.1021/jo0510890}, DOI={10.1021/jo0510890}, abstractNote={[reaction: see text] An approach to the synthesis of new imidazoline nitroxides bearing an N',N'-disubstituted amidine group is reported. The approach is based on the alkylation of diamagnetic 4-R-amino-1,2,2,5,5-pentamethyl-3-imidazolines with bromoacetic acid ethyl ester; the products of alkylation are further oxidized to the corresponding nitroxides. The approach allows a variety of functional groups to be introduced into the nitroxide molecule structure. Alkylation with bromoacetic acid ethyl ester was found to proceed with high regioselectivity and afford the products of exo-alkylation. The regiochemical assignment is made on the basis of 13C NMR spectra and confirmed by X-ray diffraction study. All of the nitroxides synthesized here were shown to have pH-dependent EPR spectra with pKa ranging from 3.5 to 6.2. For nitroxides 13 bearing the carboxylic group remote to the nitroxide moiety, the changes in isotropic magnetic parameters of EPR spectra due to reversible deprotonation of the carboxylic group were found to be small. For these nitroxides, we demonstrate an alternative approach for pKa determination that is based on measuring the peak-to-peak line width of the EPR spectrum in the presence of the paramagnetic broadening agent potassium ferricyanide. The partition coefficients of nitroxides in octanol/H2O and octanol/phosphate buffer solution mixtures were measured to reveal a range of their lipophilicities.}, number={24}, journal={The Journal of Organic Chemistry}, publisher={American Chemical Society (ACS)}, author={Voinov, Maxim A. and Polienko, Julya F. and Schanding, Thomas and Bobko, Andrey A. and Khramtsov, Valery V. and Gatilov, Yury V. and Rybalova, Tatyana V. and Smirnov, Alex I. and Grigor'ev, Igor A.}, year={2005}, month={Nov}, pages={9702–9711} }
 @article{smirnov_ruuge_reznikov_voinov_grigor'ev_2004, title={Site-Directed Electrostatic Measurements with a Thiol-Specific pH-Sensitive Nitroxide:  Differentiating Local pK and Polarity Effects by High-Field EPR}, volume={126}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/ja048801f}, DOI={10.1021/ja048801f}, abstractNote={This communication describes the use of a methanethiosulfonate derivative of an imidazolidine nitroxide, methanethiosulfonic acid S-(1-oxyl-2,2,3,5,5-pentamethyl-imidazolidin-4-ylmethyl) ester, IMTSL, for site-directed pKa determination of peptides by electron paramagnetic resonance. This spin label is covalently attached to the thiol group of unique cysteines incorporated into peptide structures. The tertiary amine nitrogen N3 of the label readily participates in proton exchange reactions, which are monitored through changes in EPR spectra of nitroxide moiety. Using EPR at 95 GHz (W-band) isotropic magnetic parameters of this nitroxide, both Aiso and giso, were calibrated in solvents of different polarity and pH. Two different linear correlations between Aiso and giso for acidic and basic forms of IMTSL were observed, making it possible to differentiate effects of local polarity from N3 protonation on nitroxide EPR spectra. Titration of a synthetic P11 peptide fragment of the laminin B1 chain illustrates the utility of this method.}, number={29}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Smirnov, Alex I. and Ruuge, Andres and Reznikov, Vladimir A. and Voinov, Maxim A. and Grigor'ev, Igor A.}, year={2004}, month={Jul}, pages={8872–8873} }
 @article{voinov_grigor’ev_2002, title={A route to the synthesis of previously unknown α-heteroatom substituted nitrones}, volume={43}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037170586&partnerID=MN8TOARS}, DOI={10.1016/S0040-4039(02)00279-4}, abstractNote={α-Heteroatom substituted nitrones have been synthesized for the first time by the reaction of α-lithiated cyclic aldonitrone 1,2,2,5,5-pentamethyl-3-imidazoline 3-oxide with HgCl2, (CH3)3SiCl, (C2H5)3GeCl, (n-C4H9)3SnBr and Ph2P(O)Cl. α-Chloronitrone was prepared for the first time by direct chlorination of α-lithiated aldonitrone using TsCl.}, number={13}, journal={Tetrahedron Letters}, author={Voinov, M.A. and Grigor’ev, I.A.}, year={2002}, pages={2445–2447} }
 @article{voinov_grigor’ev_2002, title={Dipole-stabilized carbanions in the series of cyclic aldonitrones 3.* The influence of the configuration of the nitrone group on H - D exchange of the methine hydrogen atom and metallation of aldonitrones}, volume={51}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0141853817&partnerID=MN8TOARS}, number={2}, journal={Russian Chemical Bulletin}, author={Voinov, M.A. and Grigor’ev, I.A.}, year={2002}, pages={283–290} }
 @article{voinov_salnikov_genaev_mamatyuk_shakirov_grigor’ev_2001, title={Dipole-stabilized carbanions in series of cyclic aldonitrones: Part 4.<sup>13</sup>C, <sup>14</sup>N and<sup>7</sup>Li NMR spectra of dipole-stabilized organolithiums derived from a cyclic aldonitrone}, volume={39}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0006347542&partnerID=MN8TOARS}, DOI={10.1002/mrc.906}, abstractNote={Abstract}, number={11}, journal={Magnetic Resonance in Chemistry}, author={Voinov, M.A. and Salnikov, G.E. and Genaev, A.M. and Mamatyuk, V.I. and Shakirov, M.M. and Grigor’Ev, I.A.}, year={2001}, pages={681–683} }
 @article{voinov_grigor’ev_volodarsky_2000, title={Dipole-stabilized carbanions in series of cyclic aldonitrones. Part 2: Reactions of the metalated aldonitrones - Derivatives of 3-imidazoline 3- oxide and 2H-imidazole 1-oxide with aldehydes and ketones}, volume={56}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0034625365&partnerID=MN8TOARS}, DOI={10.1016/S0040-4020(00)00321-5}, abstractNote={Metalated aldonitrones of 3-imidazoline 3-oxide and 2H-imidazole 1-oxide series react with a wide variety of aldehydes and ketones leading to the unknown α-hydroxymethyl nitrones. Reaction of 2,2-dimethyl-4-phenyl-2H-imidazole 1-oxide with allylacetone spontaneously lead to tricyclic 3,3,8-trimethyl-1-phenyl-5a,6,7,8-tetrahydro-3H,5H-4-oxa-2,3a-diaza-cyclopenta[c]pentalen-8-ol. The hydroxy group of (1,2,2,5,5-pentamethyl-2,5-dihydro-1H-imidazol-3-oxide-4-yl)phenylmethanol is substituted with piperidine to give, after further transformations, (1,2,2,5,5-pentamethyl-2,5-dihydro-1H-imidazol-4-yl)phenylmethanone.}, number={24}, journal={Tetrahedron}, author={Voinov, M.A. and Grigor’Ev, I.A. and Volodarsky, L.B.}, year={2000}, pages={4071–4077} }
 @article{dikalov_grigor’ev_voinov_bassenge_1998, title={Detection of superoxide radicals and peroxynitrite by 1-hydvoxy-4-phosphonooxy-2,2,6,6-tetvamethylpipevidine: Quantification of extracellular superoxide radicals formation}, volume={248}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032551742&partnerID=MN8TOARS}, DOI={10.1006/bbrc.1998.8936}, abstractNote={The reactions of the new sterically hindered hydroxylamine 1-hydroxy-4-phosphonooxy-2,2,6,6-tetramethylpiperidine (PP-H) with superoxide radical and peroxynitrite have been studied. These reactions produce the nitroxide 4-phosphonooxy-2,2,6, 6-tetramethyl-piperidinyloxy. The rate constant for reaction of superoxide with PP-H is determined as (8.4+/-0.6).10(2) M-1s-1. It was found that PP-H provides almost the same spin trapping efficacy as 1-hydroxy-3-carboxy-pyrrolidine (CP-H). The background oxidation of PP-H in blood is much less than for CP-H. The extremely slow PP-H penetration into the cells makes possible the study of extracellular formation of superoxide radical. The acute treatment of blood with nitroglycerin is shown to induce an extracellular superoxide radical formation. PP-H is more sensitive for detection of reactive oxygen species as compared with CP-H. PP-H is an effective scavenger of superoxide radical and of peroxynitrite, and can be used to quantify the extracellular formation of these reactive oxygen species.}, number={2}, journal={Biochemical and Biophysical Research Communications}, author={Dikalov, S. and Grigor’ev, I.A. and Voinov, M. and Bassenge, E.}, year={1998}, pages={211–215} }
 @article{voinov_grigor’ev_volodarsky_1998, title={Dipole stabilized carbanions in series of cyclic aldonitrones. Part I (1). Aldonitrones metallation and dimerization in LDA and n-Buli solutions}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032368875&partnerID=MN8TOARS}, DOI={10.1515/HC.1998.4.3.261}, abstractNote={Cyclic aldonitrones 1 (Figure 1) of pyrroline, 3-imidazoline and 2H-imidazole series was shown to be lithiated in LDA and π-BuLi ether solutions yielding dipole-stabilized carbanions 2In the absence of an "external" electrophile the latter ones react with aldonitrone group of a non-metallated molecule to give dimer products with the structure depending on the initial nitrone structure. Nitrones with non-conjugated aldonitrone group 4a. b yield dinitrones 8a.b (Scheme 2), whereas conjugated nitrones 5a.b yield iminonitrones 10a.b (Scheme 3). Acyclic aldonitrone 2 (PBN) does not undergo lithiation under the same conditions, which is likely to be due to its Z-configuration. Introduction: Possibilities of aldonitrone group chemical transformations are related as a rule to nucleophilic addition (2) and 1,3-dipolar cycloaddition (3) reactions. Aldonitrone group deprotonation can yield carbanion 2, stabilized with A/-oxide group inductive effect [cf. (4)]. Previously the necessary conditions providing the existence of carbanions of general formula 3 (Figure 1) are stated (5): group Y must possess a rather strong acidifying effect; be free of hydrogen atoms, which might compete with CH-protons for a base; be a poor leaving group to prevent /^-elimination in the resulting anion; the molecule must not involve groups, which are electrophilic towards deprotonating reagent or towards the anion formed. Ο Ο © θ γ . W θ 0 J θ Μ © ν ' Ö ^ d ö ö I 2 3 Figure 1 The first three conditions are fulfilled in a full measure in the case of aldonitrones. As for aldonitrone group electrophilicity, its effect could be minimized by means of a non-nucleophilic}, number={3}, journal={Heterocyclic Communications}, author={Voinov, M.A. and Grigor’ev, I.A. and Volodarsky, L.B.}, year={1998}, pages={261–270} }
 @article{voinov_volodarsky_1997, title={Synthesis and properties of N-[1-hydroxyimino-2-methyl-1-(2-pyridyl)prop-2-yl]hydroxyiamine and heterocyclic derivatives based on it}, volume={46}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031482225&partnerID=MN8TOARS}, DOI={10.1007/BF02495361}, number={1}, journal={Russian Chemical Bulletin}, author={Voinov, M.A. and Volodarsky, L.B.}, year={1997}, pages={126–132} }
 @article{voinov_martin_volodarskii_1992, title={Reactions of aldonitrones (3-imidazoline-3-oxides) with isothiocyanates}, volume={41}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037529642&partnerID=MN8TOARS}, DOI={10.1007/BF00863378}, number={11}, journal={Bulletin of the Russian Academy of Sciences Division of Chemical Science}, author={Voinov, M.A. and Martin, V.V. and Volodarskii, L.B.}, year={1992}, pages={2091–2095} }
 @article{martin_volodarskii_voinov_berezina_lelyukh_1988, title={1,3-Dipolar cycloaddition of 3-imidazoline-3-oxide nitroxyl radical to dipolarophiles containing carbon-carbon double bonds}, volume={37}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33750060391&partnerID=MN8TOARS}, DOI={10.1007/BF00961121}, number={8}, journal={Bulletin of the Academy of Sciences of the USSR Division of Chemical Science}, author={Martin, V.V. and Volodarskii, L.B. and Voinov, M.A. and Berezina, T.A. and Lelyukh, T.F.}, year={1988}, pages={1677–1683} }