@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={https://doi.org/10.1021/acs.jpcc.9b08007}, 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{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={https://doi.org/10.1021/acsmacrolett.9b00783}, 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{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{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_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{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}, 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}, 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} }
 @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{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}, number={5}, journal={Analyst [London]}, author={Woldman, Y. Y. and Semenov, S. V. and Bobko, A. A. and Kirilyuk, I. A. and Polienko, J. F. and Voinov, M. A. and Bagryanskaya, E. G. and Khramtsov, V. V.}, year={2009}, pages={904–910} }
 @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{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} }