@article{nevzorov_marek_milikisiyants_smirnov_2021, title={Characterization of photonic band resonators for DNP NMR of thin film samples at 7 T magnetic field}, volume={323}, ISSN={["1096-0856"]}, DOI={10.1016/j.jmr.2020.106893}, abstractNote={Polarization of nuclear spins via Dynamic Nuclear Polarization (DNP) relies on generating sufficiently high mm-wave B1e fields over the sample, which could be achieved by developing suitable resonance structures. Recently, we have introduced one-dimensional photonic band gap (1D PBG) resonators for DNP and reported on prototype devices operating at ca. 200 GHz electron resonance frequency. Here we systematically compare the performance of five (5) PBG resonators constructed from various alternating dielectric layers by monitoring the DNP effect on natural-abundance 13C spins in synthetic diamond microparticles embedded into a commercial polyester-based lapping film of just 3 mil (76 μm) thickness. An odd-numbered configuration of dielectric layers for 1D PBG resonator was introduced to achieve further B1e enhancements. Among the PBG configurations tested, combinations of high-ε perovskite LiTaO3 together with AlN as well as AlN with optical quartz wafers have resulted in ca. 40 to over 50- fold gains in the average mm-wave power over the sample vs. the mirror-only configuration. The results are rationalized in terms of the electromagnetic energy distribution inside the resonators obtained analytically and from COMSOL simulations. It was found that average of B1e2 over the sample strongly depends on the arrangement of the dielectric layers that are the closest to the sample, which favors odd-numbered PBG resonator configurations for their use in DNP.}, journal={JOURNAL OF MAGNETIC RESONANCE}, author={Nevzorov, Alexander A. and Marek, Antonin and Milikisiyants, Sergey and Smirnov, Alex I.}, year={2021}, month={Feb} }
 @article{kovaleva_molochnikov_tambasova_marek_chestnut_osipova_antonov_kirilyuk_smirnov_2020, title={Electrostatic properties of inner nanopore surfaces of anodic aluminum oxide membranes upon high temperature annealing revealed by EPR of pH-sensitive spin probes and labels}, volume={604}, ISSN={["1873-3123"]}, url={https://doi.org/10.1016/j.memsci.2020.118084}, DOI={10.1016/j.memsci.2020.118084}, abstractNote={Anodic aluminum oxide (AAO) membranes are versatile nanomaterials that combine the chemically stable and mechanically robust properties of ceramics with homogeneous nanoscale organization that can be tuned to desirable pore diameters and lengths. The AAO substrates feature high surface area that is readily accessible to large and small molecules, making these nanostructures uniquely suited for many possible applications. Examples include templated self-assembly of macroscopically aligned biological membranes and substrates for heterogeneous catalysis. For further development of such applications, one would like to characterize and tune the electrostatic properties of the inner pore surface as well as the local acidity within the nanochannels. Here, we employed electron paramagnetic resonance (EPR) spectroscopy of a small molecule – ionizable nitroxide – as a reporter of the average local acidity in the nanochannels and the local electrostatic potential in the immediate vicinity of the pore surface. The former was achieved by measuring EPR spectra of this molecular probe diffusing in an aqueous phase confined in the AAO nanochannels while for the latter the nitroxide was covalently attached to the hydroxyl group of the alumina surface. We show that the local acidity within the nanochannels is increased by as much as ≈1.48 pH units vs. the pH of bulk solution by decreasing the pore diameter down to ca. 31 nm. Furthermore, the positive surface charge of the as-prepared AAO could be decreased and even switched to a negative surface charge upon annealing the membranes first to 700 °C and then to 1200 °C. For as-prepared AAO, the local electrostatic potential reaches ψ= (163 ± 5) mV for the nitroxide label covalently attached to AAO and located about 0.5 nm away from the surface. Overall, we demonstrate that the acid-based properties of the aqueous volume confined by the AAO nanopores pores can be tuned by either changing the pore diameter from ca. 71 to 31 nm or by thermal annealing to switch the sign of the surface charge. These observations provide a simple and robust means to tailor these versatile high-surface-area nanomaterials for specific applications that depend on acid-base equilibria.}, journal={JOURNAL OF MEMBRANE SCIENCE}, publisher={Elsevier BV}, author={Kovaleva, Elena G. and Molochnikov, Leonid S. and Tambasova, Daria and Marek, Antonin and Chestnut, Melanie and Osipova, Victoria A. and Antonov, Denis O. and Kirilyuk, Igor A. and Smirnov, Alex I}, year={2020}, month={Jun} }
 @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{nevzorov_milikisiyants_marek_smirnov_2018, title={Multi-resonant photonic band-gap/saddle coil DNP probehead for static solid state NMR of microliter volume samples}, volume={297}, ISSN={["1096-0856"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85055585775&partnerID=MN8TOARS}, DOI={10.1016/j.jmr.2018.10.010}, abstractNote={The most critical condition for performing Dynamic Nuclear Polarization (DNP) NMR experiments is achieving sufficiently high electronic B1e fields over the sample at the matched EPR frequencies, which for modern high-resolution NMR instruments fall into the millimeter wave (mmW) range. Typically, mmWs are generated by powerful gyrotrons and/or extended interaction klystrons (EIKs) sources and then focused onto the sample by dielectric lenses. However, further development of DNP methods including new DNP pulse sequences may require B1e fields higher than one could achieve with the current mmW technology. In order to address the challenge of significantly enhancing the mmW field at the sample, we have constructed and tested one-dimensional photonic band-gap (PBG) mmW resonator that was incorporated inside a double-tuned radiofrequency (rf) NMR saddle coil. The photonic crystal is formed by stacking ceramic discs with alternating high and low dielectric constants and thicknesses of λ/4 or 3λ/4, where λ is the wavelength of the incident mmW field in the corresponding dielectric material. When the mmW frequency is within the band gap of the photonic crystal, a defect created in the middle of the crystal confines the mmW energy, thus forming a resonant structure. An aluminum mirror in the middle of the defect has been used to substitute one-half of the structure with its mirror image in order to reduce the resonator size and simplify its tuning. The latter is achieved by adjusting the width of the defect by moving the aluminum mirror with respect to the dielectric stack using a gear mechanism. The 1D PBG resonator was the key element for constructing a multi-resonant integrated DNP/NMR probehead operating at 190–199 GHz EPR/300 MHz 1H/75.5 MHz 13C NMR frequencies. Initial tests of the multi-resonant DNP/NMR probehead were carried out using a quasioptical mmW bridge and a Bruker Biospin Avance II spectrometer equipped with a standard Bruker 7 T wide-bore 89 mm magnet parked at 300.13 MHz 1H NMR frequency. The mmW bridge built with all solid-state active components allows for the frequency tuning between ca. 190 and ca. 199 GHz with the output power up to 27 dBm (0.5 W) at 192 GHz and up to 23 dBm (0.2 W) at 197.5 GHz. Room temperature DNP experiments with a synthetic single crystal high-pressure high-temperature (HPHT) diamond (0.3 × 0.3 × 3.0 mm3) demonstrated dramatic 1500-fold enhancement of 13C natural abundance NMR signal at full incident mmW power. Significant 13C DNP enhancement (of about 90) have been obtained at incident mmW powers of as low as <100 μW. Further tests of the resonator performance have been carried out with a thin (ca. 100 μm thickness) composite polystyrene-microdiamond film by controlling the average mmW power at the optimal DNP conditions via a gated mode of operation. From these experiments, the PBG resonator with loaded Q ≃ 250 and finesse F≈75 provides up to 12-fold or 11 db gain in the average mmW power vs. the non-resonant probehead configuration employing only a reflective mirror.}, journal={JOURNAL OF MAGNETIC RESONANCE}, author={Nevzorov, Alexander A. and Milikisiyants, Sergey and Marek, Antonin N. and Smirnov, Alex I.}, year={2018}, month={Dec}, pages={113–123} }
 @article{kovaleva_molochnikov_antonov_stepanova_hartmann_tsmokalyuk_marek_smirnov_2018, title={Proton Activity in Nanochannels Revealed by Electron Paramagnetic Resonance of Ionizable Nitroxides: A Test of the Poisson-Boltzmann Double Layer Theory}, volume={122}, ISSN={["1932-7447"]}, url={https://doi.org/10.1021/acs.jpcc.8b04938}, DOI={10.1021/acs.jpcc.8b04938}, abstractNote={Chemical and physical processes occurring within the nanochannels of mesoporous materials are known to be determined by both the chemical nature of the solution inside the pores/channels as well as the channel surface properties, including surface electrostatic potential. Such properties are important for numerous practical applications such as heterogeneous catalysis and chemical adsorption including chromatography. However, for solute molecules diffusing inside the pores, the surface potential is expected to be effectively screened by counter ions for the distances exceeding the Debye length. Here, we employed electron paramagnetic resonance spectroscopy of ionizable nitroxide spin probes to experimentally examine the conditions for the efficient electrostatic surface potential screening inside the nanochannels of chemically similar silica-based mesoporous molecular sieves (MMS) filled with water at ambient conditions and a moderate ionic strength of 0.1 M. Three silica MMS having average channel diamet...}, number={35}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, publisher={American Chemical Society (ACS)}, author={Kovaleva, Elena G. and Molochnikov, Leonid S. and Antonov, Denis O. and Stepanova, Darya P. Tambasova and Hartmann, Martin and Tsmokalyuk, Anton N. and Marek, Antonin and Smirnov, Alex I.}, year={2018}, month={Sep}, pages={20527–20538} }
 @article{acharya_chestnut_marek_smirnov_krim_2017, title={A Combined QCM and AFM Study Exploring the Nanoscale Lubrication Mechanism of Silica Nanoparticles in Aqueous Suspension}, volume={65}, ISSN={["1573-2711"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85026840191&partnerID=MN8TOARS}, DOI={10.1007/s11249-017-0898-5}, number={3}, journal={TRIBOLOGY LETTERS}, author={Acharya, B. and Chestnut, M. and Marek, A. and Smirnov, A. I. and Krim, J.}, year={2017}, month={Sep} }
 @article{curtis_marek_smirnov_krim_2017, title={A comparative study of the nanoscale and macroscale tribological attributes of alumina and stainless steel surfaces immersed in aqueous suspensions of positively or negatively charged nanodiamonds}, volume={8}, journal={Beilstein Journal of Nanotechnology}, author={Curtis, C. K. and Marek, A. and Smirnov, A. I. and Krim, J.}, year={2017}, pages={2045–2059} }
 @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}, 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{kovaleva_molochnikov_venkatesan_marek_stepanova_kozhikhova_mironov_smirnov_2016, title={Acid-Base Properties of Nanoconfined Volumes of Anodic Aluminum Oxide Pores by EPR of pH-Sensitive Spin Probes}, volume={120}, ISSN={["1932-7447"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84958092831&partnerID=MN8TOARS}, DOI={10.1021/acs.jpcc.5b10241}, abstractNote={Anodic aluminum oxide (AAO) ceramic membranes with macroscopically aligned and hexagonally packed nanopore architecture are attractive substrates for forming nanotubular lipid bilayers as well as sorption and catalytic media because of a tunable pore diameter, robust pore structure, and low fabrication cost. Here we employed continuous wave X-band (9 GHz) EPR of two pH-sensitive nitroxide radicals to assess acid–base properties AAO membranes prepared from low-cost commercial grade aluminum and compared those with commercial Anodisc membranes from Whatman, Ltd. The AAO membranes with pore diameters ≥58 ± 8 nm showed essentially the same pH inside the pores, pHint, as the bulk external solution, pHext, over the 0.1–3.0 M range of ionic strength. However, the apparent pKa of nitroxide probes inside the pores deviated from the bulk values for the nanopores of smaller diameters of ca. 29 and 18 nm. Specifically, for the latter nanopores the values of pHint were found to be 0.5–0.8 pH unit lower than the bulk p...}, number={5}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, publisher={American Chemical Society (ACS)}, author={Kovaleva, Elena G. and Molochnikov, Leonid S. and Venkatesan, Umamaheswari and Marek, Antonin and Stepanova, Darya P. and Kozhikhova, Ksenia V. and Mironov, Maxim A. and Smirnov, Alex I.}, year={2016}, month={Feb}, pages={2703–2711} }
 @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} }
 @article{marek_tang_milikisiyants_nevzorov_smirnov_2015, title={Nanotube Array Method for Studying Lipid-Induced Conformational Changes of a Membrane Protein by Solid-State NMR}, volume={108}, ISSN={["1542-0086"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84921058848&partnerID=MN8TOARS}, DOI={10.1016/j.bpj.2014.11.011}, abstractNote={Anodic aluminum oxide substrates with macroscopically aligned homogeneous nanopores of 80 nm in diameter enable two-dimensional, solid-state nuclear magnetic resonance studies of lipid-induced conformational changes of uniformly (15)N-labeled Pf1 coat protein in native-like bilayers. The Pf1 helix tilt angles in bilayers composed of two different lipids are not entirely governed by the membrane thickness but could be rationalized by hydrophobic interactions of lysines at the bilayer interface. The anodic aluminum oxide alignment method is applicable to a broader repertoire of lipids versus bicelle bilayer mimetics currently employed in solid-state nuclear magnetic resonance of oriented samples, thus allowing for elucidation of the role played by lipids in shaping membrane proteins.}, number={1}, journal={BIOPHYSICAL JOURNAL}, publisher={Elsevier BV}, author={Marek, Antonin and Tang, Wenxing and Milikisiyants, Sergey and Nevzorov, Alexander A. and Smirnov, Alex I.}, year={2015}, month={Jan}, pages={5–9} }
 @article{lagerstedt_petrlova_hilt_marek_chung_sriram_budamagunta_desreux_thonon_jue_et al._2013, title={EPR assessment of protein sites for incorporation of Gd(III) MRI contrast labels}, volume={8}, ISSN={["1555-4309"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84876786430&partnerID=MN8TOARS}, DOI={10.1002/cmmi.1518}, abstractNote={We have engineered apolipoprotein A-I (apoA-I), a major protein constituent of high-density lipoprotein (HDL), to contain DOTA-chelated Gd(III) as an MRI contrast agent for the purpose of imaging reconstituted HDL (rHDL) biodistribution, metabolism and regulation in vivo. This protein contrast agent was obtained by attaching the thiol-reactive Gd[MTS-ADO3A] label at Cys residues replaced at four distinct positions (52, 55, 76 and 80) in apoA-I. MRI of infused mice previously showed that the Gd-labeled apoA-I migrates to both the liver and the kidney, the organs responsible for HDL catabolism; however, the contrast properties of apoA-I are superior when the ADO3A moiety is located at position 55, compared with the protein labeled at positions 52, 76 or 80. It is shown here that continuous wave X-band (9 GHz) electron paramagnetic resonance (EPR) spectroscopy is capable of detecting differences in the Gd(III) signal when comparing the labeled protein in the lipid-free with the rHDL state. Furthermore, the values of NMR relaxivity obtained for labeled variants in both the lipid-free and rHDL states correlate to the product of the X-band Gd(III) spectral width and the collision frequency between a nitroxide spin label and a polar relaxation agent. Consistent with its superior relaxivity measured by NMR, the rHDL-associated apoA-I containing the Gd[MTS-ADO3A] probe attached to position 55 displays favorable dynamic and water accessibility properties as determined by X-band EPR. While room temperature EPR requires >1 m m Gd(III)-labeled and only >10 µ m nitroxide-labeled protein to resolve the spectrum, the volume requirement is exceptionally low (~5 µl). Thus, X-band EPR provides a practical assessment for the suitability of imaging candidates containing the site-directed ADO3A contrast probe.}, number={3}, journal={CONTRAST MEDIA & MOLECULAR IMAGING}, publisher={Wiley-Blackwell}, author={Lagerstedt, Jens O. and Petrlova, Jitka and Hilt, Silvia and Marek, Antonin and Chung, Youngran and Sriram, Renuka and Budamagunta, Madhu S. and Desreux, Jean F. and Thonon, David and Jue, Thomas and et al.}, year={2013}, pages={252–264} }