@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{chekmenev_hu_gor'kov_brey_cross_ruuge_smirnov_2005, title={N-15 and P-31 solid-state NMR study of transmembrane domain alignment of M2 protein of influenza A virus in hydrated cylindrical lipid bilayers confined to anodic aluminum oxide nanopores}, volume={173}, ISSN={["1096-0856"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-14844354285&partnerID=MN8TOARS}, DOI={10.1016/j.jmr.2004.12.006}, abstractNote={This communication reports the first example of a high resolution solid-state 15N 2D PISEMA NMR spectrum of a transmembrane peptide aligned using hydrated cylindrical lipid bilayers formed inside nanoporous anodic aluminum oxide (AAO) substrates. The transmembrane domain SSDPLVVA(A-15N)SIIGILHLILWILDRL of M2 protein from influenza A virus was reconstituted in hydrated 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine bilayers that were macroscopically aligned by a conventional micro slide glass support or by the AAO nanoporous substrate. 15N and 31P NMR spectra demonstrate that both the phospholipids and the protein transmembrane domain are uniformly aligned in the nanopores. Importantly, nanoporous AAO substrates may offer several advantages for membrane protein alignment in solid-state NMR studies compared to conventional methods. Specifically, higher thermal conductivity of aluminum oxide is expected to suppress thermal gradients associated with inhomogeneous radio frequency heating. Another important advantage of the nanoporous AAO substrate is its excellent accessibility to the bilayer surface for exposure to solute molecules. Such high accessibility achieved through the substrate nanochannel network could facilitate a wide range of structure-function studies of membrane proteins by solid-state NMR.}, number={2}, journal={JOURNAL OF MAGNETIC RESONANCE}, publisher={Elsevier BV}, author={Chekmenev, EY and Hu, J and Gor'kov, PL and Brey, WW and Cross, TA and Ruuge, A and Smirnov, AI}, year={2005}, month={Apr}, pages={322–327} }