@article{koroloff_tesch_awosanya_nevzorov_2017, title={Sensitivity enhancement for membrane proteins reconstituted in parallel and perpendicular oriented bicelles obtained by using repetitive cross-polarization and membrane-incorporated free radicals}, volume={67}, ISSN={["1573-5001"]}, DOI={10.1007/s10858-017-0090-0}, abstractNote={Multidimensional separated local-field and spin-exchange experiments employed by oriented-sample solid-state NMR are essential for structure determination and spectroscopic assignment of membrane proteins reconstituted in macroscopically aligned lipid bilayers. However, these experiments typically require a large number of scans in order to establish interspin correlations. Here we have shown that a combination of optimized repetitive cross polarization (REP-CP) and membrane-embedded free radicals allows one to enhance the signal-to-noise ratio by factors 2.4-3.0 in the case of Pf1 coat protein reconstituted in magnetically aligned bicelles with their normals being either parallel or perpendicular to the main magnetic field. Notably, spectral resolution is not affected at the 2:1 radical-to-protein ratio. Spectroscopic assignment of Pf1 coat protein in the parallel bicelles has been established as an illustration of the method. The proposed methodology will advance applications of oriented-sample NMR technique when applied to samples containing smaller quantities of proteins and three-dimensional experiments.}, number={2}, journal={JOURNAL OF BIOMOLECULAR NMR}, author={Koroloff, Sophie N. and Tesch, Deanna M. and Awosanya, Emmanuel O. and Nevzorov, Alexander A.}, year={2017}, month={Feb}, pages={135–144} }
 @article{koroloff_nevzorov_2015, title={Optimization of cross-polarization at low radiofrequency fields for sensitivity enhancement in solid-state NMR of membrane proteins reconstituted in magnetically aligned bicelles}, volume={256}, ISSN={["1096-0856"]}, DOI={10.1016/j.jmr.2015.03.016}, abstractNote={Solid-state NMR (ssNMR) of oriented membrane proteins (MPs) is capable of providing structural and dynamic information at nearly physiological conditions. However, NMR experiments performed on oriented membrane proteins generally suffer from low sensitivity. Moreover, utilization of high-power radiofrequency (RF) irradiations for magnetization transfer may give rise to sample heating, thereby decreasing the efficiency of conventional cross-polarization schemes. Here we have optimized the recently developed repetitive cross-polarization (REP-CP) sequence (Tang et al., 2011) to further increase the magnetization transfer efficiency for membrane proteins reconstituted in magnetically aligned bicelles and compared its performance to single-contact Hartmann–Hahn cross-polarization (CP), CP-MOIST and the adiabatic transfer. It has been found that employing the REP-CP sequence at RF amplitudes of 19 kHz instead of the commonly used higher RF fields (>45 kHz) enhances the efficiency of REP-CP. An additional 30% signal can be obtained as compared to the previously published REP-CP, and 20% when compared to the re-optimized REP-CP at 50 kHz RF fields. Moreover, the 15N signal gain of low-power REP-CP was found to be 40% over the adiabatic CP and up to 80% over CP-MOIST. Thus, the low-power REP-CP sequence surpasses all of the previous CP schemes in addition of having the tremendous advantage of reducing the RF powers by a factor of seven, thereby preserving the liquid-like bicelle sample. By contrast, in purely static (NAL crystal) and semi-rigid systems (Pf1 phage), the adiabatic CP was found to be more effective. Periodic oscillations of the intensity profile (distinct from the transient oscillations) as a function of the CP contact time and B1 RF field strengths were observed during the REP-CP optimization with the oscillations becoming more pronounced with lower RF fields. Many-spin simulations were performed to explain the oscillations and their periodicity.}, journal={JOURNAL OF MAGNETIC RESONANCE}, author={Koroloff, Sophie N. and Nevzorov, Alexander A.}, year={2015}, month={Jul}, pages={14–22} }