title: Optimization of cross-polarization at low radiofrequency fields for sensitivity enhancement in solid-state NMR of membrane proteins reconstituted in magnetically aligned bicelles.
abstract: 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 19kHz instead of the commonly used higher RF fields (>45kHz) 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 50kHz RF fields. Moreover, the (15)N 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.
authors.affiliation: Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, NC 27695-8204, USA.
authors.forename: Sophie N
authors.initials: SN
authors.lastname: Koroloff
authors.affiliation: Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, NC 27695-8204, USA. Electronic address: alex_nevzorov@ncsu.edu.
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Nevzorov
mesh_terms: D000465:Algorithms; D003198:Computer Simulation; D008051:Lipid Bilayers / Q000737:chemistry*; D060526:Magnetic Fields; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D008565:Membrane Proteins / Q000737:chemistry* / Q000528:radiation effects / Q000648:ultrastructure*; D008956:Models, Chemical; D008958:Models, Molecular; D011487:Protein Conformation; D011846:Radio Waves; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
chemical_list: D008051:Lipid Bilayers; D008565:Membrane Proteins
keywords: Low RF powers; Membrane proteins; Multiple CP contacts; Oriented samples; Repetitive cross-polarization (REP-CP); Sensitivity enhancement; Solid-state NMR