Homonuclear Chemical Shift Correlation in Solids Under MAS by Fast Cross-Relaxation Driven Spin Diffusion.

In this study, we report a two-dimensional NMR technique that correlates the chemical shifts of homonuclear spin systems in solids under MAS. The pulse sequence employs double spin-lock RF pulses to facilitate magnetization exchange among low-γ nuclei (such as 13C or 15N) through cross-relaxation driven by a combination of spin diffusion and RF field. We systematically investigate how the efficiency of the magnetization exchange depends on the Hartmann-Hahn mismatch and the MAS frequency. Experimental results obtained from 13C-labeled Fmoc-Leucine powder sample, 15N-labeled L-histidine amino acid (pH 6.3) powder sample, and uniformly-15N-labeled aquaporin reconstituted in DMPC lipid vesicles are reported. The results reveal a rapid spin-exchange process, with transfer rates that qualitatively correlate with the internuclear distances of the participating low-γ nuclei. It is remarkable that a 50 ms DARR mixing resulted in no cross peaks in the 2D 15N-15N chemical shift correlation spectrum of uniformly-15N-labeled aquaporin, whereas as short as 5.0 ms mixing duration using the double spin-lock mixing yielded 2D spectrum exhibiting cross peaks between neighboring amino acid residues. Our results demonstrate that the proposed approach can be utilized to enable magnetization exchange between nearby 15N or 13C nuclei, which is highly desirable for accomplishing resonance assignments in the structural studies of proteins.