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.