Two-Dimensional Homonuclear Chemical Shift Correlation through Cross-Polarization Matching Condition in the PAR Scheme under Magic-Angle-Spinning of Solids.

Correlating 15N resonances is attractive for establishing sequential connectivity between neighboring residues in structural studies of biomolecules. However, 15N-15N correlation experiments remain challenging because of extremely weak dipolar couplings between 15N spins. Here, we revisit the proton-assisted recoupling (PAR) scheme that facilitates magnetization exchange among dilute spins such as 13C or 15N. By selecting carbonyl carbons and monitoring magnetization transfers to other carbons under different RF field strengths under magic angle spinning (MAS), we find that the regular cross-polarization matching condition, i.e., νH - νC = ±νr, where νH and νC are the spin-lock RF field strengths for 1H and 13C, respectively, and νr is the MAS frequency, leads to rapid spin-exchange processes. Furthermore, we demonstrate that this optimized condition can be directly used to obtain efficient 15N-15N correlations with mixing times of only a few milliseconds in uniformly 15N-labeled aquaporin reconstituted in POPC/POPG lipid vesicles.