Sensitivity enhancement in two-dimensional solid-state NMR spectroscopy by transverse mixing.

The sensitivity of two-dimensional (2D) 13C-13C solid-state NMR spectroscopy under magic-angle spinning (MAS) is shown to be enhanced by the use of transverse polarization transfer in place of the conventional longitudinal polarization transfer. Experimental results are reported for 2D spectroscopy of a 20-residue, filament-forming peptide derived from the E. Coli RecA protein, containing five uniformly 13C-labeled residues, performed at 14.1 T with high-speed MAS and with finite-pulse radio-frequency-driven recoupling of dipolar interactions in the mixing period. Significant sensitivity enhancements observed at short mixing periods results from a more rapid build-up of cross-peaks under transverse mixing than under longitudinal mixing and from the 2 gain inherent in 2D measurements in which both orthogonal transverse polarization components in the t1 period contribute to each free-induction decay signal detected in the t2 period.