1H, 13C-1H, 1H dipolar cross-correlated spin relaxation in methyl groups

Relaxation in methyl groups is strongly influenced by cross-correlated interactions involving the methyl dipoles. One of the major interference effects results from intra-methyl H-1-C-13, H-1-H-1 dipolar interactions, leading to significant differences in the relaxation of certain multiplet components that contribute to double- and zero-quantum H-1-C-13 spectra. NMR experiments are presented for the measurement of this differential relaxation effect. It is shown that this difference in relaxation between double- and zero-quantum multiplet components can be used as a sensitive reporter of side chain dynamics and that accurate methyl axis order parameters can be measured in proteins that tumble with correlation times greater than approximately 5 ns.