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 1H-13C, 1H-1H dipolar interactions, leading to significant differences in the relaxation of certain multiplet components that contribute to double- and zero-quantum 1H-13C 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.