Probing Side-Chain Dynamics in Proteins by NMR Relaxation of Isolated C Magnetization Modes in CH Methyl Groups.

The dynamics of methyl-bearing side chains in proteins were probed by C relaxation measurements of a number of C magnetization modes in selectively CH-labeled methyl groups of proteins. We first show how C magnetization modes in a CH spin-system can be isolated using acute-angle H radio-frequency pulses. The parameters of methyl-axis dynamics, a measure of methyl-axis ordering () and the correlation time of fast local methyl-axis motions (τ), derived from C relaxation in CH groups are compared with their counterparts obtained from C relaxation in CHD methyl isotopomers. We show that in high-molecular-weight proteins, excellent correlations are obtained between the [CHD]-derived values and those extracted from relaxation of the C magnetization of the = 1/2 manifold in CH methyls. In smaller proteins, a certain degree of anticorrelation is observed between the and τ values obtained from C relaxation of the = 1/2 manifold magnetization in CH methyls. These parameters can be partially decorrelated by inclusion in the analysis of relaxation data of the = 3/2 manifold C magnetization.