Probing Side-Chain Dynamics in Proteins by NMR Relaxation of Isolated 13C Magnetization Modes in 13CH3 Methyl Groups.

The dynamics of methyl-bearing side chains in proteins were probed by 13C relaxation measurements of a number of 13C magnetization modes in selectively 13CH3-labeled methyl groups of proteins. We first show how 13C magnetization modes in a 13CH3 spin-system can be isolated using acute-angle 1H radio-frequency pulses. The parameters of methyl-axis dynamics, a measure of methyl-axis ordering (Saxis2) and the correlation time of fast local methyl-axis motions (τf), derived from 13C relaxation in 13CH3 groups are compared with their counterparts obtained from 13C relaxation in 13CHD2 methyl isotopomers. We show that in high-molecular-weight proteins, excellent correlations are obtained between the [13CHD2]-derived Saxis2 values and those extracted from relaxation of the 13C magnetization of the I = 1/2 manifold in 13CH3 methyls. In smaller proteins, a certain degree of anticorrelation is observed between the Saxis2 and τf values obtained from 13C relaxation of the I = 1/2 manifold magnetization in 13CH3 methyls. These parameters can be partially decorrelated by inclusion in the analysis of relaxation data of the I = 3/2 manifold 13C magnetization.