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