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 (S_axis²) and the correlation time of fast local methyl-axis motions (τ_f), 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 S_axis² 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 S_axis² and τ_f 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.