Probing methyl dynamics from 13C autocorrelated and cross-correlated relaxation.

An understanding of side-chain motions in protein is of great interest since side chains often play an important role in protein folding and intermolecular interactions. A novel method for measuring dipole-dipole cross-correlated relaxation in methyl groups of uniformly 13C-labeled proteins without deuteration has been developed by our group. The excellent agreement between dynamic parameters of methyl groups in ubiquitin obtained from the cross-correlated relaxation and 13C spin-lattice relaxation and those derived previously from 2H relaxation data demonstrates the reliability of the method. This method was applied to the study of side-chain dynamics of human intestinal fatty acid binding protein (IFABP) with and without its ligand. Binding of oleic acid to the protein results in decreased mobility of most of the methyl groups in the binding region, whereas no significant change in mobility was observed for methyl groups in the nonbinding region.