Methyl motions in 13C-methylated concanavalin as studied by 13C magnetic resonance relaxation techniques

The carbon- 13 spin-lattice relaxation times and nuclear Overhauser enhancements of the N epsilon-monomethyllysine, N epsilon,N epsilon-dimethyllysine, and N alpha,N alpha-dimethylalanine resonances of 13C-methylated concanavalin A have been measured at three carbon frequencies and compared to the relaxation parameters predicted by several motional models. The experimental parameters cannot be reproduced by a simple dipolar relaxation model which includes isotropic reorientation of the protein plus free internal rotational diffusion of the methyl groups but are well predicted by a wobble in a cone model which includes isotropic reorientation of the protein at 33 ns, free internal rotational diffusion of the methyl groups, and a wobble diffusion which reflects the net motion of the amino acid side chains. The analysis indicates that the methylated epsilon-amino side chains exhibit only slightly more motional freedom than does the methylated N-terminal alpha-amino group and suggests some restriction of methyl group rotation in the dimethylamino residues.