Solid-state H-2 NMR study of methyl-d(3)-cobalamin

A solid-state H-2 NMR study of methyl-d(3)-cobalamin has been performed as a function of temperature to provide information concerning the character and energetics of the motion performed by this unique bioorganometallic methyl group. Analysis of the H-2 NMR line shape indicates that the methyl group undergoes rapid three-fold rotation, and that the Co-C-H-2 angle lies between 105.9 and 109.5 degrees. Determination of the spin-lattice relaxation times T-1 shows that the relaxation is anisotropic, consistent with a ``jumping'' motion of the methyl group rather than rotational diffusion. This also provides the activation energy to methyl jumps as 8.3 +/- 1.3 kJ/mol. It is proposed that this energetic barrier may be a useful probe of changes in the electronic character of the Co-C bond that accompany the biological role of this molecule in such enzymes as methionine synthase.