Abstract
A solid-state 2 H NMR study of methyl-d3-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 2 H NMR line shape indicates that the methyl group undergoes rapid three-fold rotation, and that the Co-C- 2 H angle lies between 105.9 and 109.5°. Determination of the spin-lattice relaxation times T1 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.
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