METHYLCOBALAMINS FULL-STRENGTH VS HALF-STRENGTH COBALT-CARBON SIGMA-BONDS AND BOND-DISSOCIATION ENTHALPIES - A-GREATER-THAN-10(15) CO-CH3 HOMOLYSIS RATE ENHANCEMENT FOLLOWING ONE-ANTIBONDING-ELECTRON REDUCTION OF METHYLCOBALAMIN

Methylcobalamin (MeCbl, MeB12) thermolyzed in ethylene glycol from 120 to 141-degrees-C with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) as a Me. trap gives the homolysis products Co(II)B-12r and TEMPO-Me quantitatively. The 5,6-dimethylbenzimidazole axial-base-off-base-on equilibrium in ethylene glycol has an enthalpy change of -5.1 (+/- 2) kcal mol-1 and an entropy change of -10.5 (+/- 4) cal mol-1 K-1, equilibrating between the 5,6-dimethylbenzimidazole-coordinated base-on form and the two distinct yet similar non-coordinated forms: the base-off and the so-called ''tuck-in'' forms. The MeB12 Co-CH3 homolysis rates indicate an activation enthalpy of 41 +/- 3 kcal mol-1, an activation entropy of 24 +/- 6 cal mol-1 K-1, and an estimated methylcob(III)alamin Co-CH3 bond dissociation energy of 37 +/- 3 kcal mol-1. This is the strongest Co-C cobamide bond measured. Comparison of the MeCbl homolysis rate constant (10(-19 +/- 4) s-1) extrapolated to -30-degrees-C with the known reduced-methylcobamide-radical-anion values indicates rate enhancements of 10(22 +/- 4) (at -30-degrees-C) following electrochemical reduction, or still over 10(15) at 25-degrees-C. Such reduction provides an antibonding electron which weakens the Co-C bond from 37 kcal mol-1 down to ca. 12 kcal mol-1. These rate enhancements are greater than the analogous enzyme-induced Co-C cleavage rate enhancements in adenosylcobalamin (Coenzyme B-12, AdoCbl)-dependent enzymes. However, electron transfer is not predicted for the mechanism of any adenosylcobalamin-dependent or methylcobalamin-dependent enzymes.