LIGAND SUBSTITUTION-REACTIONS OF AQUACOBALAMIN - EVIDENCE FOR A DISSOCIATIVE INTERCHANGE MECHANISM

The kinetics of substitution of bound H2O in aquacobalamin (vitamin B12a) by hydroxylamine, methyl glycinate, pyridine, 4-methylpyridine, imidazole and histamine (imidazole-4-ethanamine) was investigated as a function of ligand concentration and temperature by stopped-flow spectrophotometry at constant ionic strength (1.0 mol dm-3) and pH. In all six cases the observed pseudo-first-order rate constants, corrected for protonation of the N-donor of the ligand, L, and the ionisation of bound H2O in B12a, showed saturation at high ligand concentrations. There is a compensating change in the DELTA-H double dagger and DELTA-S double dagger values of the saturating rate constant, k(sat), but there is no general isokinetic relationship for the ligands. The dependence of the value of k(sat) and, in particular, the dependence of DELTA-H double dagger and DELTA-S double dagger for this rate constant on the entering ligand, L, indicate that the rate-limiting step in the reaction is not unimolecular release of H2O from B12a. The results are interpreted in terms of a dissociative interchange mechanism with nucleophilic participation of L in the transition state. The compensating effect is explained on the basis of the extent of bond formation between Co and L in the transition state. The steric bulk of L, quantified using molecular mechanics techniques in terms of the cone angle subtended by co-ordinated L at Co(III), and molecular volume calculations, appears to play an important role in controlling its rate of reaction with the metal centre.