KINETIC EVIDENCE FOR UNEXPECTEDLY STRONG PRECURSOR FORMATION IN LIGAND SUBSTITUTION-REACTIONS OF AQUACOBALAMIN (VITAMIN-B(12A))

A detailed kinetic study of the substitution of aquacobalamin by a variety of substituted, performed as function of entering ligand concentration, pH, temperature and pressure. The kinetic data for the complex formation and reverse aquation reactions confirm previous findings on the operation of a dissociative interchange mechanism. Typical DELTAV(double dagger) values for complex formation are between +4, and +7 cm3 mol-1. The thermodynamic data reflect significant precursor formation for this special class of ligands investigated, with formation constants that vary between 1 and 5 M-1. This is ascribed to an interaction of the pi-system on the pyridine ligands with either the corrin moiety or other functional groups on the cobalamin molecule. Activation and thermodynamic parameters for all reaction steps involved are reported and discussed with reference to available data from the literature.