Notes. Activation parameters for the reaction of aquocobalamin (vitamin B12a) with small anionic and neutral ligands

Abstract

From the temperature dependence of the rate constants, activation enthalpies and entropies were determined for the reaction of aquocobalamin (vitamin B12a) with SCN–, S2O32–, NO2–, SO32– and HSO3–(ionic strength, I= 1.0 mol dm–3, KCl). These, and previously determined activation parameters for CN–, HCN, N3– and HN3, demonstrate the very small differences that exist between most small anionic and neutral ligands in their reactions with B12a. Exceptions are CN– and SO32– which have, respectively, anomalously small and large values for ΔH‡ and ΔS‡. These effects are possibly due to nucleophilic participation of these ligands in the transition state of the reaction.