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 B_12a) with SCN^–, S₂O₃^2–, NO₂^–, SO₃^2– 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 B_12a. Exceptions are CN^– and SO₃^2– 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.