Effects of solvent and ionic medium on the kinetics of axial ligand substitution in vitamin B 12. Part II. the reaction between aquocobalamin and thiourea in dioxane-water and acetonitrile-water mixtures

Abstract

The rate constants for the reaction of aquocobalamin with thiourea were measured as a function of ionic strength, pH and solvent composition in dioxane-water and acetonitrile-water mixtures. With the help of solubility measurements a complete quantitative analysis of solvent effects on the reaction profile could be made. The transfer Gibbs energy of the initial state strongly depends on solvent composition. Because the transition state and the final state closely follow the initial state, this is not reflected in the rate constants. For the acetonitrile-water mixtures the transfer enthalpy and transfer entropy were determined and were found to exhibit the familiar compensation effect. It is concluded that, when the solvent changes, vitamin B 12 creates its own micro-environment around the active metal site, so that the reactivity is effectively solvent independent. The mechanism of activation is dissociative.