The Kinetic Activation Volumes for the Binding of Iodide to Cobalamin (Vitamin B₁₂) Studied on a High Pressure Laser Temperature Jump Apparatus

Abstract

The relaxation kinetics of the reaction of iodide with cobalamin (vitamin B12) were studied on a high pressure laser temperature jump apparatus in aqueous solution at 25" and ionic strength 0.2 M . Analysis of the pressure dependence of the formation and dissociation rate constants gave their respective volumes of activation to be: ~V I * = 5.5 * 0.8 cm"ol-' and IV,,:': = 11.5 f 1.6 cm:' mol-'. The positive activation volume for formation of the complex, A V , * , after appropriate correction for the volume change due to formation of an outer sphere complex, is consistent with a dissociative type mechanism in which the metal ion -water bond is stretched in the activated complex. IntroductionThe macrocyclic corrin compound confers unusual properties to the ligand substitution kinetics of the Co(II1) derivative. For example, the second order rate constant for ligand substitution is about lo7 faster on cobalamin (vitamin B,,) than on other simple Co(II1) compounds (1). Previous work has measured the kinetics of the reaction of cobalamin with N3-, OCN-, and SCN-(2, 3) as well as more recently the kinetics and activation parameters for I-, S, and SO,5).The benzimidazole side chain on cobalamin coordinates with the Co3+ in one of the axial positions preventing formation of 2 : 1 complexes with small ligands; this is often a complicating factor in studying the ligand substitution kinetics on other metal ion macrocyclic complexes such as porphyrins (1).By measuring the influence of pressure on the rate constant of a reaction,