Cobalamin substitution kinetics

Cobalamin and Cobaloxime Substitution Reactions.— The substitution kinetics of vitamin Biaa with 8-azaguanine and 6-mercaptopurine have been investigated in aqueous media at 25 °C and ionic strength 0.1 M. The complex pH dependences are interpreted in terms of reactions of the base on aquocobalamin with protonated and deprotonated forms of the ligands. Rate constants are listed in Table 7 and are consistent with a dissociative interchange mechanism. A similar mechanism is likely in the substitution reaction of cyanocobalamin with [Fe(CN)e] . Both base on and base off forms of the coenzyme are reactive. 

A perspective report emphasised the key role of the application of pressure in kinetic studies in bringing clarity to understanding the mechanism of substitution reactions of cobalamins.193 The effect of various alkyl substituents in the trans position on the kinetic, thermodynamic and ground-state properties has been studied. Cobalamins featuring in these studies were cyanocobalamin (vitamin Bi2), aquacobalamin and the complex formed when the cyano or water ligand is replaced... 

Thusius has studied the kinetics of cobalamin-ligand substitution reactions trans to the pendant 5,6-dimethylbenzimidazole ligand (Eqn. 59) for a variety of inorganic ligands including thiocyanate, azide, iodide and bromide ions [103]. 

Substitution reactions of cobalamins have attracted considerable attention from kineticists in recent years. In the majority of these studies conventional kinetic techniques have been employed, and kinetic data reported as a function of concentration, pH, ionic strength, and solvent concentration. The mechanistic interpretation of these data has been restricted by a shortage of activation parameters for the rate-determining step, and as a result differentiation between an or a /) mechanism for substitution reactions has been difficult. 

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