Kinetic and thermodynamic studies on ligand substitution reactions and base-on/base-off equilibria of cyanoimidazolylcobamide, a vitamin B12 analog with an imidazole axial nucleoside

Abstract

Ligand substitution reactions of the vitamin B₁₂ analog cyanoimidazolylcobamide, CN(Im)Cbl, with cyanide were studied. Cyanide substitutes imidazole (Im) in the α-position more slowly than it substitutes dimethylbenzimidazole in cyanocobalamin (vitamin B₁₂). The kinetics of the displacement of Im by CN⁻ showed saturation behaviour at high cyanide concentration; the limiting rate constant was found to be 0.0264 s⁻¹ at 25 °C and is characterized by the activation parameters: ΔH^≠ = 111 ± 2 kJ mol⁻¹, ΔS^≠ = +97 ± 6 J K⁻¹ mol⁻¹, and ΔV^≠ = +9.3 ± 0.3 cm³ mol⁻¹. These parameters are interpreted in terms of an I_d mechanism. The equilibrium constant for the reaction of CN(Im)Cbl with CN⁻ was found to be 861 ± 75 M⁻¹, which is significantly less than that obtained for the reaction of cyanocobalamin with CN⁻ (viz. 10⁴ M⁻¹). pK_base-off for the base-on/base-off equilibrium was determined spectrophotometrically and found to be 0.99 ± 0.05, which is about 0.9 pH units higher than that obtained previously in the case of cyanocobalamin. In addition, the kinetics of the base-on/base-off reaction was studied using a pH-jump technique and the data obtained revealed evidence for an acid catalyzed reaction path. The results obtained in this study are discussed in reference to those reported previously for cyanocobalamin.