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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2005-02, Vol.2005 (4), p.782-787
• Main Authors: Hamza, Mohamed S A, Zou, Xiang, Banka, Rakesh, Brown, Kenneth L, van Eldik, Rudi
• Format: Article
• Language: English
• Subjects: Cobamides - chemistry; Cobamides - metabolism; Cyanides - chemistry; Cyanides - metabolism; Imidazoles - chemistry; Kinetics; Ligands; Models, Chemical; Nucleosides - chemistry; Thermodynamics; Vitamin B 12 - analogs & derivatives; Vitamin B 12 - chemistry; Vitamin B 12 - metabolism
• ISSN: 1477-9226; 0300-9246; 1477-9234
• DOI: 10.1039/b414092c

Ligand substitution reactions of the vitamin B12 analog cyanoimidazolylcobamide, CN(Im)Cbl, with cyanide were studied. Cyanide substitutes imidazole (Im) in the alpha-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 degrees C and is characterized by the activation parameters: DeltaH(not =) = 111 +/- 2 kJ mol(-1), DeltaS(not =) = +97 +/- 6 J K(-1) mol(-1), and DeltaV(not =) = +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. 10(4) M(-1)). pKbase-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.