title: 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 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.
authors.affiliation: Institute for Inorganic Chemistry, University of Erlangen-Nurnberg, Egerlandstr.1, 91058, Erlangen, Germany.
authors.forename: Mohamed S A
authors.initials: MS
authors.lastname: Hamza
authors.forename: Xiang
authors.initials: X
authors.lastname: Zou
authors.forename: Rakesh
authors.initials: R
authors.lastname: Banka
authors.forename: Kenneth L
authors.initials: KL
authors.lastname: Brown
authors.forename: Rudi
authors.initials: R
authors.lastname: van Eldik
mesh_terms: D003038:Cobamides / Q000737:chemistry / Q000378:metabolism; D003486:Cyanides / Q000737:chemistry* / Q000378:metabolism; D007093:Imidazoles / Q000737:chemistry*; D007700:Kinetics; D008024:Ligands; D008956:Models, Chemical; D009705:Nucleosides / Q000737:chemistry*; D013816:Thermodynamics; D014805:Vitamin B 12 / Q000031:analogs & derivatives* / Q000737:chemistry* / Q000378:metabolism
chemical_list: D003038:Cobamides; D003486:Cyanides; D007093:Imidazoles; D008024:Ligands; D009705:Nucleosides; C029899:imidazole; D014805:Vitamin B 12