THE COORDINATION OF IMIDAZOLE AND ITS DERIVATIVES BY AQUOCOBALAMIN

Formation constants have been determined for the substitution of coordinated water in aquocobalamin (vitamin B12a) by imidazole and a number of its derivatives at 25 °C, μ = 1.0 M (KCl) in the pH range 5.5 to 10.5. The observed formation constants are all strongly pH dependent. Since the binding constant for 2-methylimidazole (log K = 1.15) is low compared to that for imidazole (log K = 4.59) because of steric repulsion between the annular substituent and the corrin ring, only the five-substituted tautomers of asymmetrically substituted imidazoles are considered to be significantly coordinated by Co(III). Taking this and the state of ionisation of the pendant amino group of histamine and histidine into account, equations were derived to explain the pH dependence of the observed formation constants. From these the following pH independent formation constants (log K) were deduced: imidazole, 4.59; 2-methylimidazole, 1.15; 5-methylimidazole, 4.90; N-methylimidazole, 4.90; 1,5-dimethylimidazole, 4.82; imidazole lactic acid, 4.86; cationic histamine, 4.43; neutral histamine, 4.71; neutral histidine, 4.30; and anionic histidine, 4.60. The magnitude of the formation constant is virtually independent of the basicity of the ligand. HPLC evidence is presented to show that at sufficiently high concentrations, these imidazoles displace 5,6-dimethylbenzimidazole (dbzm) from the fifth coordination position of Co(III) with log K = 1.4 to <-1. At high pH, ionisation of bound imidazole labilises the trans 5,6-dimethylbenzimidazole ligand which is substituted by an imidazole from solution. From fits of the relevant equations to the experimental data, and from direct spectrophotometric titrations, it is shown that coordination decreases the pKa of the imino group of the imidazoles from >14 to c. 10 due to polarisation by the metal ion. Furthermore, the pKas of the pendant amino groups of histamine and histidine decrease from 9.96 and 9.18 to 8.89 and 7.99, respectively, on coordination. This is ascribed to destabilisation of the conjugate acids by coulombic repulsion with the residual positive charge at the metal centre. The possible biological significance of these results are discussed. © 1990.