The chemistry of vitamin B12. Part 18. Nature of the equilibria exhibited by organocobalamins

Abstract

The equilibria between the ‘base-on’ and unprotonated ‘base-off’ forms have been studied, and values of ΔH and ΔS determined, for a series of organocobalamins. The equilibrium is increasingly displaced in favour of the ‘base-off’ form as the ligand is varied in the order: cyclopropyl ⩽ methyl < 5′-deoxy-5′-adenosyl (in the dimethylbenzimidazolylcobamide coenzyme, dbc) < ethyl ∼ propyl < isobutyl ∼ cyclobutyl < neopentyl ∼ cyclopentyl < isopropyl ∼ cyclohexyl. The equilibrium constants and the spectra of the ‘base-off’ forms are anomalous for complexes such as ethylcobalamin, and it is suggested that in their unprotonated ‘base-off’ forms the heterocyclic base interacts with the corrin ring and its substituents through hydrophobic bonding. The experimental evidence for the various forms exhibited by organocobalamins is summarised and assessed.