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

Publisher Website
Google Scholar
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.

This publication has 0 references indexed in Scilit: