The effect of chemical modifications of cobamide co-enzyme on the rate of substitution of trans-axial ligands

Abstract

The effect of aglycone substitution in the 5′-desoxynucleoside part of cobamide co-enzyme 1 1 Notation : cobamide co-enzyme [α(5,6-dimethylbenzimidazolyl)-Co-5′-desoxyadenosylcobamide, DBC]. , modifications in the sugar part of desoxynucleoside ligand, the effect of desoxyglycosyl ligands and modifications in the “lowel” axial position on the stability of CoC and CoN Bz bonds in the cyanide cleavage reactions have been studied. Cyanide reactions involving octahedral derivatives of cobamide co-enzymes proceed via two steps: a fast step ( k 1) corresponding to the substitution of the “lower” 5,6-dimethylbenzimidazole ligand, and a slow step ( k 2) corresponding to the simultaneous cleavage of the CoC and Nglycoside bonds. Other mechanisms are involved in cyanide reactions with analogues modified at the “lower” axial ligand which presumably possess a similar structure to penta-coordinated complexes.