Comparative study of reaction of cobalamin and cobinamide with thiocyanate.

The interaction of Co(III) and Co(II) cobalamin (Cbl) and cobinamide (Cbi) with thiocyanate was examined with UV-vis and EPR spectra. S/N-linkage isomerism was explored on Co(III) and Co(II) Cbl and Cbi models using density functional theory (DFT; BP86, B3LYP). Performed calculations suggest the prevalence of isothiocyanato isomers over thiocyanato complexes on both Co(III) and Co(II) centers. The formation of Cbl(II) complex with thiocyanate was observed at high ligand concentrations which was proposed to be hexacoordinated. DFT data maintain the possibility of hexacoordinated Co(II) complexes with thiocyanate in which one of extra-ligands is weakly coordinated. It is found that high thiocyanate concentrations could retard cyanide binding to cobalamin but not to cobinamide.