Probing the cis and trans Influence in Cobalamin Chemistry by Electronic Spectroscopy

Abstract

The electronic spectra of 11 cobalamin complexes were analysed by Gaussian analysis. It is shown that the αβ-region of the spectrum, sometimes attributed to a forbidden π --> π* transition and its vibrational fine structure actually consists of several overlapping transitions, of which between three and five could be resolved in each spectrum. The λ-band region could be successfully modelled in each case with three overlapping transitions. It is shown that there is no fundamental difference between the \'typical\' and \'atypical\' spectra of the cobalamins, and that the difference in appearance is merely a consequence of the relative positions of the λ-region components of the spectrum. The electronic transitions of the cobalaminsmoveto longer wavelengths as the donor power of the axial ligand increases. This cis influence, as measured by the average position of the Gaussian components of the αβ-region, correlates with the trans influence, as measured by the Co-N bond length to the trans dimethylbenzimidazole ligand of the corrins.


Keywords: Vitamin B12; Cobalamins; Electronic spectroscopy; Cis effect; Trans effect


South African Journal of Chemistry Vol.58 2005: 9-15