Cobalamins and the spectrochemical series

Abstract

UV-visible-NIR spectra of a variety of cobalamins were run in water and methanol. A broad absorption band (band A) with extinction coefficients of about an order of magnitude less than those of the αβ bands was found in the red and NIR regions for Cl-cobalamin (Cl-cbl), Br-cbl, I-cbl, SC(NH₂)₂-cbl⁺ and SeCN-cbl. OCrO₃-cbl⁻, which also has a broad absorption band in the NIR was prepared for the first time. After deconvolution, similar broad bands were seen in the visible region for many other cobalamins. The wavelengths for band A placed the cobalamins in an order similar to the spectrochemical series but different from that of the αβ and γ bands (π–π* transitions), which follow the nephelauxetic series. Band A was ascribed to a ligand-to-metal charge transfer (LMCT) transition from a π orbital in the corrin ring to Co(III). This is the first systematic study of LMCT bands in cobalamins.