Electron addition to alkyl cobalamins, coenzyme B12 and vitamin B12. A radiolytic and electron spin resonance study

Abstract

Exposure of dilute solutions of methyl and ethyl cobalamins and coenzyme B₁₂ in dilute solutions (D₂O + CD₃OD) to ⁶⁰Co γ-rays at 77 K gave a single broad feature in the free-spin region assigned to electron-capture species with the excess electron largely confined to a π^* corrin orbital. On warming above 77 K the methyl derivative gave a novel species with spectral features characteristic of an unpaired electron in the Co(d_x²–y²) orbital. The other two substrates gave spectra due to Co^II B_12r, both on warming and after photolyses with visible light.

The acetyl derivative gave an electron-capture species whose e.s.r. spectrum was characteristic of an electron in the Co(d_z²) orbital, which on warming above 77 K changed to the normal Co^II B_12r spectrum. The cyano derivative (vitamin B₁₂) gave electron addition into the Co(d_z²) orbital, as evidenced by the large hyperfine coupling to ¹³C from ¹³CN ligands. On annealing, cyanide ions were lost irreversibly, B_12r being detected by e.s.r. spectroscopy. In contrast, the dicyano derivative on electron addition at 77 K gave a species containing only one ¹³CN ligand. Hence in this case one CN^– ligand was lost at 77 K, with no return of the dimethylbenzimidazole ligand.

These results are discussed in terms of a new mechanism for electron-addition to alkyl cobalamins.