The B12 cofactors methylcobalamin (MeCbl) and 5′-deoxyadenosylcobalamin (AdoCbl) have long fascinated chemists because of their complex structures and unusual reactivities in biological systems; however, their electronic absorption (Abs) spectra have remained largely unassigned. In this study, we have used Abs, circular dichroism (CD), magnetic CD (MCD), and resonance Raman spectroscopic techniques to probe the electronic excited states of Co3+Cbl species that differ with respect to their upper axial ligand, including MeCbl, AdoCbl, aquacobalamin (H2OCbl+), and vitamin B12 (cyanocobalamin, CNCbl). Also included to probe the effect of the lower axial ligand on the electronic properties of Cbls is Adocobinamide (AdoCbi+), an AdoCbl derivative that lacks the tethered base 5,6-dimethylbenzimidazole (DMB) and instead binds a water molecule in the lower axial position. Spectroscopic data for each species are analyzed within the framework of time-dependent density functional theory (TD-DFT) to assign the major spectral features (the so-called α/β, D/E, and γ bands) and to generate experimentally validated electronicstructure descriptions. These studies reveal that the "unique" Abs spectra of MeCbl and AdoCbl, which differ considerably from the "typical" Abs spectra of H2OCbl+ and CNCbl, reflect the high degree of σ-donation from the alkyl ligand to the Co center and the consequent destabilization of all Co 3d orbitals. They reveal further that with increasing σ-donor strength of the upper axial ligand, the contribution from the formally unoccupied Co 3dz2 orbital to the HOMO increases, which induces a strong Co-NDMB σ-antibonding interaction, consistent with the experimentally observed lengthening of this bond from H2OCbl+ to CNCbl and MeCbl. Alternatively, our spectroscopic and computational data for MeCbl and MeCbi+ reveal that substitution of the DMB by a water molecule in the lower axial position has negligible effects on the Co-C bond. A simple model is presented that explains why the identity of the upper axial ligand has a major effect on the Co-Nax bond strength, whereas the lower axial ligand does not appreciably modulate the nature of the Co-C bond. Implications of these results with respect to enzymatic Co-C bond activation are discussed.
ASJC Scopus subject areas
- Colloid and Surface Chemistry