Combined Spectroscopic/Computational Studies of Metal Centers in Proteins and Cofactors: Application to Coenzyme B12

This article illustrates how the combined computational/spectroscopic methodology used in our studies of metal centers in proteins and cofactors can be applied to vitamin B12 and its biologically relevant derivatives. The B12 cofactors 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, Abs, circular dichroism (CD), magnetic CD (MCD), and resonance Raman spectroscopic techniques are used to probe the electronic excited states of various Co3+ Cbl species that differ with respect to their upper axial ligand. Spectroscopic data for each species are analyzed within the framework of time-dependent density functional theory (TD-DFT) to assign the major spectral features and to generate experimentally validated bonding descriptions. 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. Impli- cations of these results with respect to enzymatic Co–C bond activation are discussed.