Spectroscopic and Computational Studies of Glutathionylcobalamin: Nature of Co–S Bonding and Comparison to Co–C Bonding in Coenzyme B₁₂

Glutathionylcobalamin (GSCbl) is a unique, biologically relevant cobalamin featuring an axial Co–S bond that distinguishes it from the enzymatically active forms of vitamin B₁₂, which possess axial Co–C bonds. GSCbl has been proposed to serve as an intermediate in cobalamin processing and, more recently, as a therapeutic for neurological disorders associated with oxidative stress. In this study, GSCbl and its close relative cysteinylcobalamin (CysCbl) were investigated using electronic absorption, circular dichroism, magnetic circular dichroism, and resonance Raman spectroscopies. The spectroscopic data were analyzed in the framework of density functional theory (DFT) and time-dependent DFT computations to generate experimentally validated electronic structure descriptions. Although the change in the upper axial ligand from an alkyl to a thiol group represents a major perturbation in terms of the size, basicity, and polarizability of the coordinating atom, our spectroscopic and computational results reveal striking similarities in electronic structure between methylcobalamin (MeCbl) and GSCbl, especially with regard to the σ donation from the alkyl/thiol ligand and the extent of mixing between the cobalt 3d and the ligand frontier orbitals. A detailed comparison of Co–C and Co–S bonding in MeCbl and GSCbl, respectively, is presented, and the implications of our results with respect to the proposed biological roles of GSCbl are discussed.