The effect of solvent on the electrode process of methylcobalamine as studied by cyclic voltammetry

The cyclic voltammetry (CV) of methylcobalamine, Me–Cbl, was investigated as a function of solvent with different ratios of DMF/MeOH and for neat N,N-dimethylformamide (DMF) and neat dimethylsulfoxide (DMSO) in 0.1 M tetrabutylammonium fluoroborate electrolyte vs. a Ag/AgCl/Cl− reference electrode. The effect of the solvent slows the following chemical process down for DMF/MeOH ratios equal to or greater than 50% DMF. The electrode process is a chemically irreversible EC mechanism at DMF/MeOH ratios of 40:60 and 20:80 However, for the 80:20 DMF/MeOH solvent mixture, the return wave shows reoxidation of the methylcobalamine radical anion at scan rates ⩾ 100 V s−1 at room temperature. The features of this system were investigated by digital simulation of a multi-step EC mechanism of the electrode process which incorporates a scheme of squares for electroreduction of base-on and base-off forms of the MeCbl species to the base-on and base-off forms of its radical anion. The mechanism contains a series of following homogeneous reactions including Me–Co bond breaking in a solvent cage from the base-off radical anion and a final irreversible step of break-up of the caged species which gives the channel out of the scheme of squares. Simulation shows that the systems is sensitive to key parameters and gives a reasonably good fit to the experimental CV curve for the 80:20 DMF/MeOH mixture. This allows a complete parameter set of heterogeneous and homogeneous constants to be evaluated. DFT calculations (B3LYP/ LANL2DZ) indicate that the dissociation energy of Co–C bond breaking from the radical anion is less for the base-off species.