ELECTROCHEMICAL STUDY OF METHYLCOBALAMIN - DETERMINATION OF THE REDUCTION POTENTIAL FOR A QUASI-REVERSIBLE SYSTEM WITH A FAST FOLLOWING REACTION

The reductive electrochemistry of methylocobalamin in nonaqueous solution is typical of many electrochemical mechanisms in that the initial electron transfer at the electrode is followed by a fast chemical reaction. The rate constant of the following chemical step, methyl radical cleavage, was measured by double potential step chronoamperometry to be 590 sec-1 in a solvent mixture (DMF 40%, methanol 60%, at -30-degrees). The cyclic voltammetric response in the slow scan-rate regime was analyzed by the simulation-fitting program CVFIT to extract the remaining parameters of the electrode reaction chemical reaction mechanism: the formal reduction potential (E0, = -1.529 +/- 0.004 V), the standard heterogeneous rate-constant (k0 = 0.012 +/- 0.002 cm/sec), and the transfer coefficient (alpha = 0.78 +/- 0.02). This method of analysis allows for the rigorous determination of reduction potentials under conditions where the cyclic voltammetric response appears irreversible (no reverse peak is observed). A detailed analysis of the actual reversibility of the system and its effect on the apparent transfer coefficient is presented.