Evaluation of electrochemical parameters for an EC mechanism from a global analysis of current-potential-time data: application to reductive cleavage of methylcobalamin.

Abstract

Simultaneous evaluation of electron-transfer rate constant, k degree, following chemical reaction rate constant, kf, electron-transfer coefficient, alpha, and standard potential, E degree', for electron transfer coupled to a following chemical reaction (EC mechanism) is described. A mathematical model for the current response to a potential step is developed by incorporating the appropriate concentration terms into the Butler-Volmer equation. Experimental current-potential-time (i-E-t) surfaces are fit to this model to evaluate the parameters. Fitting individual i-t or i-E curves did not yield unique parameter values whereas an i-E-t surface constituted by several i-t or i-E curves could be fitted to obtain unique values. A generalized kinetic zone diagram for the EC reaction is drawn by examining the limiting forms of the expression for current. Theoretical limits of measurable rate constants are estimated from the zone diagram. The three-dimensional electro-chemistry described above was used to study the reductive cleavage of methylcobalamin in dimethyl sulfoxide (DMSO) solvent and 0.1 M tetrabutyl-ammonium perchlorate supporting electrolyte. The parameters estimated are as follows: alpha = 0.552 +/- 0.004; k degree = 0.011 +/- 0.0015 cm s-1; kf = 1500 +/- 140 s-1; E degree' = -1.54 +/- 0.01 V. The rate constant for the following reaction, kf, in DMSO solvent is approximately 4000-fold faster than the similar process in aqueous medium. It is suggested that this enhancement is relevant to methyl group transfer in enzymatic reactions, e.g., methionine synthase, if the enzyme mechanism involves a reductive cleavage which produces a methyl radical.