Electrochemical study of methylcobalamin Determination of the reduction potential for a quasireversible 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 (E(0') = -1.529 +/- 0.004 V), the standard heterogeneous rate-constant (k(0) = 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.

Hemin-catalyzed decomposition of artemisinin (qinghaosu).

Artemisinin (qinghaosu) and its derivatives represent an important new class of antimalarial drugs. Previous work suggests that the antimalarial activity of artemisinin may be mediated by a reaction with intraparasitic hemin. Using cyclic voltammetry, artemisinin and dihydroartemisinin were irreversibly reduced at approximately -1 V. In the presence of concentrations of hemin as low as 50 nM, the reduction took place at much lower potentials (-0.435 to -0.460 V). Both reductions took place after adsorption onto the electrode surface. The shift of the reduction potential to more positive values is indicative of a catalytic process similar to that seen with hydrogen peroxide. The catalytic decomposition of artemisinin may play a role in the antimalarial activity of artemisinin.

A PC-based general program for the simulation and analysis of cyclic voltammetric experiments.

A PC-based program, a General Program for Simulation of Cyclic Voltammetric Experiments (GPS-CV), is described. GPS-CV, written in Turbo Pascal 5.5, utilizes advances in simulation methodology that increase efficiency, allow for generality of mechanism, and include IR drop and capacitive current effects. An accessible user interface is used, and a graphical analysis program is provided. Cyclic voltammograms for nearly any mechanism can be simulated by the user. A companion program, CVFIT, combines the GPS-CV program with a least-squares fit by simplex minimization to give the best-fit parameters, with error estimates. The use of CVFIT is demonstrated with a three-parameter fit of experimental cyclic voltammograms.