The electrochemical (EC) reduction mechanism of methylcobalamin (Me-Cbl) in a mixed DMF/MeOH solvent in 0.2 M tetrabutylammonium fluoroborate electrolyte was studied as a function of temperature and solvent ratio vs a nonaqueous Ag/AgCl/Cl(-) reference electrode. Double-potential-step chronoamperometry allowed the rate constant of the subsequent homogeneous reaction to be measured over the temperature range from 0 to -80 degrees C in 40:60 and 50:50 DMF:MeOH ratios. Activation enthalpies are 5.8 +/- 0.5 and 7.6 +/- 0.3 kcal/mol in the 40:60 and 50:50 mixtures of DMF/MeOH, respectively. Digital simulation and curve-fitting for an EC mechanism using a predetermined homogeneous rate constant of 5.5 x 10(3) s(-1) give E degrees' = -1.466 V, k degrees = 0.016 cm/s, and alpha = 0.77 at 20 degrees C for a quasi-reversible electrode process. Digital simulation of the results of Lexa and Savéant (J. Am. Chem. Soc. 1978, 100, 3220-3222) shows that the mechanism is a series of stepwise homogeneous equilibrium processes with an irreversible step following the initial electron transfer (ET) and allows estimation of the equilibrium and rate constants of these reactions. An electron coupling matrix element of H(kA) = (4.7 +/- 1.1) x 10(-4) eV ( approximately 46 J/mol) is calculated for the nonadiabatic ET step for reduction to the radical anion. A reversible bond dissociation enthalpy for homolytic cleavage of Me-Cbl is calculated as 31 +/- 2 kcal/mol. The voltammetry of the ethyl-, n-propyl-, n-butyl-, isobutyl-, and adenosyl-substituted cobalamin was studied, and estimated reversible redox potentials were correlated with Co-C bond distances as determined by DFT (B3LYP/ LANL2DZ) calculations.

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Stable, functional composite films were made from the ionomer Nafion, water-insoluble surfactants, and heme proteins or the enzyme alcohol dehydrogenase. Rates of electron transfer between myoglobin (Mb) and hemoglobin (Hb) and edge plane pyrolytic graphite (PG) electrodes in these composite films were much larger than those for Hb and Mb in solution on Nafion-coated or bare PG electrodes. Films containing Nafion, didodecydimethylammonium bromide (DDAB), and hemoglobin or myoglobin retained about 90% of the initial redox activity during 4 weeks of storage at 4°C. DDAB is arranged in lamellar liquid crystal bilayers in these films at 25°C, which were more stable than films of the proteins and DDAB alone. Films of Nafion-lecithin-Cyt c on edge plane PG also had good electrochemical properties but were much less stable. Electronic and vibrational spectra indicate that the heme proteins are not denatured in the composite films, but conformational differences from the native state may exist. Linear dichroism suggests that the proteins are specifically oriented in these films. After treatment with a cross-linking agent, Nafion-lipid films containing the enzyme alcohol dehydrogenase retained twice the activity of the enzyme in solution (4°C) when stored for 2 days at room temperature.

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