The electron transfer kinetics between the hydrogenase from Desulvovibrio vulgaris (strain Hildenborough) and three different viologen mediators has been investigated by cyclic voltammetry. The mediators methyl viologen, di(n-aminopropyl) viologen and propyl viologen sulfonate differ in redox potential and in net charge. Dependent on the pH both the one-and two-electron-reduced forms or only the two-electron-reduced form of the viologens are effective in electron exchange with hydrogenase. Calculations of the second-order rate constant k for the reaction between reduced viologen and hydrogenase are based on the theory of the simplest electrocatalytic mechanism. Values for k are in the range of 106-107 M-' s-' and increase in the direction propyl viologen sulfonate + methyl viologen + di(n-aminopropyl) viologen. An explanation is based on electrostatic interactions. It is proposed that the electron transfer reaction is the rate-determining step in the catalytic mechanism.In recent years the study of electron-transfer processes in biological systems by means of electrochemical methods has received increasing attention. For a number of small-sized redox proteins (e. g. cytochrome c, ferredoxines, and azurine) a direct electron transfer at metal oxide electrodes [l] or at gold electrodes modified with organic non-electroactive organic 'promotors' [2] has been achieved. The electrode kinetics of these systems can be investigated by standard techniques like cyclic voltammetry, rotating-disk electrode or impedance measurements. Still, for most (larger-sized) redox enzymes no direct electron transfer at electrodes has been described yet. The literature on the direct electron transfer between redox enzymes and electrodes is still scarce. Some examples are hydrogenase at the dropping mercury electrode in the presence of polylysine [3], covalently modified glucose oxidase at metal Correspandence to J. C. Hoogvliet,
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