Electron transfer between the hydrogenase from Desulfovibrio vulgaris (Hildenborough) and viologens

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Abstract
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−1 s−1 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.