NEW PHOTOSYSTEM I ELECTRON ACCEPTORS: IMPROVEMENT OF HYDROGEN PHOTOPRODUCTION BY CHLOROPLASTS *

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Abstract
Four novel electron carriers (two zwitterionic bipyridyls, dicarboxyl colbalticinium and sodium metatungstate), which are negatively charged in their reduced form, have been tested as photo‐system I acceptors and as mediators of H2evolution. Measurements of O2uptake, anaerobic photoreduction rates and stationary concentrations of reduced species under continuous illumination indicate that Coulombic interactions control the electron transfer between the photosynthetic membrane and the mediators. Both rates of forward transfer and back reaction (electron cycling) seem to depend on the charge of the electron carrier. The low concentration of anionic species in the diffuse layer associated with the membrane could explain our results. Hydrogen evolution rates obtained with these four mediators used as electron relays between the photosynthetic membrane and colloidal platinum catalyst are higher than with methylviologen. This improvement of the conversion efficiency parallels the high steady state accumulation of reduced carriers favoured by their negative charge. It is also shown that these synthetic mediators, except metatungstate, are able to evolve hydrogen with an hydrogenase isolated fromDesulfovibrio desulfuricans.

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