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LIGHT‐DRIVEN H2 PRODUCTION WITH PROFLAVIN AND HYDROGENASE: COMPARISON OF CYTOCHROME C3 AND METHYL VIOLOGEN AS e‐ MEDIATORS

We have studied the hydrogenase‐catalyzed production of H, when either its natural electron mediator cytochrome c3 (c3) or the artificial mediator methyl viologen (MV) was reduced by illumination of proflavin (PF) in the presence of ethylenediaminetetraacetate (EDTA). The reduction rates of MV and c3 were comparable at equivalent concentrations of PF, taking into account the four redox sites of c3. However, when the concentration of c3 exceeded that of PF, the reduction rate decreased. We explain this by light quenching. In the H2‐producing system, MV was more efficient than c3 as electron mediator when the initial reaction rates were compared. However, under certain conditions with MV, the rate of H2 production decreased rapidly with time of illumination, whereas with c3 it consistently remained stable. Possible reasons for this difference are discussed.