Photoelectrochemical Properties of Electrodes Coated with Photoactive-Membrane Vesicles Isolated from Photosynthetic Bacteria

Chromatophore and sphericle vesicles, isolated from the photosynthetic bacterium, Rhodopseudomonas sphaeroides R-26, were dried as a film on tin oxide electrodes, and their response to red light was examined in a liquid-junction, photoelectrochemical cell. Steady-state photocurrents demonstrate that electron transfer must occur across both the SnO₂/chromatophore interface and the chromatophore film itself. The photoelectrochemical cell functions only when the chromatophore quinone pool is oxidized and cytochrome c₂ is reduced. Proton transport is not involved. Coated SnO₂ electrodes act as photocathodes instead of photoanodes, which is the case for uncoated SnO₂. The addition of electron carriers to photoelectrochemical cells incorporating chromatophore- and sphericle-coated electrodes suggest that factors other than the orientation of the reaction center within the vesicle membrane play an important part in governing the net electrical responses. A model is proposed to explain how electron transport occurs across a vesicle-coated electrode.