Electrochemical approach to the development of a photoelectrode on the basis of photosynthetic reaction centers

Experimental approaches to the coupling of photoinduced charge separation in reaction centers (RCs) of the photosynthetic bacterium Rhodobacter sphaeroides R-26 with electron transfer to an electrode are discussed. Exogenous quinones are used as an electron transfer mediator. With the use of photopolarography it is shown that water-soluble ubiquinone can serve as a diffusionally mobile mediator of electron transfer. Some methods of quinone immobilization at an electrode have been developed to obtain a non-diffusional mediator of electron transfer. Quasi-reversible electrochemical kinetics were observed for aminonaphthoquinone immobilized as a monolayer at a Pt electrode. The ubiquinone-depleted RCs were subjected to affinity immobilization at these chemically modified electrodes probably due to the insertion of the immobilized quinone into the primary quinone Q_A binding site. The quantum efficiency of photocurrent formation was ca. 5% for the photoelectrode obtained. The electrochemical process of the immobilized quinone is shown to be the stage that limits electron transfer from RCs to the electrode.