The primary photoreactions in the complex cytochrome-P-890 · P-760 (bacteriopheophytin760) of Chromatium minutissimum at low redox potentials

Experimental evidence for electron transfer, photosensitized by bacteriochlorophyll, from cytochrome c to a pigment complex P-760 (involving bacteriopheophytin-760 and also bacteriochlorophyll-800) in the reaction centers of Chromatium minutissimum has been described. This photoreaction occurs between 77 and 293 °K at a redox potential of the medium between ?250 and ?530 mV. Photoreduction of P-760 is accompanied by development of a wide absorption band at 650 nm and of an EPR signal with g = 2.0025±0.0005 and linewidth of 12.5±0.5 G, which are characteristic of the pigment radical anion.It is suggested that the photoreduction of P-760 occurs under the interaction of reduced cytochrome c with the reaction center state P⁺-890 · P^?-760 which is induced by light. The existence of short-lived state P⁺-890 · P^?-760 is indicated by the recombination luminescence with activation energy of 0.12 eV and $\text{τ}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{< 6}\text{ns}$. This luminescence is excited and emitted by bacteriochlorophyll and disappears when P-760 is reduced.At low redox potentials, the flash-induced absorbance changes related to the formation of the carotenoid triplet state with $\text{τ}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 6 μ}\text{s}$ at 20 °C are observed. This state is not formed when P-760 is reduced at 293 and 160 °K. It is assumed that this state is formed from the reaction center state P⁺-890 · P^?-760, which appears to be a primary product of light reaction in the bacterial reaction centers and which is probably identical with the state P^F described in recent works.