Light-induced absorption changes in Chromatium subchromatophore particles exhaustively extracted with non-polar solvents

Abstract

Chromatium subchromatophore particles completely freed of ubiquinone were prepared by Triton treatment preceeded by an exhaustive extraction with non-polar solvents at different stages of their preparation. Reconstituted particles were prepared by recondensing appropriate amounts of the quinone extract onto the extracted particles. The following observations are made and conclusions drawn from the light-induced absorption changes. 1. 1. In the extracted particles, where cytochrome-422 is present in the oxidized state, light causes charge separation, which is followed by charge recombination in the dark with a half time of 20 ms; the light- minus-dark difference spectrum between 240 and 950 nm is essentially the same as that of the unextracted subchromatophore particles. 2. 2. In the reconstituted particles, presumably because of the presence of ubiquinone as the secondary electron acceptor, charge recombination is prevented. After addition of 1 mM o-phenanthroline, which blocks electron transfer from the primary reductant to the secondary acceptor, charge recombination is re-established, as indicated by the return of the decay kinetics to the 20-ms half time. 3. 3. The room-temperature light- minus-dark difference spectrum of the extracted particles indicates that the majority of the absorption decrease at 280 nm may be attributed to P890 photooxidation and not ubiquinone reduction. This is further confirmed by the fact that the light- minus-dark difference spectrum at 77 °K is essentially the same for extracted and unextracted particles. 4. 4. If the oxidized cytochrome-422 in the extracted or reconstituted particles is reduced chemically, the reduced cytochrome can couple to the photooxidized P890 +, leading to its own oxidation and the reduction of P890 + in the dark. 5. 5. With the reconstituted particles, a steady, slow absorption decrease associated with the reduction of ubiquinone can be observed in the ultraviolet region when an electron donor such as N, N, N′ N′,-tetramethylphenylenediamine is present. No such reaction occurs in the extracted particles. 6. 6. Studies with the extracted particles have provided evidence that endogenous ubiquinone does not play the role of a primary electron acceptor in bacterial photosynthesis. Recent trends in the studies of the primary electron acceptor and possible approaches to the problem are briefly commented upon.