Toward the isolation of a photochemical reaction center in Rhodopseudomonas spheroides

Abstract

Chromatophores of Rhodopseudomonas spheroides show reversible light-induced absorbancy changes corresponding to the oxidation of P870 (a pigment resembling bacteriochlorophyll), the oxidation of one or more cytochromes, and the reduction of ubiquinone. The light-reacting P870 is accompanied by a much larger amount of “light-harvesting” bacteriochlorophyll. Upon prolonged anaerobic incubation in the light, most of the bacteriochlorophyll in cells of blue-green mutant R. spheroides is converted in situ to bacteriopheophytin. In chromatophores from such cells the light-reactions of P870, cytochrome, and ubiquinone have the same size and character as in chromatophores from “fresh” cells. Under suitable conditions P870 can be isolated as a spectrophotometric entity; i.e., as an absorption band at 870 mμ that is bleached completely and reversibly by light. Excitation spectra for fluorescence of bacteriochlorophyll (at 900 mμ) and bacteriopheophytin (at 780 mμ), and for generation of the light-reactions of P870 and cytochrome, show the occurrence of energy transfer from bacteriopheophytin to bacteriochlorophyll and from bacteriopheophytin to P870, as well as from bacteriochlorophyll to P870. Kinetics of the 900-mμ fluorescence indicate that bacteriochlorophyll is fluorescent but P870 is not Extraction and analysis of pigments in “pheophytinized” chromatophores indicates that P870 is simply bacteriochlorophyll in a specialized environment. These experiments define a photochemical reaction center built around P870 as a terminal acceptor of excitation energy. The reaction center is served by light-harvesting molecules; these can be either bacteriochlorophyll or bacteriopheophytin.