The Reaction Center and Antenna Pigments of Green Photosynthetic Bacteria

Abstract

AbstractExperiments are reported on the optical properties and orientation of reaction center pigments of the green photosynthetic bacterium Prosthecochloris aestuarii. Analysis of the light‐induced absorption difference spectra of P840, the primary electron donor, and of reaction center triplet formation indicates that the bleaching of the P840 band at 836 nm is accompanied by band shifts of bacteriochlorophylls absorbing at 797, 816 and 833 nm at 80 K. The Qy transitions of the long‐wave bacteriochlorophylls were found to be oriented more or less parallel to the membrane, the Qx transitions were more or less perpendicularly oriented. Fluorescence emission spectra of bacteriochlorophyll a in intact cells and isolated pigment‐protein complexes are presented for the temperature range 295‐4 K. Experiments with different wavelengths of excitation indicate that in the PP and RCPP complexes light absorbed by bacteriochlorophyll a produces mainly emission by light‐harvesting bacteriochlorophyll, whereas light absorbed by pigments absorbing at 670 nm (bacteriopheophytin c or a possibly related pigment) predominantly excites long‐wave fluorescence, which appears to be emitted by pigments more closely associated with the photochemical reaction center. The emission spectra of the light‐harvesting bacteriochlorophyll a complex at various temperatures indicated temperature equilibrium between the emitting species. In intact cells, an increase in bacteriochlorophyll c fluorescence upon cooling indicates a decrease in the efficiency of energy transfer between the chlorosome and bacteriochlorophyll a.