Primary Photochemistry and Electron Transport in Rhodospirillum rubrum

The quantum yield for photoxidation of the primary electron donor molecule in bacterial photosynthesis has been determined for a variety of chromatophore fractions obtained from Rhodospirillum rubrum. The most accurate value was obtained by measurement of a reaction, the photoxidation of cytochrome c₂, which can be closely coupled to the primary event. The quantum yield for this reaction was 0.85 ± 0.05 as determined with the supernatant fraction. It was demonstrated that for this system the rate of cytochrome c₂ oxidation exactly matches that for the dark conversion of the primary electron donor molecule back into its reduced form. It was also demonstrated that many of these supernatant systems displayed tight coupling so that 75-95% of all primary pigment photoxidized subsequently interacted with reduced cytochrome c₂. Knowing the degree of this coupling and the quantum yield for cytochrome c photoxidation, a quantum yield of 0.95 ± 0.05 was determined for production of the primary oxidized species called P 870 or P0.44. The extent of the cytochrome c photoxidation could be significantly increased by adding horse heart ferrocytochrome c and 1.5% Triton X-100 and/or 6 M urea. However, the quantum yield was unchanged. The quantum yield for photooxidation of the primary electron donor molecule was also determined by direct measurement of absorbance change in the near-infrared region for washed chromatophore fractions and using an assumed molar extinction. These results were in good agreement with those of the coupled system. Identical results were obtained for all measurements whether conducted with 887- or 865-mμ exciting light. The results are discussed with regard to possible mechanisms for the trapping act and the per cent of light energy conserved as chemical potential.