UBIQUINONE AND PHOTOCHEMICAL ACTIVITY IN RHODOSPIRILLUM RUBRUM

Abstract— An analytical procedure was developed for quantitatively determining the ubiquinone content of bacterial photosynthetic material. High sensitivity was obtained by supplementing growth media with¹⁴C‐labelled p‐hydroxybenzoic acid which was specifically incorporated into the benzoquinones, ubiquinone and rhodoquinone. Sufficiently radioactive p‐hydroxybenzoic acid was used so that 10 picomoles of ubiquinone could be quantitatively measured. High accuracy was obtained by using a carrier technique which was employed during the extraction and isolation of quinones for analyses. Purification of quinones was achieved by selective solvent extraction and thin layer chromatography in two different solvent systems. This analytical procedure was employed to determine the ubiquinone content of R. rubrum chromatophores following hydrocarbon solvent extraction to remove ubiquinone from the chromatophores. Ubiquinone was found to exist in at least two pools: a loosely bound pool which easily extracted with hydrocarbon solvents, and a tightly bound pool which required addition of some polar solvent for its removal. It was demonstrated that ubiquinone would not exchange readily between these two pools. Careful analyses of all protein fractions showed that less than 0.06 ubiquinone per primary electron donor unit (P865) could be covalently bound to these components. From the average of a number of measurements, the tightly bound ubiquinone was determined as very nearly one‐half ubiquinone per phototrap (0.48 ± 0.05 S.D.). Removal of the tightly bound ubiquinone was linearly related to the loss of primary photochemical capacity. These results are consistent with the ‘duplex model’ for primary photochemical events but would seem to rule out an iron‐ubiquinone complex where the ubiquinone can function only between the semiquinone and the fully oxidized oxidation state.