Inhibition of the Primary Photochemical Events in Rhodospirillum Rubrum by Ubiquinone Analogues

Twenty-one quinone compounds have been tested for their ability to inhibit the primary photochemical events when added to R. rubrum chromatophores. This was determined by measuring their effect on the light-induced absorbance change at 605 nm and their ability to quench the variable portion of fluorescence (Fv). Benzoquinone structures with one or more bromine or iodine substituents and a bulky group (isopropyl, t-butyl, n-hexyl) were particularly effective inhibitors. The most potent of these were 2,3,5-tribromo-6-n-hexyl-1,4-benzoquinone and 2,3-diiodo-5-t-butyl-1,4-benzoquinone which caused 50% inhibition at less than μM quantities (or about 5 to 10 fold above the phototrap concentrations). From control experiments with egg yolk phosphatidylcholine liposomes containing bacteriochlorophyll, it was shown that although the effective inhibitors also quenched fluorescence when added to this model system, quenching depended on the viscosity of the bilayer and was not very sensitive to structural parameters. In the in vivo system, phototrap activity was inhibited at 77 K to the same extent as room temperature, indicating diffusion was not important. In the control experiments with liposomes, the concentration for which 50% quenching was observed by 2,5-dibromo-3,6-dimethyl-1,4-benzoquinone and 2,3,5-tribromo-6-n-hexyl-1,4-benzoquinone only differed by two fold whereas in the chromatophore system, the difference was over 1000 fold; no effect was seen in vivo by the former compound and the latter was the most effective. It is suggested that most of the instantaneous fluorescence (Fo) is from a bacteriochlorophyll component(s) which does not communicate with the reaction center and a higher concentration of added quinone is required before this pigment fluorescence is quenched.