Thermodynamic and Kinetic Characterization of Electron‐Transfer Components in situ in Rhodopseudomonas spheroides and Rhodospirillum rubrum

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Abstract
The b‐ and c‐type cytochromes in chromatophores from photosynthetically grown Rhodospirillum rubrum and Rhodopseudomonas spheroides have been characterized in situ in terms of their oxidation‐reduction potential properties.In R. rubrum only one cytochrome c measured at 551—540 nm was resolved potentiometrically; the midpoint potential at pH 7.2, Em7.2, is +293 mV. Three cytochrome species measured at 562—540 nm were resolved; they had Em7.2 values + 170 mV, −5 mV and −105 mV and were tentatively identified with cytochrome b, cc′ and b respectively; they contributed to the over‐all absorbance change in the ratio 8:25:17, respectively.In Rps. spheroides the Em7.0 of cytochrome c is +295 mV. Three cytochromes tentatively considered to be of the b‐type have Em7.2 values of +155 mV, +50 mV and −90 mV; their approximate reduced minus oxidized maxima were determined to be 558—9 nm, 560 nm and 564 nm, respectively. The approximate relative absorbance ratio given at the reduced minus oxidized maxima was 8:21:14. The rapid oxidation‐reduction reactions of reaction‐center bacteriochlorophyll, cytochrome c and cytochromes bin Rps. spheroides measured following a 20‐nsec laser flash or a 200‐μsec xenon flash have been studied in the absence and presence of antimycin A as a function of oxidation‐reduction potential. The results indicated that the reaction‐center bacteriochlorophyll (Em7.0+ 450 mV) serves to oxidize the cytochrome c with a midpoint potential of 295 mV, cytochrome c295, (half‐time t1/2↗100 μsec with a slow phase t1/2∼2 msec) which in turn can oxidize cytochrome b155t1/2 1—2 msec) in a reaction which is antimycin‐A‐sensitive. Cytochrome b50 becomes reduced following a flash with a t1/2 1–1.5 msec probably via the primary electron acceptor (Em7.0∼−20 mV) of the reaction‐center bacteriochlorophyll. Antimycin A stimulates the extent of cytochrome b50 reduction following a 200‐μsec xenon flash but has no detectable effect following a 20‐nsec laser flash. A working model for electron transfer in Rps. spheroides based on the measured thermodynamic and kinetic properties is presented.