The primary electron acceptor in the photo-synthetic bacteria Rhodopseudomonas spheroides and Rhodospirillum rubrum has been studied by methods of oxidation-reduction potentiometry combined with instrumentation for measuring photoinduced absorbance changes and photoinduced electron paramagnetic resonance signals. The Em for the oxidation-reduction dependence of photoinduced changes at pH 7.62 and 25° was found to be—0.022 v for both whole cells of R. rubrum and their chromatophores at high ionic strength. Chemically induced absorbance changes, which reproduced the light-induced changes between 740 and 770 mμ, were titrated and found to have an Em value=—0.06 v at pH 7.65 and 25° for chromatophores of R. rubrum at high ionic strength. The location of the absorbance changes suggests that the pigment observed is a porphyrin derivative. The data presented, together with earlier results, are viewed as defining the oxidation-reduction range in which primary events operate in these bacteria. A unified concept of primary oxidation-reduction reactions in bacterial photosynthesis is offered and related to photophosphorylation.
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