Abstract
1. Kinetics and difference spectra of light-induced absorbance changes were measured in reaction center preparations from Rhodopseudomonas spheroides and Rhodospirillum rubrum. 2. The results indicate the photoreduction of an electron acceptor X by P870. X is subsequently reoxidized in the light by a secondary endogenous electron acceptor A. The half times of the back reaction from X − to P870 + and from A − to P870 + are 0.12–0.15 s and 20–60 s, respectively. Photoreduced X − can also be reoxidized by ferricyanide, the net result being a reversible photooxidation of P870 by ferricyanide. Alternatively, photooxidized P870 + can be re-reduced by ascorbate, the net result being photoreduction of X, or of X and A, by ascorbate. 3. On the basis of these results, reduced minus oxidized difference spectra were obtained for P870, X and A. There is only one mole of X per mole of P870. The difference spectra indicate that X is ubiquinone and X − is the semiquinone anion. The absorbance of A does not alter markedly upon oxidation or reduction.
Published Version
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