KINETICS OF PHOTO-OXIDATION OF C-TYPE CYTOCHROMES BY RHODOSPIRILLUM-RUBRUM REACTION CENTERS

The kinetics of photooxidation of c-type cytochromes from horse heart, Rhodospirillum rubrum, and Rhodopseudomonas capsulata by purified reaction centers from R. rubrum have been investigated. The kinetic mechanism was found to be complex with a second-order step (complex formation) followed by a rate limiting first-order step. Based on studies of the reaction as a function of pH, ionic strength, and detergent concentration, it appears that the complex formation step is largely electrostatically controlled with only portions of the surfaces of the interacting molecules participating. Further, the first-order process observed at high cytochrome concentration appears to result from solvent reorganization and/or a conformational change following complex formation. Based on data analysis in terms of outersphere electron transfer, it is proposed that another first-order process exists which is not rate limiting and is the electron transfer step. Finally, it was found that the detergent concentration can have a profound effect on both the oxidation-reduction potential of the cytochromes and the kinetics of photooxidation. These results limit the detergent concentration range over which experiments can be conducted and interpreted. © 1979.