Kinetic studies on the reaction center protein from Rhodopseudomonas sphaeroides: the temperature and free energy dependence of electron transfer between various quinones in the QA site and the oxidized bacteriochlorophyll dimer

Abstract

The temperature and free energy dependence of electron transfer from the primary semiquinone (Q/sub A/sup .-/) to the oxidized bacteriochlorophyll dimer ((BChl)/sub 2//sup .+/ have been measured in the reaction center protein from Rhodopseudomonas sphaeroides in which the native Q/sub A/, ubiquinone-10, has been removed and replaced by one of 11 substituted 9,10-anthraquinones, seven 1,4-naphthoquinones, 1,2-naphthoquinone, or five 1,4-benzoquinones. For 19 of these quinones an in situ midpoint potential value at 295 K for the Q/sub A//Q/sub A//sup .-/ couple was available providing a series of reaction center proteins with a variation of reaction -..delta..G/sup 0/ from 0.49 to 0.81 eV. The E/sub 1/2/ values for the remaining Q/sub As/ were estimated from measurements on quinones in solution, extending the reaction -..delta..G/sup 0/ range from 0.11 to 0.94 eV. The rates of intraprotein electron transfer from the various Q/sub A//sup .-/ molecules to (BChl)/sub 2//sup .+/ were found to be virtually independent of temperature from 5 to 100 K and, where measured, to decrease severalfold from 100 to 300k, a pattern well-known for native reaction center protein. Preliminary attempts have been made to model the observed dependence of the electron-transfer rate on the -..delta..G/sup 0/ and temperature in terms ofmore » current theories that describe electron-transfer reactions as nonadiabatic, multiphonon, nonradiative decay processes.« less