A MECHANISM OF TEMPERATURE-DEPENDENT ELECTRON-TRANSFER REACTIONS IN BIOLOGICAL-SYSTEMS

A mechanism contributing to the temperature dependence of electron transfer reactions in biological systems, based on conformational control of electron transfer, is considered. It is shown that such a mechanism can account for the observed temperature dependence of electron transfer between cytochrome c and the special pair of bacteriochlorophylls in the reaction center of several photosynthetic bacteria. It is also shown that a parallel path model, recently proposed as an alternative to the vibronic coupling analysis because of criticism against the latter, cannot explain the observations on some of these systems. In those systems for which a complete structure has been determined, a specific structural component is suggested to be involved in the conformational control of electron transfer. A model system of the reaction center of the photosynthetic bacterium Rhodopseudomonas viridis is shown to have a conformational state at an energy above the ground state in reasonable agreement with a preliminary experimental value of the high temperature activation energy of electron transfer and with an enhanced electron transfer capability compared to the ground state in accordance with the proposed mechanism. Suggestions for further experimental and theoretical investigations of the mechanism are provided.