A MOLECULAR MECHANISM OF CONFORMATIONAL GATING OF ELECTRON-TRANSFER IN PHOTOSYNTHETIC REACTION CENTRA

A mechanism based on conformational control of electron transfer provides a reinterpretation of the observed temperature dependence of electron transfer from cytochrome c to the special pair of bacteriochlorophylls in the reaction center of several photosynthetic bacteria. More generally, it constitutes an alternative contribution to the temperature dependence of electron transfer reactions in biological systems compared to the vibronic coupling and parallel path mechanisms. Starting from the crystallographic structure of the reaction center of Rhodopseudomonas viridis, a detailed molecular mechanism of conformational gating of electron transfer is derived by studies of conformational states and electronic structure. The low temperature, unactivated, electron transfer is assigned to a direct, superexchange, mechanism involving intermediate orbitals on a bridging tyrosine. In the high temperature, activated, region, hydrogen bond formation and proton transfer between the bridging tyrosine and an asparagine residue, in a high energy conformational state of the reaction center, makes sequential electron transfer dominate the rate. This mechanism predicts the formation of a neutral tyrosine radical during electron transfer, which should be possible to detect by timeresolved spectroscopy. The mechanism also is of interest with respect to coupling of electron and proton transfer, gating of electron transfer in biological energy transduction, the role of bridging aromatic amino acids in long-range electron transfer, and molecular electronic devices.