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Science 6 October 1989:
Vol. 246. no. 4926, pp. 112 - 116
DOI: 10.1126/science.2675313
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Articles

Science, Vol 246, Issue 4926, 112-116
Copyright © 1989 by American Association for the Advancement of Science

articles

Dispersed polaron simulations of electron transfer in photosynthetic reaction centers

A Warshel, ZT Chu, and WW Parson

Department of Chemistry, University of Southern California, Los Angeles 90007.

A microscopic method for simulating quantum mechanical, nuclear tunneling effects in biological electron transfer reactions is presented and applied to several electron transfer steps in photosynthetic bacterial reaction centers. In this "dispersed polaron" method the fluctuations of the protein and the electron carriers are projected as effective normal modes onto an appropriate reaction coordinate and used to evaluate the quantum mechanical rate constant. The simulations, based on the crystallographic structure of the reaction center from Rhodopseudomonas viridis, focus on electron transfer from a bacteriopheophytin to a quinone and the subsequent back-reaction. The rates of both of these reactions are almost independent of temperature or even increase with decreasing temperature. The simulations reproduce this unusual temperature dependence in a qualitative way, without the use of adjustable parameters for the protein's Franck-Condon factors. The observed dependence of the back-reaction on the free energy of the reaction also is reproduced, including the special behavior in the "inverted region."


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