Insights into the factors controlling the rates of the deactivation processes that compete with charge separation in photosynthetic reaction centers

Abstract

This article addresses the factors that underlie the unity quantum yield of charge separation in the bacterial photosynthetic reaction center (RC). Of particular interest are the factors that suppress charge recombination and other deactivation processes which could compete effectively with charge separation. Studies on mutant RCs with enhanced charge recombination show that rates of decay processes can be increased tenfold or more upon a relatively small fractional increase in the free energy gap to the ground state. This behavior is opposite to that expected on the basis of Franck-Condon effects, where an increase in Δ G should decrease the deactivation rate. In the mutants, it is proposed that the enhanced deactivation rates of the intermediate charge-separated states result from (i) increased quantum mechanical mixing with electronic states at slightly higher energy that have inherently strong deactivation properties, and (ii) increased thermal repopulation of such strongly deactivated states. The thermal repopulation mechanism is proposed to play an important role in the heterodimer mutants studied here. This mechanism requires that the free energy gaps between the charge-separated and strongly deactivated states must decrease with temperature to be consistent with the weakly temperature-dependent deactivation behavior. Potential mediating states include the excited dimeric primary electron donor and a state involving the oxidized dimer and the reduced accessory bacteriochlorophyll molecule. It is concluded that increasing the free energy of charge-separated intermediates in order to minimize the Franck-Condon factors can enhance the deactivation rates and thus reduce the quantum yield of charge separation.