Role of Charge Transfer States of the Special Pair in Primary Photosynthetic Charge Separation

Abstract

We propose a model for primary charge separation in the reaction centers of photosynthetic bacteria based upon an internal charge transfer state of the special pair dimer. In our model, this state serves as a trigger for primary electron transfer, linking the excited state of the special pair (with which it is nearly in resonance) to a superexchange pathway coupled to the initial bacteriopheophytin acceptor. The recombination reaction is not similarly facilitated because the charge transfer state is not close in energy to the ground state; this could be a key factor in explaining the high quantum yield of the primary electron transfer event. The model provides a qualitative explanation for three important experimental measurements (Stark effect, photochemical holeburning, singlet-triplet splitting of the charge separated species) on the reaction center. In addition, its predictions are in excellent accord with recent studies of a genetically modified reaction center in which one bacteriochlorophyll molecule of the special pair is changed to a pheophytin. For this system, one can directly observe the proposed charge transfer state participating in the electron transfer kinetics, located at an energy compatible with our model calculations.