The simulation of interquinone charge transfer in a bacterial photoreaction center highlights the central role of a hydrogen-bonded non-heme iron complex.

We consider electron transfer between the quinones Q(A) and Q(B), one of the final steps in the photoinduced charge separation in the photoreaction center of Rhodobacter sphaeroides. The system is described by a model with atomic resolution using classical force fields and a carefully parameterized tight-binding Hamiltonian. The rates estimated for direct interquinone charge transfer hopping involving a non-heme iron complex bridging the quinones and superexchange based on the geometry of the photochemically inactive dark state are orders of magnitude smaller than those obtained experimentally. Only if the iron complex is attached to both quinones via hydrogen bonds - as characteristic of the charge transfer active light state - the computed rate for superexchange involving the histidine ligands of the complex will become comparable to the experimental value of k(CT)=10⁵s⁻¹.
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