Charge transfer through a cytochrome multiheme chain: theory and simulation.

We study sequential charge transfer within a chain of four heme cofactors located in the c-type cytochrome subunit of the photoreaction center of Rhodopseudomonas viridis from a theoretical perspective. Molecular dynamics simulations of the thermodynamic integration type are used to compute two key energies of Marcus' theory of charge transfer, the driving force ∆G and the reorganization energy λ. Due to the small exposure of the cofactors to the solvent and to charged amino acids, the outer sphere contribution to the reorganization energy almost vanishes. Interheme effective electronic couplings are estimated using ab initio wave functions and a well-parameterized semiempirical scheme for long-range interactions. From the resulting charge transfer rates, we conclude that at most the two heme molecules closest to the membrane participate in a fast recharging of the photoreaction center, whereas the remaining hemes are likely to have a different function, such as intermediate electron storage. Finally, we suggest means to verify or falsify this hypothesis.

Recent progress in biological charge transfer: theory and simulation.

In this contribution, we discuss three recent developments in atomistic biological charge transfer theory. First, in the context of Marcus' classical theory of charge transfer, key quantities of the theory such as driving forces and reorganization enthalpies are now accessible by thermodynamic integration schemes within standard molecular dynamics simulations at high accuracy. Second, direct simulations of charge transfer enable the computation of fast charge transfer reaction rates without having to resort to Marcus' theory. Finally, exploring the electronic structure beyond that of hitherto presumed centers of localization helps to identify new stepping stones of charge transfer reactions. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

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.