Investigating Primary Charge Separation in the Reaction Center of Heliobacterium modesticaldum

We compute the primary charge separation step in the homodimeric reaction center (RC) of Heliobacterium modesticaldum from first principles. Using time-dependent density functional theory with the optimally tuned range-separated hybrid functional omega PBE, we calculate the excitations of a system comprising the special pair, the adjacent accessory bacteriochlorophylls, and the most relevant parts of the surrounding protein environment. The structure of the excitation spectrum can be rationalized from coupling of the individual bacteriochlorophyll pigments similar to molecular J- and H-aggregates. We find excited states corresponding to forward-charge transfer along the individual branches of the RC of H. modesticaldum. In the spectrum, these are located at an energy between the coupled Q(y) and Q(x) transitions. With ab initio Born-Oppenheimer molecular dynamics simulations, we reveal the influence of thermal vibrations on the excited states. The results show that the energy gap between the coupled Q(y) and the forward-charge transfer excitations is similar to 0.4 eV, which we consider to conflict with the concept of a direct transfer mechanism. Our calculations, however, reveal a certain spectral overlap of the forward-charge transfer and the coupled Q(x) excitations. The reliability and robustness of the results are demonstrated by several numerical tests.

Automated summary

In this study, the primary charge separation step in the homodimeric reaction center of Heliobacterium modesticaldum was calculated from first principles. The results showed a certain spectral overlap between the forward-charge transfer and the coupled Q(x) excitations, indicating a complex mechanism for charge transfer in the system. The influence of thermal vibrations on the excited states were also revealed through ab initio Born-Oppenheimer molecular dynamics simulations, demonstrating the reliability and robustness of the findings.