Effect of the Environment on the Excitation Energy and Charge Transfer Pathways of the <i>Heliobacterium modesticaldum</i> Reaction Center

The spectroscopic properties and charge transfer (CT) processes in a tetrameric model of the homodimeric reaction center of <i>Heliobacterium modesticaldum</i> (HbRC) are presented. HbRC represents the simplest known analogue of other photosynthetic systems and comprises the special pair, <i>SPP</i> dimer molecule, adjacent cofactors <i>Acc</i> and <i>Aoh</i>, key amino acid residues (<i>His</i>537, <i>Gln</i>458, and <i>Ser</i>545), and two water molecules. Therefore, to assess the influence of the nearby surrounding environment, we compare a ″naked″ model─excluding all amino acid residues─with a ″full″ model that incorporates all nearby residues. We identify several bright states in the reaction center that trigger subsequent CT processes beyond that of the <i>SPP</i> dimer. Notably, one of the bright states, characterized as <i>(SPP AccAoh)</i>*, aligns with experimental observations. We also analyze the charge-separated configurations <i>(SPPAcc)</i>⁺<i>(Aoh)</i>^- as the charge separation initial state (CSI) and <i>(SPP)</i>⁺ <i>Acc (Aoh)</i>^- as the charge separation final (CSF) state, along with intermediate processes such as <i>(SPP)</i>⁺<i>(Acc)</i>^-<i>Aoh</i> and <i>SPP(Acc)</i>⁺<i>(Aoh)</i>^-. A key finding is that most of the calculated CSF states are energetically lower than those of the bright states, enabling electron transfer through a downhill CT pathway. Additionally, CSI states are energetically much lower in the full model, which in turn decreases the driving force energy that explicitly affects the rate. We have employed first-principles calculations based on Fermi's golden rule for the two models. Most rates are found to lie in the picosecond time scale, which is in line with observations. All electronic parameters are derived by using an optimally tuned screened range-separated hybrid functional within a polarizable continuum model, accounting for varying dielectric constants.