Multiscale Simulation of Primary Charge Separation Mechanisms in an LH1-RC Complex

The light-harvesting complex II (LH2) of purple bacteria captures solar energy using bacteriochlorophyll (BChl) pigments. This energy is then transferred to the LH1 complex and subsequently to the embedded reaction center (RC). The initial separation of charges instigates a series of subsequent processes, ultimately culminating in the synthesis of adenosine triphosphate (ATP). While the excitation transfer process within the LH2 complex has been thoroughly characterized, the atomistic mechanism of charge separation in the RC remains unresolved. In this study, we employed a combination of classical molecular dynamics (MD), ab initio quantum mechanics/molecular mechanics (QM/MM) MD, and time-dependent density functional theory (TD-DFT) to delineate the excitation funnel within the LH1 ring of Thermochromatium tepidum, which is instrumental in facilitating charge separation in the RC. The analysis of the excitation profile indicates that the process of charge separation does not originate from the so-called "special pair" (P), but rather from the adjacent P/B BChl pair located on the active branch. The protein environment has been determined to play a pivotal role in this process. A competing low-lying charge-transfer state on the inactive branch exhibits inconsistent directionality, thereby rendering it an inefficient route. This study presents the first comprehensive analysis of an entire LH1-RC complex, and the findings challenge traditional models, highlighting the role of the protein scaffold as a crucial factor for charge separation in bacterial photosynthesis.