Charge-Transfer Processes within the Isolated Tetramer Models of the Reaction Center Rhodobacter sphaeroides

Over the last two decades, advancements in structural resolution and spectral characterization have significantly enhanced our understanding of photosynthesis. However, the complexity of photosystem (PS) supercomplexes still presents challenges. In the Rhodobacter sphaeroides reaction center (RSRC), the charge separation process begins with a charge-transfer (CT) step at the special pair (P), a dimer of bacteriochlorophyll a (BChl), which acts as the donor, and continues with electron transport through the active pigments. Our computational study explores CT rectification in RSRC. We find that the CT rate is faster in the A branch compared with the B branch, which can be attributed to the orientation of the pigments near P and the influence of the surrounding protein complex on the dielectric constant. The calculated rate constants are derived using Fermi's golden rule, with a first-principles approach that employs an optimally tuned screened range-separated hybrid functional within a polarizable continuum model (SRSH-PCM).