Molecular basis for alternative charge separation pathways in type-II photosynthetic reaction centers

In photosynthetic reaction centers from purple bacteria, bacteriochlorophyll a (the special pair P_AP_B and accessory bacteriochlorophyll B_A) and bacteriopheophytin a (H_A) form the active electron-transfer branch and uniquely contain a polar methyl-keto (3-acetyl) group. However, despite its importance in defining local polarity, even the currently available highest-resolution structures (∼2 Å) cannot unambiguously distinguish the methyl carbon from the keto oxygen, limiting insight into its functional role. Here, we investigate how the methyl-keto orientations of the P_B and H_A cofactors influence the energetics of charge-separated intermediates, using a quantum mechanical/molecular mechanical approach. We identify two kinetically isolated, metastable methyl-keto conformations of P_B, Tyr-OH…B_A and Tyr-OH…P_B, each associated with a distinct charge-separation pathway: the canonical [P_AP_B]* → [P_AP_B]^•+B_A ^•- and alternative B_A* → B_A ^•+H_A ^•- pathways, respectively. For H_A, methyl-keto reorientation stabilizes H_A ^•- when forward transfer to the primary quinone (Q_A) is inhibited. These results show that distinct methyl-keto conformations selectively tune charge-separation routes while also contributing to the oxidative robustness of bacteriochlorin macrocycles.