Abstract
In photosynthetic reaction centers from purple bacteria, bacteriochlorophyll a (the special pair P(A)P(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(A)P(B)]* → [P(A)P(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.