Differential Charge Polarization of Axial Histidines in Bacterial Reaction Centers Balances the Asymmetry of the Special Pair

摘要

In photosynthesis, light energy is transformed into chemical energy that sustains most forms of life on earth. The initial process of light-induced electron transfer occurs in photosynthetic reaction centers (RCs). The structures of the RCs of several purple bacteria have been resolved by X-ray diffraction and resemble the photosystem II in higher plants. The RC complex of Rhodobacter sphaeroides consists of three polypeptides (H, L, and M) and several cofactors that are arranged in two nearly symmetric branches (Figure 1A). These cofactors inc ude a dimer of excitonically coupled bacteriochlorophyll a (BChl a) molecules (designated P1 and P2) constituting the primary electron donor (P), two accessory mono-meric BChl a molecules, two bacteriopheophytin a molecules, two ubiquinones, a non-heme iron atom, and a carotenoid molecule.