Antagonistic Effects of Point Mutations on Charge Recombination and a New View of Primary Charge Separation in...

Light-induced electron-transfer reactions were investigated in wild-type and three mutant Rhodobacter sphaeroides reaction centers with the secondary electron acceptor (ubiquinone Q_A) either removed or permanently reduced. Under such conditions, charge separation between the primary electron donor (bacteriochlorophyll dimer, P) and the electron acceptor (bacteriopheophytin, H_A) was followed by P⁺H_A^- → PH_A charge recombination. Two reaction centers were used that had different single amino-acid mutations that brought about either a 3-fold acceleration in charge recombination compared to that in the wild-type protein, or a 3-fold deceleration. In a third mutant in which the two single amino-acid mutations were combined, charge recombination was similar to that in the wild type. In all cases, data from transient absorption measurements were analyzed using similar models. The modeling included the energetic relaxation of the charge-separated states caused by protein dynamics and evidenced the appearance of an intermediate charge-separated state, P⁺B_A^-, with B_A being the bacteriochlorophyll located between P and H_A. In all cases, mixing of the states P⁺B_A^- and P⁺H_A^- was observed and explained in terms of electron delocalization over B_A and H_A. This delocalization, together with picosecond protein relaxation, underlies a new view of primary charge separation in photosynthesis.