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

Abstract

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.

Understanding Primary Charge Separation in the Heliobacterial Reaction Center

“…In view of the geometric relations apparent from the crystal structure and the earlier experimental studies, ,,− , Gisriel et al proposed a two-step mechanism for the formation of the SPP + EC3 – state, either with SPP + EC2 – or EC2 + EC3 – as the intermediate step. Both variants are present in different types of photosynthetic RCs. − ,− Our results from TDDFT and BOMD calculations on the heliobacterial RC strongly favor the second mechanism with EC2 + EC3 – as an intermediate step. In all our calculationswith different parts of the protein environment and for different BOMD geometrieswe consistently obtain an energy gap of several tenths of an eV between the EC2 + EC3 – and SPP + EC2 – excitations.…”