Abstract
Light-induced electron-transfer reactions were investigated in wild-type and three mutant Rhodobacter sphaeroides reaction centers with the secondary electron acceptor (ubiquinone QA) either removed or permanently reduced. Under such conditions, charge separation between the primary electron donor (bacteriochlorophyll dimer, P) and the electron acceptor (bacteriopheophytin, HA) was followed by P+HA– → PHA 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+BA–, with BA being the bacteriochlorophyll located between P and HA. In all cases, mixing of the states P+BA– and P+HA– was observed and explained in terms of electron delocalization over BA and HA. This delocalization, together with picosecond protein relaxation, underlies a new view of primary charge separation in photosynthesis.
Highlights
Studies of light-induced electron-transfer (ET) reactions in photosynthetic proteins are important because similar ET processes occur commonly in many other proteins and play vital functional roles
We extend our previous kinetic model of primary charge separation and charge recombination in closed reaction centers (RCs), that is, in RCs with a blocked ET from HA− to QA,[32,48] based on high-quality transient absorption data and their detailed global and target analyses
Purified RCs were prepared according to procedures described earlier.[33,49−51] In addition to WT RCs, the following mutant complexes were studied: ELL (Glu L104 replaced by Leu), AMW (Ala M260 replaced by Trp), and ELL/AMW
Summary
Studies of light-induced electron-transfer (ET) reactions in photosynthetic proteins are important because similar ET processes occur commonly in many other proteins and play vital functional roles. The transformation of an excited electronic state into a charge-separated state is a crucial reaction in many artificial photovoltaic devices; so, understanding this natural process may provide important guidelines on how to construct such devices for efficient energy conversion. Protein complexes in which the energy of absorbed light is transformed into that of charge-separated states.[1−4] The primary charge separation in RCs, the initial step of ET, occurs between a (bacterio)chlorophyll species, denoted P, and a nearby (bacterio)chlorin acceptor within a few picoseconds.[5−7]. Three redox centers embedded in the protein are involved in the primary charge separation reaction, namely, the dimeric.
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