Time-Resolved Vibrational Analysis of Quinone Binding and Interaction in the Electron Transfer of Bacterial Photosynthetic Reaction Centers

Abstract

The understanding of the sequence of events, the mechanisms of reactions and the identity of reaction partners in the primary electron transfer of photosynthesis mainly arises from time-resolved spectroscopic techniques (for a review, see [1]). These primary steps of energy conversion take place in reaction centers (RC) which contain well-organized pigments and redox components. In purple photosynthetic bacteria, electron transport upon photoexcitation proceeds within 2–3 picoseconds from the primary electron donor “P” (a BChl₂) to the monomelic BPheo “H” and from H to the primary acceptor quinone within approx. 200 psec. In the presence of the secondary quinone Q_A, electron transfer from Q_A to Q_B proceeds within approx. 150 μsec. Q_A and Q_B are the first electron acceptors stable in the msec-to-second range. If the redox energy stored in the P⁺Q_B ⁻ state is not used by secondary redox reactions, charge recombination with a half-time of 0.5-1 sec observed In the absence of Q_B, charge recombination from Q_A proceeds within ca. 70 msec.