PARENT SESSION
Posters P4Ab Type II reaction centres: Acceptor side. Abstracts (272-288)

Investigating the energetic of electrons and protons transfer in bacterial photosynthetic reaction center of Rhodobacter sphaeroides. Zhenyu Zhu¹, M. Gunner^*,1, ¹ Physics Department, City College of New York., New York, NY, USA

ABSTRACT- Multiple Conformational Continuum Electrostatics (MCCE) was used to study the quinone electrochemical midpoints and quinone pK's for one and two electron reduction of ubiquinone at the Q_A and distal and proximal Q_B sites in Rb. sphaeroides Reaction Centers (RCs). (1) The first reduction: At pH 7, the Em of Q_A/Q_A– is –40 and proximal Q_B/Q_B– –10 mV in reasonable agreement with experimental values. This assumes an Em of –145 mV in aqueous solution. Reduction of Q_B requires significant changes in proton distribution in the Q_B site and reorientation of SerL223. The Em for distal Q_B/Q_B– is –260 mV, supporting views that quinone caonnt not be reduced here. (2) Protonating the semiquinone: In Q_A and proximal Q_B sites QH is at higher energy than Q– because the protein backbone and nearby residues stabilize the anion more than the loss of solvation energy stabilizes the neutral semiquinone. In the distal Q_B site the neutral semiquinone is lower in energy because this site destabilizes an anion. (3) The second reduction: The Em for Q–/Q–² is –700, –490, and –654 mV for Q_A, proximal and distal Q_B (Em solution=–170 mV). In each site the dianion is very high in energy because the stabilizing interactions with residues and backbone increase with the charge (q) while the loss of solvation energy increases with q². (4) Binding the first proton: A pathway forming QH prior to the second reduction is preferable to forming the dianion in each site. QH– is favored in the proximal Q_B site, largely because a favorable hydrogen bond can be formed with SerL223. In Q_A QH–is at higher energy then Q–. (5) Forming the dihydroquinone: QH₂ is higher in energy at the Q_B site than it is in solution because the site has weak affinity for neutral quinones and because the second proton must be bound near HisL190 which is a hydrogen bond donor to the proximal Q_B. Thus, the reduced quinone binds poorly. QH₂ is even less stable in the Q_A site. In contrast, QH₂ is more stable at the distal Q_B site showing the preference of this site for the dihydroquinone. Other systems: Reaction energetics in bacterial reaction centers will be compared with PSI and PSII. Supported by the National Science Foundation MCB 0212696.

KEY WORDS: quinone, reaction center, electrochemistry, proton transfer