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Proton transfer to ubiquinone Q B in the photosynthetic reaction center of Rps. Viridis : The role of electrostatic interactions
Author(s) -
ComettaMorini C.,
Scharnagl C.,
Fischer S. F.
Publication year - 1993
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560480712
Subject(s) - chemistry , photosynthetic reaction centre , protonation , electron transfer , proton coupled electron transfer , proton , poisson–boltzmann equation , photochemistry , electrostatics , acceptor , proton affinity , electron acceptor , computational chemistry , ion , organic chemistry , physics , quantum mechanics , condensed matter physics
Electrostatic calculations of the p K a of ionizable groups in the reaction center of Rhodopseudomonas (Rps.) viridis were carried out to investigate three possible mechanisms for proton transfer to the singly reduced acceptor ubiquinone Q B . The program DelPhi, which solves the Poisson–Boltzmann equation given the distribution of charges and dielectric boundaries, was used to determine the electrostatic potential. The shift in p K a of the titratable residues in the Q B binding pocket in response to the one‐electron reduction and following protonation of Q B was obtained from calculated interactions with the reaction field, background protein dipoles, charged cofactors, and other ionizable residues. A limited number of bound waters was also included in the computations as titrating sites. Their titration behavior was shown to be strongly coupled to neighboring ionizable sites. The results show that strong electrostatic interaction between the radical anion Q B −· and a neighboring serine residue (SER L 223) as well as the protein environment stabilize a system in which the incoming proton is localized on serine and only shared in a hydrogen‐bonding relationship with Q B −· . These results hint to the possibility that actual proton transfer to Q B −· only occurs after a second negative charge has been added to the system through transfer of a second electron either to the menaquinone Q A , with formation of the Q B −· Q A −· system, or to Q B −· , leading to the doubly reduced Q B 2− . This preposition is consistent with spectroscopical and electron nuclear double resonance (ENDOR) experimental results for bacterial reaction centers (RCs) failing to find evidence for the existence of the protonated semiquinone Q B H·. © 1993 John Wiley & Sons, Inc.