title: The 2-Methoxy Group Orientation Regulates the Redox Potential Difference between the Primary (QA) and Secondary (QB) Quinones of Type II Bacterial Photosynthetic Reaction Centers.
abstract: Recent studies have shown that only quinones with a 2-methoxy group can act simultaneously as the primary (QA) and secondary (QB) electron acceptors in photosynthetic reaction centers from purple bacteria such asRb. sphaeroides.13C HYSCORE measurements of the 2-methoxy group in the semiquinone states, SQAand SQB, were compared with DFT calculations of the13C hyperfine couplings as a function of the 2-methoxy dihedral angle. X-ray structure comparisons support 2-methoxy dihedral angle assignments corresponding to a redox potential gap (ΔEm) between QAand QBof 175-193 mV. A model having a methyl group substituted for the 2-methoxy group exhibits no electron affinity difference. This is consistent with the failure of a 2-methyl ubiquinone analogue to function as QBin mutant reaction centers with a ΔEmof ∼160-195 mV. The conclusion reached is that the 2-methoxy group is the principal determinant of electron transfer from QAto QBin type II photosynthetic reaction centers with ubiquinone serving as both acceptor quinones.
authors.affiliation: LQC-MM, Departamento de Química, ICEx, Universidade Federal de Minas Gerais (UFMG) , Campus Pampulh, Belo Horizonte, MG 31.910-270, Brazil.
authors.forename: Wagner B
authors.initials: WB
authors.lastname: de Almeida
authors.affiliation: Center for Biophysics and Computational Biology, Department of Veterinary Clinical Medicine, and Department of Biochemistry University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
authors.forename: Alexander T
authors.initials: AT
authors.lastname: Taguchi
authors.affiliation: Center for Biophysics and Computational Biology, Department of Veterinary Clinical Medicine, and Department of Biochemistry University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
authors.forename: Sergei A
authors.initials: SA
authors.lastname: Dikanov
authors.affiliation: Center for Biophysics and Computational Biology, Department of Veterinary Clinical Medicine, and Department of Biochemistry University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States ; Center for Biophysics and Computational Biology, Department of Veterinary Clinical Medicine, and Department of Biochemistry University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.
authors.forename: Colin A
authors.initials: CA
authors.lastname: Wraight
authors.affiliation: School of Chemistry, The University of Manchester , Manchester M13 9PL, United Kingdom.
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: O'Malley