title: Hydrogen bond network around the semiquinone of the secondary quinone acceptor Q(B) in bacterial photosynthetic reaction centers.
abstract: By utilizing a combined pulsed EPR and DFT approach, the high-resolution structure of the QB site semiquinone (SQB) was determined. The development of such a technique is crucial toward an understanding of protein-bound semiquinones on the structural level, as (i) membrane protein crystallography typically results in low resolution structures, and (ii) obtaining protein crystals in the semiquinone form is rarely feasible. The SQB hydrogen bond network was investigated with Q- (∼34 GHz) and X-band (∼9.7 GHz) pulsed EPR spectroscopy on fully deuterated reactions centers from Rhodobacter sphaeroides. Simulations in the SQB g-tensor reference frame provided the principal values and directions of the H-bond proton hyperfine tensors. Three protons were detected, one with an anisotropic tensor component, T = 4.6 MHz, assigned to the histidine NδH of His-L190, and two others with similar anisotropic constants T = 3.2 and 3.0 MHz assigned to the peptide NpH of Gly-L225 and Ile-L224, respectively. Despite the strong similarity in the peptide couplings, all hyperfine tensors were resolved in the Q-band ENDOR spectra. The Euler angles describing the series of rotations that bring the hyperfine tensors into the SQB g-tensor reference frame were obtained by least-squares fitting of the spectral simulations to the ENDOR data. These Euler angles show the locations of the hydrogen bonded protons with respect to the semiquinone. Our geometry optimized model of SQB used in previous DFT work is in strong agreement with the angular constraints from the spectral simulations, providing the foundation for future joint pulsed EPR and DFT semiquinone structural determinations in other proteins.
authors.affiliation: #Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.|‡Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
authors.forename: Alexander T
authors.initials: AT
authors.lastname: Taguchi
authors.affiliation: ⊥School of Chemistry, The University of Manchester, Manchester M13 9PL, U.K.
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: O'Malley
authors.affiliation: #Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.|§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: ‡Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
authors.forename: Sergei A
authors.initials: SA
authors.lastname: Dikanov
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D003198:Computer Simulation; D006860:Hydrogen Bonding; D016018:Least-Squares Analysis; D016014:Linear Models; D008956:Models, Chemical; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D011522:Protons; D011809:Quinones / Q000737:chemistry*; D012242:Rhodobacter sphaeroides; D012997:Solvents / Q000737:chemistry; D013057:Spectrum Analysis; D014867:Water / Q000737:chemistry
chemical_list: D001426:Bacterial Proteins; D045322:Photosynthetic Reaction Center Complex Proteins; D011522:Protons; D011809:Quinones; D012997:Solvents; D014867:Water