Theoretical investigation of Q A−· -ligand interactions in bacterial reaction centers ofRhodobacter sphaeroides

Abstract

Density functional theory was used to calculate magnetic resonance parameters for the primary stable electron acceptor anion
radical (QA−·) in its binding site in the bacterial reaction center (bRC) ofRhodobacter sphaeroides. The models used for the calculations of the QA−· binding pocket included all short-range interactions of the ubiquinone with the protein surroundings in a gradual manner
and thus allowed a decomposition and detailed analysis of the different specific interactions. Comparison of the obtained
hyperfine and quadrupole couplings with experimental data demonstrates the feasibility and reliability of calculations on
such complex biologically relevant systems. With these results, the interpretation of previously published 3-pulse electron
spin echo envelope modulation data could be extended and an assignment of the observed double quantum peak to a specific amino
acid is proposed. The computations provide evidence for a slightly altered binding site geometry for the QA ground state as investigated by X-ray crystallography with respect to the QAt-· anion radical state as accessible via EPR spectroscopy. This new geometry leads to improved fits of the W-band correlated-coupled
radical pair spectra of QA−·-P865+· compared to orientation data from the crystal structure. Finally, a correlation of the14N quadrupole parameters of His219 with the hydrogen bond geometry and a comparison with previous systematic studies on the
influence of hydrogen bond geometry on quadrupole coupling parameters (J. Fritscher: Phys. Chem. Chem. Phys. 6, 4950–4956,
2004) is presented.