Theoretical investigation of Q(A)(-)(center dot) - Ligand interactions in bacterial reaction centers of Rhodobacter sphaeroides

Density functional theory was used to calculate magnetic resonance parameters for the primary stable electron acceptor anion radical (Q(A)(-.)) in its binding site in the bacterial reaction center (bRC) of Rhodobacter sphaeroides. The models used for the calculations of the Q(A)(-.) 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 Q, ground state as investigated by X-ray crystallography with respect to the Q(A)(-.) 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 Q(A)(-.)-P-865(+.) compared to orientation data from the crystal structure. Finally, a correlation of the N-14 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.