Protein-Cofactor Interactions in Bacterial Reaction Centers from Rhodobacter sphaeroides R-26: II. Geometry of the Hydrogen Bonds to the Primary Quinone QA- by 1H and 2H ENDOR Spectroscopy

Abstract

The geometry of the hydrogen bonds to the two carbonyl oxygens of the semiquinone QA- in the reaction center (RC) from the photosynthetic purple bacterium Rhodobacter sphaeroides R-26 were determined by fitting a spin Hamiltonian to the data derived from 1H and 2H ENDOR spectroscopies at 35 GHz and 80 K. The experiments were performed on RCs in which the native Fe2+ (high spin) was replaced by diamagnetic Zn21 to prevent spectral line broadening of the QA- due to magnetic coupling with the iron. The principal components of the hyperfine coupling and nuclear quadrupolar coupling tensors of the hydrogen-bonded protons (deuterons) and their principal directions with respect to the quinone axes were obtained by spectral simulations of ENDOR spectra at different magnetic fields on frozen solutions of deuterated QA- in H2O buffer and protonated QA- in D2O buffer. Hydrogen-bond lengths were obtained from the nuclear quadrupolar couplings. The two hydrogen bonds were found to be nonequivalent, having different directions and different bond lengths. The H-bond lengths rO...H are 1.73 6 0.03 A° and 1.60 6 0.04 A° , from the carbonyl oxygens O1 and O4 to the NH group of Ala M260 and the imidazole nitrogen Nd of His M219, respectively. The asymmetric hydrogen bonds of QA- affect the spin density distribution in the quinone radical and its electronic structure. It is proposed that the H-bonds play an important role in defining the physical properties of the primary quinone, which affect the electron transfer processes in the RC.