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

Abstract

The geometry of the hydrogen bonds to the two carbonyl oxygens of the semiquinone Q A ⋅ ⁡ − 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 Fe 2+ (high spin) was replaced by diamagnetic Zn 2+ to prevent spectral line broadening of the Q A ⋅ ⁡ − 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 Q A ⋅ ⁡ − in H 2O buffer and protonated Q A ⋅ ⁡ − in D 2O 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 r O⋯H are 1.73 ± 0.03 Å and 1.60 ± 0.04 Å, from the carbonyl oxygens O 1 and O 4 to the NH group of Ala M260 and the imidazole nitrogen N δ of His M219, respectively. The asymmetric hydrogen bonds of Q A ⋅ ⁡ − 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.