The primary acceptor quinone QA in reaction centers of Rhodobacter sphaeroides R26 is hydrogen bonded to the Nσ(1)-H of His M219. An electron spin echo study of QA−•

An electron spin echo envelope modulation (ESEEM) study is performed on the reduced primary electron-accepting ubiquinone-10 (QA−•) in Zn-substituted reaction centers of the photosynthetic bacterium Rhodobacter sphaeroides R26. The ESEEM spectra showed hyperfine and quadrupolar couplings of QA−• to nitrogens in the protein matrix. Simulation of the spectra revealed the following 14N coupling parameters: hyperfine interaction: Aiso = 1.85 MHz, T11 = 0.32, α = 00, β = 450; nuclear quadrupole interaction: e2qQ/h = 1.52 MHz, η = 0.82. Comparison of the quadrupole values with data in the literature shows that QA−• is coupled to the Nσ(1)-H group of the M219 heterocycle, most probably through a hydrogen bond with the 4-C carbonyl group of the quinone.