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

Abstract

An electron spin echo envelope modulation (ESEEM) study is performed on the reduced primary electron-accepting ubiquinone-10 (Q A −•) in Zn-substituted reaction centers of the photosynthetic bacterium Rhodobacter sphaeroides R26. The ESEEM spectra showed hyperfine and quadrupolar couplings of Q A −• to nitrogens in the protein matrix. Simulation of the spectra revealed the following 14N coupling parameters: hyperfine interaction: A iso = 1.85 MHz, T 11 = 0.32, α = 0 0, β = 45 0; nuclear quadrupole interaction: e 2 qQ/ h = 1.52 MHz, η = 0.82. Comparison of the quadrupole values with data in the literature shows that Q A −• is coupled to the N σ(1) -H group of the M219 heterocycle, most probably through a hydrogen bond with the 4-C carbonyl group of the quinone.