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... Show moreAn 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 N-14 coupling parameters: hyperfine interaction: A(iso)=1.85 MHz, T-11=0.32, alpha=0(0), beta=45(0); nuclear quadrupole interaction: e(2)qQ/h=1.52 MHz, eta=0.82. Comparison of the quadrupole values with data in the literature shows that Q(A)(-.) is coupled to the N-delta(1)-H group of the M219 heterocycle, most probably through a hydrogen bond with the 4-C carbonyl group of the quinone. Show less
