THE PRIMARY ACCEPTOR QUINONE Q(A) IN REACTION CENTERS OF RHODOBACTER-SPHAEROIDES R26 IS HYDROGEN-BONDED TO THE N-DELTA(1)-H OF HIS M219 - AN ELECTRON-SPIN ECHO STUDY OF Q(A)(-.)

An electron spin echo envelope modulation (ESEEM) study is performed o n the reduced primary electron-accepting ubiquinone-10 (Q(A)(-.)) in Z n-substituted reaction centers of the photosynthetic bacterium Rhodoba cter sphaeroides R26. The ESEEM spectra showed hyperfine and quadrupol ar couplings of Q(A)(-.) to nitrogens in the protein matrix. Simulatio n of the spectra revealed the following N-14 coupling parameters: hype rfine 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. Comparis on 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, mo st probably through a hydrogen bond with the 4-C carbonyl group of the quinone.