ASYMMETRIC BINDING OF THE PRIMARY ACCEPTOR QUINONE IN REACTION CENTERS OF THE PHOTOSYNTHETIC BACTERIUM RHODOBACTER-SPHAEROIDES R26, PROBED WITH Q-BAND (35 GHZ) EPR SPECTROSCOPY

The reaction center (RC)-bound primary acceptor quinone Q(A) of the ph otosynthetic bacterium Rhodobacter sphaeroides R26 functions as a one- electron gate. The radical anion Q(A)(.-) is proposed to have an asymm etric electron distribution, induced by the protein environment. We re place the native ubiquinone-10 (UQ10) with specifically C-13-labelled UQ10, and use Q-band (35 GHz) EPR spectroscopy to investigate this phe nomenon in closer detail. The direct observation of the C-13-hyperfine splitting of the g(z)-component of UQ10(A)(.-) in the RC and in froze n isopropanol shows that the electron spin distribution is symmetric i n the isopropanol glass, and asymmetric in the RC. Our results allow q ualitative assessment of the spin and charge distribution for QA(.-) i n the RC. The carbonyl oxygen of the semiquinone anion nearest to the S = 2Fe(2+)-ion and Q(B) is shown to acquire the highest (negative) ch arge density.