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 photosynthetic bacterium Rhodobacter sphaeroides R26 functions as a one-electron gate. The radical anion Q(A)(.-) is proposed to have an asymmetric electron distribution, induced by the protein environment. We replace the native ubiquinone-10 (UQ10) with specifically C-13-labelled UQ10, and use Q-band (35 GHz) EPR spectroscopy to investigate this phenomenon 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 frozen isopropanol shows that the electron spin distribution is symmetric in the isopropanol glass, and asymmetric in the RC. Our results allow qualitative assessment of the spin and charge distribution for QA(.-) in 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) charge density.