MAGNETIC INTERACTION BETWEEN Q(A)(-CENTER-DOT) AND THE TRIPLET-STATE OF THE PRIMARY DONOR IN MODIFIED REACTION CENTERS OF THE PHOTOSYNTHETIC BACTERIUM RHODOBACTER-SPHAEROIDES R26

The interaction between the reduced primary acceptor quinone (Q(A)(-.) ), and the triplet state of the primary donor D-3 is investigated with time-resolved continuous-wave EPR, The trough at high-field in the Q( A)(-.) electron-spin polarized X-band EPR-spectrum at early delay time s after the laser flash [De Groot et al. (1985) Biochim. Biophys. Acta 808, 13-20] is studied as a function of temperature and of the delay, in zinc-reconstituted reaction centers, with and without carotenoid. In all cases investigated a decrease in D-3 concentration is accompani ed by a simultaneous attenuation of the high-field trough in the Q(A)( -.) EPR spectrum. These observations confirm the hypothesis that the l ine-shape of Q(A)(-.) at short delay times is influenced by a magnetic interaction with D-3. The line-shape of the Q(A)(-.) electron-spin po larized EPR spectrum directly after the laser flash, could be very wel l simulated using an extension of the model of Hore et al. [Here, P.J. et al. (1993) Biochim. Biophys. Acta 1141, 221-230], with a dipolar c oupling between Q(A)(-.) and D-3 Of -0.125 mT.