ELECTRONIC-STRUCTURE OF Q(A)(-) IN REACTION CENTERS FROM RHODOBACTER-SPHAEROIDES .1. ELECTRON-PARAMAGNETIC-RESONANCE IN SINGLE-CRYSTALS

The magnitude and orientation of the electronic g-tensor of the primary electron acceptor quinone radical anion, Q(A)(-), has been determined in Single crystals of zinc-substituted reaction centers of Rhodobacter sphaeroides R-26 at 275 K and at 80 K. To obtain high spectral resolution, EPR experiments were performed at 35 GHz and the native ubiquinone-10 (UQ(10)) in the reaction center was replaced by fully deuterated UQ(10). The principal values and the direction cosines of the g-tensor axes with respect to the crystal axes a, b, c were determined. Freezing of the single crystals resulted in only minor changes in magnitude and orientation of the g-tensor. The orientation of Q(A)(-) as determined by the g-tensor axes deviates only by a few degrees (less than or equal to 8 degrees) from the orientation of the neutral Q(A) obtained from an average of four different x-ray structures of Rb. sphaeroides reaction centers. This deviation lies within the accuracy of the x-ray structure determinations. The g-tensor values measured in single crystals agree well with those in frozen solutions. Variations in g-values between Q(A)(-), Q(B)(-), and UQ(10) radical ion in frozen solutions were observed and attributed to different environments.