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 primar y electron acceptor quinone radical anion, Q(A)(-), has been determine d in Single crystals of zinc-substituted reaction centers of Rhodobact er sphaeroides R-26 at 275 K and at 80 K. To obtain high spectral reso lution, EPR experiments were performed at 35 GHz and the native ubiqui none-10 (UQ(10)) in the reaction center was replaced by fully deuterat ed UQ(10). The principal values and the direction cosines of the g-ten sor axes with respect to the crystal axes a, b, c were determined. Fre ezing of the single crystals resulted in only minor changes in magnitu de and orientation of the g-tensor. The orientation of Q(A)(-) as dete rmined 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) obtain ed from an average of four different x-ray structures of Rb. sphaeroid es reaction centers. This deviation lies within the accuracy of the x- ray structure determinations. The g-tensor values measured in single c rystals agree well with those in frozen solutions. Variations in g-val ues between Q(A)(-), Q(B)(-), and UQ(10) radical ion in frozen solutio ns were observed and attributed to different environments.