Electronic structure of Q-A in reaction centers from Rhodobacter sphaeroides. I. 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 (UQ10) in the reaction center was replaced by fully deuterated UQ10. 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 (< or = 8 degrees) from the orientation of the neutral QA 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 UQ10 radical ion in frozen solutions were observed and attributed to different environments.
Department of Physics, University of California, San Diego, La Jolla 92093, USA.
R A
RA
Isaacson
F
F
Lendzian
E C
EC
Abresch
W
W
Lubitz
G
G
Feher
D000838:Anions / Q000737:chemistry; D055592:Biophysical Phenomena; D001703:Biophysics; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D015394:Molecular Structure; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry*; D011809:Quinones / Q000737:chemistry*; D012242:Rhodobacter sphaeroides / Q000737:chemistry*
D000838:Anions; D005609:Free Radicals; D045322:Photosynthetic Reaction Center Complex Proteins; D011809:Quinones