Structure of the Charge Separated State in the Photosynthetic Reaction Centers of Rhodobacter sphaeroides by Quantum Beat Oscillations and High-Field Electron Paramagnetic Resonance:  Evidence for Light-Induced Reorientation

• Published in: Journal of the American Chemical Society Vol. 129; no. 51; pp. 15935 - 15946
• Main Authors: Heinen, Ulrich, Utschig, Lisa M, Poluektov, Oleg G, Link, Gerhard, Ohmes, Ernst, Kothe, Gerd
• Format: Journal Article
• Language: English
• Published: American Chemical Society 26-12-2007
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja075065h

The structure of the secondary radical pair, , in fully deuterated and Zn-substituted reaction centers (RCs) of the purple bacterium Rhodobacter sphaeroides R-26 has been determined by high-time resolution and high-field electron paramagnetic resonance (EPR). A computer analysis of quantum beat oscillations, observed in a two-dimensional Q-band (34 GHz) EPR experiment, provides the orientation of the various magnetic tensors of with respect to a magnetic reference frame. The orientation of the g-tensor of in an external reference system is adapted from a single-crystal W-band (95 GHz) EPR study [Klette, R.; Törring, J. T.; Plato, M.; Möbius, K.; Bönigk, B.; Lubitz, W. J. Phys. Chem. 1993, 97, 2015−2020]. Thus, we obtain the three-dimensional structure of the charge separated state on a nanosecond time scale after light-induced charge separation. Comparison with crystallographic data reveals that the position of the quinone is essentially the same as that in the X-ray structure. However, the head group of has undergone a 60° rotation in the ring plane relative to its orientation in the crystal structure. Analysis suggests that the two different QA conformations are functionally relevant states which control the electron-transfer kinetics from to the secondary quinone acceptor QB. It appears that the rate-limiting step of this reaction is a reorientation of in its binding pocket upon light-induced reduction. The new kinetic model accounts for striking observations by Kleinfeld et al. who reported that electron transfer from to QB proceeds in RCs cooled to cryogenic temperature under illumination but does not proceed in RCs cooled in the dark [Kleinfeld, D.; Okamura, M. Y.; Feher, G. Biochemistry 1984, 23, 5780−5786].