Structure of the Charge Separated State in the Photosynthetic Reaction Centers of Rhodobacter sphaeroides by Quantum Beat Oscillations and High-Field Electron Paramagnetic Resonance: Ev
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- 作者：Ulrich Heinen ; Lisa M. Utschig ; Oleg G. Poluektov ; Gerhard Link ; Ernst Ohmes ; Gerd Kothe
- 刊名：Journal of the American Chemical Society
- 出版年：2007
- 出版时间：December 26, 2007
- 年：2007
- 卷：129
- 期：51
- 页码：15935 - 15946
- 全文大小：289K
- 年卷期：v.129,no.51(December 26, 2007)
- ISSN：1520-5126
文摘
The structure of the secondary radical pair, , in fully deuterated and Zn-substitutedreaction centers (RCs) of the purple bacterium Rhodobacter sphaeroides R-26 has been determined byhigh-time resolution and high-field electron paramagnetic resonance (EPR). A computer analysis of quantumbeat oscillations, observed in a two-dimensional Q-band (34 GHz) EPR experiment, provides the orientationof the various magnetic tensors of with respect to a magnetic reference frame. The orientation ofthe 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 ananosecond time scale after light-induced charge separation. Comparison with crystallographic data revealsthat the position of the quinone is essentially the same as that in the X-ray structure. However, the headgroup 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 theelectron-transfer kinetics from to the secondary quinone acceptor QB. It appears that the rate-limitingstep of this reaction is a reorientation of in its binding pocket upon light-induced reduction. The newkinetic 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 inRCs cooled in the dark [Kleinfeld, D.; Okamura, M. Y.; Feher, G. Biochemistry 1984, 23, 5780-5786].