EPR Study of the Molecular and Electronic Structure of the Semiquinone Biradical QA-QB
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- 作者：Rafael Calvo ; Edward C. Abresch ; Robert Bittl ; George Feher ; Wulf Hofbauer ; Roger A. Isaacson ; Wolfgang Lubitz ; Melvin Y. Okamura ; and Mark L. Paddock
- 刊名：Journal of the American Chemical Society
- 出版年：2000
- 出版时间：August 2, 2000
- 年：2000
- 卷：122
- 期：30
- 页码：7327 - 7341
- 全文大小：270K
- 年卷期：v.122,no.30(August 2, 2000)
- ISSN：1520-5126
文摘
The photocycle of bacterial photosynthetic reaction centers (RCs) involves electron transfer betweentwo quinone molecules, QA and QB. The semiquinone biradical QA-
QB- forms an intermediate state in thisprocess. We trapped the biradical at low temperature (77 K) and investigated its EPR spectra at three microwavefrequencies, 9.6, 35, and 94 GHz, at temperatures between 1.5 and 100 K. The spectra were described with aspin Hamiltonian that contained, in addition to the Zeeman terms, dipolar and exchange interactions, and werefitted using the simulated annealing method (Kirkpatrick et al. Science 1983, 220, 671). From the parametersderived from the fit, information about the spatial and electronic structure was obtained. The relative positionand orientation of the two quinones, determined from the EPR spectra, compared well with those obtainedfrom X-ray diffraction of RCs in the QAQB- state (Stowell et al. Science 1997, 276, 812). The values of thedipolar coupling and of the exchange interaction obtained from the fits were Ed/h = (10.3 ± 0.1) MHz andJo/h = (-60 ± 20) MHz, respectively. The value of Jo was used to estimate a maximum electron-transfer rate,kET, (QA-QB- 
QAQB=) of ~109 s-1. This agrees within an order of magnitude with the value derived fromkinetics experiments (Graige et al. Biochemistry 1999, 38, 11465).
QB- forms an intermediate state in thisprocess. We trapped the biradical at low temperature (77 K) and investigated its EPR spectra at three microwavefrequencies, 9.6, 35, and 94 GHz, at temperatures between 1.5 and 100 K. The spectra were described with aspin Hamiltonian that contained, in addition to the Zeeman terms, dipolar and exchange interactions, and werefitted using the simulated annealing method (Kirkpatrick et al. Science 1983, 220, 671). From the parametersderived from the fit, information about the spatial and electronic structure was obtained. The relative positionand orientation of the two quinones, determined from the EPR spectra, compared well with those obtainedfrom X-ray diffraction of RCs in the QAQB- state (Stowell et al. Science 1997, 276, 812). The values of thedipolar coupling and of the exchange interaction obtained from the fits were Ed/h = (10.3 ± 0.1) MHz andJo/h = (-60 ± 20) MHz, respectively. The value of Jo was used to estimate a maximum electron-transfer rate,kET, (QA-QB- QAQB=) of ~109 s-1. This agrees within an order of magnitude with the value derived fromkinetics experiments (Graige et al. Biochemistry 1999, 38, 11465).