EPR Study of the Molecular and Electronic Structure of the Semiquinone Biradical QA-QB- in Photosynthetic Reaction Centers from Rhodobacter sphaeroides

J. Am. Chem. Soc., 122 (30), 7327 -7341, 2000. 10.1021/ja000399r S0002-7863(00)00399-1
Web Release Date: July 13, 2000

EPR Study of the Molecular and Electronic Structure of the Semiquinone Biradical Q_A^-Q_B^- in Photosynthetic Reaction Centers from Rhodobacter sphaeroides

Rafael Calvo, Edward C. Abresch, Robert Bittl, George Feher,* Wulf Hofbauer, Roger A. Isaacson, Wolfgang Lubitz, Melvin Y. Okamura, and Mark L. Paddock

Contribution from the Department of Physics, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0319, Departamento de Física, Facultad de Bioquímica y Ciencias Biológicas, Universidad Nacional del Litoral, and INTEC (CONICET-UNL), Güemes 3450, 3000 Santa Fe, Argentina, and Max-Volmer-Institut für Biophysikalische Chemie und Biochemie, Technische Universität Berlin, D-10623 Berlin, Germany

Received February 2, 2000

Revised Manuscript Received May 30, 2000

Abstract:

The photocycle of bacterial photosynthetic reaction centers (RCs) involves electron transfer between two quinone molecules, Q_A and Q_B. The semiquinone biradical Q_A^-Q_B^- forms an intermediate state in this process. We trapped the biradical at low temperature (77 K) and investigated its EPR spectra at three microwave frequencies, 9.6, 35, and 94 GHz, at temperatures between 1.5 and 100 K. The spectra were described with a spin Hamiltonian that contained, in addition to the Zeeman terms, dipolar and exchange interactions, and were fitted using the simulated annealing method (Kirkpatrick et al. Science 1983, 220, 671). From the parameters derived from the fit, information about the spatial and electronic structure was obtained. The relative position and orientation of the two quinones, determined from the EPR spectra, compared well with those obtained from X-ray diffraction of RCs in the Q_AQ_B^- state (Stowell et al. Science 1997, 276, 812). The values of the dipolar coupling and of the exchange interaction obtained from the fits were E_d/h = (10.3 ± 0.1) MHz and J_o/h = (-60 ± 20) MHz, respectively. The value of J_o was used to estimate a maximum electron-transfer rate, k_ET, (Q_A^-Q_B^- Q_AQ_B⁼) of ~10⁹ s^-¹. This agrees within an order of magnitude with the value derived from kinetics experiments (Graige et al. Biochemistry 1999, 38, 11465).

Download the full text: PDF | HTML