Electron paramagnetic resonance (EPR) spectra of the reduced quinone-iron acceptor complex in reaction centers were measured in a variety of environments and compared with spectra calculated from a theoretical model. Spectra were obtained at microwave frequencies of 1, 9, and 35 GHz and at temperatures from 1.4 to 30 K. The spectra are characterized by a broad absorption peak centered at g = 1.8 with wings extending from g approximately equal to 5 to g less than 0.8. The peak is split with the low-field component increasing in amplitude with temperature. The theoretical model is based on a spin Hamiltonian, in which the reduced quinone, Q-, interacts magnetically with Fe2+. In this model the ground manifold of the interacting Q-Fe2+ system has two lowest doublets that are separated by approximately 3 K. Both perturbation analyses and exact numerical calculations were used to show how the observed spectrum arises from these two doublets. The following spin Hamiltonian parameters optimized the agreement between simulated and observed spectra: the electronic g tensor gFe, x = 2.16, gFe, y = 2.27, gFez = 2.04, the crystal field parameters D = 7.60 K and E/D = 0.25, and the antiferromagnetic magnetic interaction tensor, Jx = -0.13 K, Jy = -0.58 K, Jz = -0.58 K. The model accounts well for the g value (1.8) of the broad peak, the observed splitting of the peak, the high and low g value wings, and the observed temperature dependence of the shape of the spectra. The structural implications of the value of the magnetic interaction, J, and the influence of the environment on the spin Hamiltonian parameters are discussed. The similarity of spectra and relaxation times observed from the primary and secondary acceptor complexes Q-AFe2+ and Fe2+Q-B leads to the conclusion that the Fe2+ is approximately equidistant from QA and QB.

The photocycle of bacterial photosynthetic reaction centers (RCs) involves electron transfer between two quinone molecules, QA and QB. The semiquinone biradical QA -•QB -• 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 QAQB -• 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, (QA -•QB -• → QAQB =) 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).