The photocycle of bacterial photosynthetic reaction centers (RCs) involves electron transfer between two quinone molecules, Qa 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(A)Q(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(A)Q(B)(=)) of similar to 10(9) s(-1) This agrees within an order of magnitude with the value derived from kinetics experiments.