ELECTRON-PARAMAGNETIC-RESONANCE OF THE PRIMARY RADICAL PAIR [D-CENTER-DOT-DOT-)] IN REACTION CENTERS OF PHOTOSYNTHETIC BACTERIA(PHI(A)(CENTER)

Using continuous wave EPR spectroscopy with a high time resolution, a new short-lived signal at g = 2 is observed in bacterial photosyntheti c reaction centers in which electron transfer past the bacteriopheophy tin Phi(A) is blocked. This signal decays with a time constant equal t o the rise time of the triplet state of the primary donor D-3, and is therefore attributed to the primary radical pair [D(.+)Phi(A)(.-)]. Ap plying the theory of spin-correlated radical pairs, the spectrum could be satisfactorily simulated, yielding the following magnetic interact ion parameters between D-.t and Phi(A)(.-): exchange interaction J(D P hi) = -0.9 +/- 0.1 mT and J(D Phi) = -1.7 +/- 0.1 mT for reaction cent ers of Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis, respe ctively, and an axial dipolar interaction D-D Phi = -3.0 +/- 0.5 mT fo r Rhodopseudomonas viridis, The implications of We magnitude and sign of the exchange parameters for the energetics of photosynthetic electr on transfer are discussed.