Electron paramagnetic resonance of the primary radical pair [D·+ φA·−] in reaction centers of photosynthetic bacteria

Using continuous wave EPR spectroscopy with a high time resolution, a new short-lived signal at g = 2 is observed in bacterial photosynthetic reaction centers in which electron transfer past the bacteriopheophytin φA, is blocked. This signal decays with a time constant equal to the rise time of the triplet state of the primary donor 3D, and is therefore attributed to the primary radical pair [D·+ φA·−]. Applying the theory of spin-correlated radical pairs, the spectrum could be satisfactorily simulated, yielding the following magnetic interaction parameters between D·+ and φA·−: exchange interaction JDφ = − 0.9 ± 0.1 mT and JDφ = −1.7 ± 0.1 mT for reaction centers of Rhodobacter sphaeroides R26 and Rhodopseudomottas viridis, respectively, and an axial dipolar interaction DDφ = − 3.0 ± 0.5 mT for Rhodopseudomonas viridis. The implications of the magnitude and sign of the exchange parameters for the energetics of photosynthetic electron transfer are discussed.