Pulsed EPR experiments on radical pairs in photosynthesis: Comparison of the donor‐acceptor distances in photosystem I and bacterial reaction centers

Pulsed EPR spectroscopy is applied to the radical pairs P Q in bacterial reaction centers of Rhodobacter sphaeroides R‐26 and P A in photosystem I from Synechococcus elongatus. For both radical pairs an out‐of‐phase echo was observed showing a deep modulation as a function of the spacing between the two microwave pulses. From the modulation frequency the dipolar and isotropic couplings between the radical pair spins were derived. For the bacterial reaction centers a dipolar coupling of D≈︁ −121 μT was obtained, resulting in a distance of about 28.4 Å between the electron spins of the radical pair. This value is in good agreement with the distance derived from the x‐ray structure of Rhodobacter sphaeroides R‐26. From the faster modulation in the PS I sample we obtained a dipolar coupling of D≈︁ −170μT, indicating a distance of 25.4 Å between the primary donor P700 and the acceptor quinone A1. For both radical pairs, only a small isotropic coupling of J ≈︁ 1 μT, consistent with transient EPR spectra, was found.