EPR study of the semiquinone biradical Q(A)(center dot-)Q(B)(center dot-) in photosynthetic reaction centers ofRhodobacter sphaeroides at 326 GHz: Determination of the exchange interaction J(o)

The magnitude of the exchange interaction J(o), defined by H-exch = -J(o)S(A)(.)S(B), between unpaired spins S-A and Sg in biological molecules is an important parameter in calculating electron-transfer rates. In a previous work, we obtained an approximate value of J(o) for the biradical Q(A)*(-)QB(*-), which forms an intermediate state in the photocycle of reaction centers (RCs) of the photosynthetic bacterium Rhodobacter sphaeroides (Calvo, R. et al. J. Am. Chem. Sec. 2000, 122, 7327). In that work J(o) was derived from EPR measurements at 35 and 94 GHz, frequencies at which the EPR spectra are relatively insensitive to J(o). In this work we obtained EPR spectra between 5 and 80 K at 326 GHz, a frequency at which the spectrum is more sensitive to J(o). Consequently, we were able to improve the accuracy of J(o) approximately by an order of magnitude. From a global fit with a spin-Hamiltonian of the spectra at five temperatures we obtained a value of J(o)/h = (-82 +/- 3) MHz. The value of J(o) is within the experimental uncertainty of the previously reported value and results in an estimated maximum electron-transfer rate between the semiquinones of k(ET)(max) approximate to 10(9) s(-1).