Semiquinone-Iron Complex of Photosystem II: EPR Signals Assigned to the Low-Field Edge of the Ground State Doublet of QA•-Fe2+ and QB•-Fe2+

The quinone-iron complex of the electron acceptor complex of Photosystem II was studied by EPR spectroscopy in Thermosynechococcus elongatus. NeW g similar to 2 features belonging to the EPR signal of the semiquinone forms of the primary and secondary quinone, i.e., Q(A)(center dot-)Fe(2+) and Q(B)(center dot-)Fe(2+), respectively, are reported. In previous studies, these signals were missed because they were obscured by the EPR signal arising from the stable tyrosyl radical, TyrD(center dot). When the TyrD(center dot) signal was removed, either by chemical reduction or by the use of a mutant lacking TyrD, the new signals dominate'd the spectrum. For Q(A)(center dot-)Fe(2+) the signal was formed by illumination at 77 K or by sodium dithionite reduction in the dark. For Q(B)(center dot-)Fe(2+), the signal showed the characteristic period-of-two variations in its intensity when generated by a series of laser flashes. The new features showed relaxation characteristics comparable to those of the well-known features of the semiquinone iron complexes and showed a temperature dependence consistent with an assignment to the low-field edge of the ground state doublet of the spin system. Spectral simulations are consistent with this assignment and with the current model of the spin system. The signal was also present in Q(B)(center dot-)Fe(2+) in plant Photosystem II, but in plants, the signal was not detected in the Q(A)(center dot-)Fe(2+) state.