The nature of the Fe(III) EPR signal from the acceptor‐side iron in photosystem II

The EPR spectrum at both X‐ and S‐band (3.94 GHz) of the oxidized acceptor‐side iron in photosystem II from spinach shows two absorption‐type peaks at g = 8.0 and 5.6. The intensities of these peaks have been measured at X‐band in the temperature range 2–10 K. All results can be fully described assuming that the EPR spectrum arises from high‐spin Fe(III) with D = 1.0±0.3 cm−1 and E/D = 0.10 ±0.01. Quantifications show that the spectrum in our case represents 0.4–0.5 Fe(III) per reaction center. The EPR parameters are consistent with the iron having bicarbonate and/or tyrosine as ligands in addition to four imidazoles.