The effects of NO and fluoride on the iron-quinone complex of the acceptor side of Photosystem II (PSII) are examined by X- and Q-band EPR spectroscopy. It is found that the EPR signal of the iron-nitrosyl complex changes upon addition of F - . The change is determined to be due to a superhyperfine interaction between the electronic spin ( S = 3 / 2 ) and the fluorine nuclear spin ( I = 1 / 2 ), indicating that both F - and NO are bound to the same nonheme iron. To the best of our knowledge, this is the first report of simultaneous binding of both F - and NO to the same iron of a mononuclear nonheme protein. On the basis of an analysis of the hyperfine interaction, a cis F−Fe−NO coordination is indicated. Upon illumination of (NO, Cl - )- or (NO, F - )-treated PSII membranes at 200 K, new X- and Q-band EPR signals are observed in B 1 || B and B 1 ⊥ B modes. Quantitative simulations of these signals provide an unambiguous assignment to the iron-quinone complex, Q A - {FeNO}, 7 of the acceptor site of PSII. The exchange interaction between the iron-nitrosyl complex ( S = 3 / 2 ) and the semiquinone Q A - radical ( S = 1 / 2 ) is determined to be J = +0.5 cm - 1 (F - ) and +1.3 cm - 1 (Cl - ) for H ex = J S 1 S 2 . A distribution in the exchange coupling is required to satisfactorily simulate the EPR spectra. This distribution is correlated to small structural variations of the iron-quinone acceptor side of PSII.
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Sanakis et al. (1999) studied this question.