Simultaneous binding of fluoride and NO to the nonheme iron of photosystemII: Quantitative EPR evidence for a weak exchange interaction between the semiquinone Q(A)(-) and the iron-nitrosyl complex

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 spectroscop y. It is found that the EPR signal of the iron-nitrosyl complex changes upo n addition of F-. The change is determined to be due to a superhyperfine in teraction between the electronic spin (S = 3/2) and the fluorine nuclear sp in (I = 1/2), indicating that both F- acid NO are bound to the same nonheme iron. To the best of our knowledge, this is the first report of simultaneo us binding of both F- and NO to the same iron of a mononuclear nonheme prot ein. On the basis of an analysis of the hyperfine interaction, a cis F-Fe-N O coordination is indicated. Upon illumination of (NO, Cl-)- or (NO, F-)-tr eated PSII membranes at 200 K, new X-and Q-band EPR signals are observed in B(1)parallel to B and B(1)perpendicular to B modes. Quantitative simulatio ns of these signals provide an unambiguous assignment to the iron-quinone c omplex, Q(A)(-){FeNO},(7) of the acceptor site of PSII. The exchange intera ction 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 = JS(1)S(2). A distribution in the exchange coupling is r equired to satisfactorily simulate the EPR spectra. This distribution is co rrelated to small structural variations of the iron-quinone acceptor side o f PSII.