FLUORIDE SUBSTITUTION IN THE MN CLUSTER FROM PHOTOSYSTEM-II - EPR AND X-RAY-ABSORPTION SPECTROSCOPY STUDIES

X-band electron paramagnetic resonance (EPR) and Mn K-edge X-ray fluorescence absorption were used to study the effects of fluoride inhibition on the Mn complex in Photosystem II. The tetrameric Mn complex, responsible for the light-induced oxidation of H2O to form molecular oxygen, is influenced by treatments in which the naturally occurring chloride salts are removed or replaced. Inhibition of the complex by fluoride is examined by parallel enzyme activity and EPR studies. It is found that, as a function of increasing fluoride concentration, the declining enzymatic activity is paralleled initially by an exchange of the S = 1/2 'multiline' EPR signal for the S > 1/2, 'g = 4' EPR signal in illuminated samples. High concentrations of fluoride induce a broad (approximate to 200 G), featureless radical signal in samples which have not been illuminated; subsequent illumination of these samples also generates the g = 4 EPR signal. X-ray absorption studies (XAS) of fluoride-inhibited samples show subtle alterations of the conformation of the Mn complex that are consistent with the presence of two dissimilar pairs of Mn atoms. The halide studies are discussed in terms of structural models for the Mn complex.