X-ray Absorption Spectroscopic Studies of the Sulfide Complexes of Hemerythrin

The structures of the dinuclear iron sites of (μ-sulfido)methemerythrin and (μ-sulfido)semi-methemerythrin are explored by using X-ray absorption spectroscopy. The analysis of pre-edge features in the XAS spectrum of (μ-S)metHr does not reveal a significant change in the coordination number/geometry of the iron centers compared with those of native oxo/hydroxo-bridged forms of the protein. The EXAFS analysis of the first coordination sphere data is consistent with the expected five- and six-coordination of the iron atoms and with the presence of a sulfido bridge, with an average Fe-S distance of 218 pm. The Fe-Fe distance in (μ-S)metHr is found to be 335 pm. Upon reduction of the protein to (μ-S)semi-metHr, the Fe-Fe distance lengthens to 359 pm, while the Fe-S distance remains unchanged. Lengthening of the Fe-Fe distance and the average Fe-O,N bond distance upon reduction is shown to be relevant to structural changes that occur upon reduction of the native oxo/hydroxo-bridged forms of the protein and the interconversion between two distinct forms of semi-metHr, (semi-met)_rHr and (semi-met)_oHr. The larger bridging sulfide apparently favors an iron site structure resembling that of (semi-met)_oHr.