Evidence for synergistic anion binding to iron in ovotransferrin complexes from resonance raman and extended X-ray absorption fine structure analysis

The 2,3-dihydroxybenzoate and thioglycolate complexes of iron(III)-ovotransferrin have been studied with resonance Raman and extended x-ray absorption fine structure spectroscopies, respectively, to obtain evidence for the coordination of the synergistic anion to the iron center. The dihydroxybenzoate complex exhibits resonance-enhanced Raman vibrations arising from both the endogenous tyrosinates and the added dihydroxybenzoate. A comparison of the extended x-ray absorption fine structure spectra of the carbonate and thioglycolate complexes shows a large feature at about 1.95 Å assigned to Fe-(O,N) interactions. The latter complex exhibits an added feature at 2.32 Å assigned to an Fe-S interaction. These experiments demonstrate that the Lewis base functions in the synergistic anions coordinate to the iron in ovotransferrin.