X-ray absorption fine-structure spectroscopy studies of Fe sites in natural human neuromelanin and synthetic analogues

X-ray absorption fine-structure spectroscopy is used to study the local env ironment of the iron site in natural (human) neuromelanin extracted from su bstantia nigra tissue and in various synthetic neuromelanins. All the mater ials show Fe centered in a nearest neighbor sixfold (distorted) oxygen octa hedron; the Fe-O distances, while slightly different in the natural and syn thetic neuromelanin, are both similar to 2.0 Angstrom. Appreciable differen ces arise, however, in the second (and higher) coordination shells. In this case the synthetic melanin has the four planar oxygens bound to carbon rin gs with Fe-C distances of similar to 2.82 and 4.13 Angstrom; the human samp le does not show the 2.82 Angstrom link but instead indicates a double shel l at approximately 3.45 and 3.78 Angstrom.