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 environment of the iron site in natural (human) neuromelanin extracted from substantia nigra tissue and in various synthetic neuromelanins. All the materials show Fe centered in a nearest neighbor sixfold (distorted) oxygen octahedron; the Fe-O distances, while slightly different in the natural and synthetic neuromelanin, are both similar to 2.0 Angstrom. Appreciable differences arise, however, in the second (and higher) coordination shells. In this case the synthetic melanin has the four planar oxygens bound to carbon rings with Fe-C distances of similar to 2.82 and 4.13 Angstrom; the human sample does not show the 2.82 Angstrom link but instead indicates a double shell at approximately 3.45 and 3.78 Angstrom.