X-RAY CHARACTERIZATION OF MELANINS .1.

The intrinsic local structure characterization of natural sepia melani n and L-dopa and tyrosine synthetic melanin powder has been carried ou t by X-ray diffraction using synchrotron radiation. The derived struct ure factor, S(q), shows six significant diffuse peaks within the q-ran ge from 0.3 Angstrom(-1) to 16 Angstrom(-1) in the reciprocal space (q = (4 pi sin theta)/lambda, 2 theta is the scattering angle). The Four ier transform of S(q), which yields the radial distribution function ( RDF), gives us information in real space of a 1.42 Angstrom distance a veraged over the C-C, C-O and C-N bond lengths as well as peaks at 2.4 0-2.41 Angstrom, 3.67-3.71 Angstrom and 4.67-4.70 Angstrom discrete ne ighbor distances. There is a great similarity in the scattering intens ity profiles of the natural and synthetic melanins indicating that the synthetically prepared material may be essentially similar to ''real' ' melanin in its local atomic arrangements. An evidence of a prepeak a t q similar or equal to 0.45 Angstrom(-1) has been confirmed which ind icates a preferred length scale of similar to 13-20 Angstrom that corr esponds to the initial particle size in colloidal melanin solutions.