Physical studies on melanins. II. X-ray diffraction.

A number of purified natural and synthetic melanins have been examined by X-ray diffraction. A consistent finding with all samples was the lack of structure in the diffraction pattern corresponding to any significant crystallinity in these melanin preparations. A diffuse ring, centered at a Bragg spacing of 3.4 A was consistently found in samples of melanin from animal sources, and a similar ring at 4.2 A in all melanins obtained from plants. Models for these two polymer types, based upon the current concept that they primarily involve indole and catechol monomeric units respectively, were then evaluated by a Monte Carlo method. From the comparison of the observed spacings with the calculated ones it was concluded that the 4.2 A spacing in the catechol melanins is probably related to the average interaction between adjacent monomeric units, with mutually random orientations. The 3.4 A spacing observed in indole melanins appears to derive from the tendency of indole monomers (probably of adjacent chains) tending to aggregate in near parallel stacks. Some randomness in the form of translations and rotations parallel to the planar groups is consistent with the diffraction patterns. An interesting finding was that the diffraction pattern of synthetic melanin prepared by the alkaline auto-oxidation of catechol gave the 3.4 A spacing found in the indole melanins of natural origin.