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

Molecular packing for planar molecules

The crystal packing of five planar molecules is considered in this paper. Each unit cell contains two non-equivalent molecules whose planes are inclined to each other. It is shown that the angle of inclination between the planes is completely determined by a simple geometrical criterion. A simple sequential arrangement of the four molecules defining the elementary parallelopiped of which the entire crystal is built leads to various configurations from which the one which has the least interaction energy can be picked out. Using a crude Lennard-Jones potential for the non-bonded interaction and a hard sphere model for the atoms, one can compute the crystal structure from the minimum energy criterion and this is found to be in fair agreement with the observed structure. This simple sequential packing with some modifications can provide an useful model for calculating the radial distribution function in amorphous solids involving planar groups