The proposed structure for a eumelanin protomolecule [11–14]
was deduced from X-ray diffraction data. The protomolecule
consists of four imperfectly stacked planar sheets with each sheet
consisting of eight indolequinone molecules linked together so that
the oxygen atoms lie on the outer edges of each sheet and the
nitrogen atoms are located in a porphyrin-like hole at the center of
each sheet. The spacing between sheets is 3.45 A, a distance
similar to that found for graphitic sheets. The stack of sheets of
melanin forms a diffracting structure which is seen as a prominent
broad peak easily discernible in the raw data. However, the
information leading to the final proposed structure of the sheets
required the subtraction of the background incoherent contributions.
X-ray diffraction studies carried out on neuromelanins
extracted from human brains (13) have shown the presence of a
robust peak similar to that found for eumelanins. The position of
the peak, however, suggests a difference in the stacking distance of
the neuromelanins compared to the eumelanins. In this study we
use X-ray diffraction techniques to compare four different types of
fungal melanins. Ours results are consistent with the proposal that
melanins are composed of planar structures that can differ in
stacking distances.