Abstract
Melanins are notoriously difficult to study because they are amorphous, insoluble and often associated with other biological
materials. Consequently, there is a dearth of structural techniques to study this enigmatic pigment. Current models of
melanin structure envision the stacking of planar structures. X ray diffraction has historically been used to deduce stacking
parameters. In this study we used X ray diffraction to analyze melanins derived from Cryptococcus neoformans, Aspergillus
niger, Wangiella dermatitides and Coprinus comatus. Analysis of melanin in melanized C. neoformans encapsulated cells was
precluded by the fortuitous finding that the capsular polysaccharide had a diffraction spectrum that was similar to that of
isolated melanin. The capsular polysaccharide spectrum was dominated by a broad non-Bragg feature consistent with origin
from a repeating structural motif that may arise from inter-molecular interactions and/or possibly gel organization. Hence,
we isolated melanin from each fungal species and compared diffraction parameters. The results show that the inferred
stacking distances of fungal melanins differ from that reported for synthetic melanin and neuromelanin, occupying
intermediate position between these other melanins. These results suggest that all melanins have a fundamental diffracting
unit composed of planar graphitic assemblies that can differ in stacking distance. The stacking peak appears to be a
distinguishing universal feature of melanins that may be of use in characterizing these enigmatic pigments.