Character of melanin

Melanins are among the most widespread natural pigments, being present in all living organisms including plants, fungi and bacteria. Plant melanins have a different biochemical derivation from animal melanins. The latter originate from the amino acid tyrosine, and they are characterized by a brown-black colour, a high molecular weight and a polymeric structure. One of the most problematic features of melanin to the scientist is its insolubility in almost all solvents. Because it is stubbornly resistant to chemical treatment melanin is difficult to purify and analyse. Both Littre and Albinus, in the eighteenth century, were amazed at their failure to extract pigment from black skin after subjecting it to prolonged immersion in water and alcohol. Even after two and a half centuries of technology there are still no general methods to solubilize natural melanin under physiological conditions. Hence the Harvard biologist Carroll Williams was moved to describe melanin as ‘a pigment of the imagination’!

The intractability of melanin, on the other hand, may prove of value to palaeontologists (Daniels, Post & Johnson, 1972). Melanin has been found in a 150-million-year-old ichthyosaur, in extinct mammoth skin and in mummy skin. The cephalopod molluscs possess an ‘ink gland’, the secretion of which (i.e. melanin) was used by the ancients as a dye (sepia). The squid stores its melanin in a reservoir and, in time of danger, squirts the pigment from a siphon so as to create a smokescreen to divert its enemy. Cephalopod melanin is invulnerable to decay and early in the nineteenth century, in the south of England, a 150-million-year-old squid was discovered whose own ink was used to make drawings of its remains (Fox & Vevers, 1960).