Human beings come in a glorious spectrum of different colors: light, dark, plain or freckly skin; black, brunette, blond, auburn, and white hair; and eyes that are blue, hazel, green, amber and brown, to name just a few. It’s amazing to realize that most of this color is attributed to a single class of pigments: the melanins.
What are melanins?
Melanins compose a class of compounds that serve predominantly as a pigment. These pigments are derivatives of the amino acid tyrosine. There are at least three types of naturally occurring melanins: eumelanin, pheomelanin and neuromelanin. Both the chemical composition and the physical properties differ for the various types of melanin, suggesting that their chemical and biological responses may behave differently when exposed to light.
Although melanins are ubiquitous in humankind, our understanding of their chemistry is surprisingly limited. Melanin pigments have a complex natural structure that has defied detailed analysis. Once separated from living tissue, they form an amorphous mass and lose their inherent structure, making them very difficult to analyze.
What are the properties of melanins?
Melanins seem to be heterogeneous, with some small regions of order at the nanometer scale. The optical properties we can see depend on the ability of monomers and oligomers (made up of small numbers of monomers) that make up melanin to absorb light, and the ability of melanin particles to reflect and scatter incident light for different wavelengths. Melanins seem to have some semiconductor properties, and none of the proposed band models adequately account for this.