The Red and the Black

Abstract

Pigmentation, which is primarily determined by the amount, the type, and the distribution of melanin, shows a remarkable diversity in human populations, and in this sense, it is an atypical trait."sE. J. Parra.Melanin is found throughout the human body, skin, eye, brain, hair, and inner ear, yet its molecular structure remains elusive. Researchers have characterized the molecular building blocks of melanin but have not been able to describe how those components fit together in the overall architecture of the pigment. Melanin is categorized into two distinct classes, pheomelanin (red) and eumelanin (black). Although these classes share a common biosynthetic origin, specific molecular reactions occurring early in pigment production differentiate these two types. Pure eumelanin is found throughout nature, which has allowed researchers to characterize and quantify its chemical properties. However, pure pheomelanin is not observed in nature and rarely makes up more than ∼25% of the total melanin present. In this Account, we explore our current understanding of the structure and reactivity of the red and black pigments.Epidemiological studies of skin and ocular cancers suggest that increasing relative proportions of pheomelanin correlate with increased risk factors for these diseases. Therefore, understanding the factors that control the relative abundance of the two pigments has become increasingly important. Consequently, researchers have worked to elucidate the chemistry of pheomelanin to determine whether the pigment could cause these cancers and, if so, by what mechanisms. The photoactivation of oxygen by pheomelanin in the UV-A range could contribute to the development of UV-induced cancers: recent measurement of the surface photoionization threshold of intact melanosomes reveals a lower photoionization potential for pheomelanin than eumelanin. A complementary study of intact human melanosomes isolated from different colored irides reveals that the absorption coefficient of the melanosome decreases with increasing pheomelanin content. These results suggest that the epidemiological data may simply result from an increased exposure of the underlying tissues to UV light.

Interpreting melanin-based coloration through deep time: a critical review

“…Several major melanin types exist in nature, but the most common are (dark brown-black) eumelanin and (red-yellow) pheomelanin [27]. Eumelanin is derived from enzyme-controlled oxidation of the amino acid tyrosine [28] to form a biochrome that is inherently resistant to degradation [25]; thus, its molecular structure is incompletely known [22,26]. Likewise, despite recognition that pheomelanin is produced from sulfur-containing benzothiazine units [28], its chemical structure is also not fully characterized [21].…”

Dynamics of Interfacial Electron Transfer from Betanin to Nanocrystalline TiO2: The Pursuit of Two-Electron Injection

“…Another possible two-electron, two-proton oxidation product is yellow neobetanin (VII of Figure 1), which has been observed to form in thermally treated extracts of red beet root. 23 Melanogenesis, or polymerization to form brown pigments similar to those found in the biological pigment eumelanin, 24 is another possible oxidative degradation product of betanin and other betacyanins. These polymers form on cross-linking of monomers of dihydroxyindole and derivatives and show spectral shifts associated with noncovalent interactions.…”

Direct chemical evidence for eumelanin pigment from the Jurassic period

“…Two forms of melanin, produced from different molecular precursors, are present in nature-eumelanin (dark brown-black in color) and pheomelanin (orange-red in color) (3). These classes of melanin serve numerous essential biological roles, including photoprotection, radioprotection, display, camouflage, and predation avoidance (4,5).…”