Melanin: The Dark Pigment’s Chemistry Beyond Suntans

In Chemistry World’s Chemistry In Its Element, Kat Arney explores melanin as a biology–chemistry powerhouse. From its tyrosine-derived polymer structure to its diverse roles across organisms, the episode reveals why melanin is more than a suntan pigment.

• Melanin is a polymer formed from modified tyrosine that absorbs across visible to ultraviolet light.
• In humans, eumelanin and pheomelanin determine brown/black versus red hues, with genetics shaping their balance and effects on skin, hair, and lips.
• Beyond UV protection, melanin acts as an antioxidant and participates in skin health, eye colour, and brain function through neuromelanin in the substantia nigra.
• Cephalopod ink provides a natural melanin source with defensive and culinary uses, including the famed sepia tint.

Introduction: Melanin as a pigment and polymer

The podcast begins by tracing the heritage of melanin, a biological pigment whose name derives from the Greek melanos, dark, and which is found across the animal kingdom, plants, bacteria and fungi. Melanin is described as a polymer built from long chains of modified amino acids, typically linked to tyrosine. The host notes that the pigment’s dark colour comes from its polymer structure, rich in carbon rings capable of absorbing a broad spectrum of light from visible to ultraviolet. An aside highlights Berzelius’s role in coining terms like polymer and protein, linking historical chemistry to the biology of melanin.

Melanin chemistry and the two main human forms

Tyrosine oxidation initiates melanin formation, followed by polymerisation and further modification to yield different colours and forms. The two most common types in humans are eumelanin (brown to black) and pheomelanin (reddish). The relative amounts of these forms are controlled by genetic variations, explaining why dark skin and hair tend to be rich in eumelanin, while pale skin and blonde hair show lower levels. The introduction of pheomelanin results in strawberry blonde or fiery red hair when present in higher amounts.

Functions beyond a sunscreen role

While melanin provides UV protection, the transcript explains that this is an overly simplistic view. Tyrosine is an aromatic amino acid, and the resulting melanin polymer contains carbon rings that absorb a wide range of wavelengths. This absorption contributes to the pigment’s darkness and UV shielding. Crucially, melanin’s structure also makes it proficient at scavenging free radicals and binding/toxins, supporting skin health in a broader sense than sunscreen alone.

Melanin is not limited to skin. In the eye, melanin influences iris colour, with more pigment producing darker brown eyes and less resulting in hazel or green eyes, while blue eyes arise from very low melanin levels. In the retina, melanin helps reduce light scattering, effectively darkening the inside of the eye much as painting the inside of a camera black would. In the brain, a form called neuromelanin resides in the substantia nigra, a region tied to Parkinson’s disease where a loss of neuromelanin may be implicated in the condition.

Beyond humans, melanin appears in the inner ear and in various animal bodies enabling colouration, protective functions and even specialized optical effects, such as the iridescent sheen observed in some beetle shells caused by layered melanin pigments.

Melanin in other species and practical implications

The discussion expands to melanin’s presence in cephalopod ink, a natural pigment produced by octopuses, squids and cuttlefish when threatened. The ink varies in hue and function, with squid ink described as velvety blue-black and octopus ink as jet black. The transcript notes that cephalopod ink can serve as a smokescreen or irritant, and some cephalopods can even eject a mucus-infused ink that forms a decoy, a pseudomorph, to aid an escape. The breadth of melanin’s roles across species underscores its fundamental place in biology.

Roots, history and the broader pigment family

An aside draws attention to the history of polymer science and pigment terminology, emphasizing melanin’s place in the long arc of chemistry from Berzelius’s era to modern polymer science. The summary closes by connecting melanin’s diverse roles—from bananas browning due to melanin formation to its multi-tissue presence in humans and animals—showing how chemistry and biology converge in everyday phenomena and exotic pigments alike.

Takeaway

The podcast reveals melanin as a multifaceted polymer that governs light absorption, protection against damage, and color across tissues and species. Its presence in cephalopod ink and in critical brain regions points to a central role in biology that transcends simple skin colour, inviting ongoing exploration of pigment chemistry and protective biology in health and disease.