When summer comes around and you find yourself in the sun – you forget your hat, perhaps, or there’s no shade to slip into – your body notes the incoming UV radiation and activates its own defence system, quickly creating a pigment called melanin in your skin.
Now scientists from the City University of New York (CUNY) have opened a path to a new kind of sun-safe cosmetics and biomedical materials by artificially mimicking the tanning and protective properties of melanin and manipulating the colour with unprecedented control. They report their findings in Science.
Melanin is produced in the body through the oxidation of the amino acid tyrosine, the molecules of which link together through an enzymatic proces to form the dark pigment that is responsible for most differences in skin colour. It’s a very effective defence, too: melanin has been reported to protect against 99.9% of absorbed UV radiation.
Unlike other bio-polymers, such as DNA and proteins, where scientists have been able to establish a direct link between molecular structure and properties, melanin has been harder to engineer in the laboratory since it has an inherently disordered molecular structure.
Lead researcher, chemist Rein Ulijn, said, “We spent six months perfecting the method and characterizing the materials, which eventually led to a pretty good understanding of how variable polymeric pigments can be produced in the lab.”
In this study, Ulijn and his team of researchers have uncovered exactly what causes the different shades of brown of melanin and developed a technique that provides unparalleled control over the colours expressed by different combinations of amino acids.
The team found that the position of tyrosine, in a sequence of just three amino acids, could strongly influence the self-assembly of the amino acids as they transformed into varying shades of brown pigment. By simply switching the order of two amino acids in the sequence the researchers were able to form an array of nanostructures that produce different shades of brown: small spheres, fibres and flat sheets.
Adam Martin, a chemist from the University of NSW not involved with the study, said, “The most surprising aspect of the results is the diversity of structures that can be obtained through the use of three amino acids and an enzyme.”
The researchers hope that this new-found level of control will inform a new generation of cosmetics that protect against harmful UV rays by mimicking the natural action of melanin. Safety will be a concern, but Ulijn is confident that “the use of simple combinations of the same amino acids that form the proteins that we eat ensures low regulatory barriers for these materials”.
Other researchers agree that this study is an important step towards creating functional materials inspired by the processes occurring in our bodies. “It bodes well for the future generation of functional libraries from ordered peptide assemblies,” said Adam Martin.