Melanin is best known as the pigment that imparts color to hair, eyes, and skin. The biopolymer also has interesting optical, mechanical, and electronic properties that could be harnessed for various applications. But it has proven difficult to make materials that mimic the properties of melanin. Researchers now introduce a method to make synthetic melanin as well as the ability to fine-tune their properties.

Melanins are a group of biological polymers that have diverse roles in organisms. In addition to providing color, melanin protects cells from UV radiation and free radicals that can damage them. Some marine animals like mussels use melanin-like materials for adhesion.

There are two challenges with synthesizing melanin-mimicking polymers, says Rein Ulijn, a nanoscientist at the City University of New York. One is that they are highly complex structures that are poorly understood. Second, while other biopolymers like DNA and proteins have a clear relationship between their structure and function, melanin is inherently disordered. “This makes them hard to study, control, and synthesize,” he says.

Researchers have, of course, tried to produce melanin. The process involves first oxidizing an amino acid called tyrosine to form a highly reactive monomer. This monomer diffuses in solution and reacts with other molecules it comes across. “There are lots of monomers reacting randomly in a poorly controlled way, and you get this black material with a messy, poorly organized polymeric structure,” Ulijn says.

Instead of tyrosine, Ulijn and his colleagues started with small peptide molecules (shorter versions of proteins) as precursors, which allowed them to guide the polymerization process and better control the properties of the resulting material.

First, the researchers identified a small set of peptides that are made of only three amino acids: tyrosine, phenylalanine, and aspartic acid. Based on the order of the amino acids, this created six different peptides. These six peptides act as tunable precursors that give different melanin-like polymers.

To make the polymers, the researchers first heated solutions of each peptide. The peptides self-assembled into different nanostructures based on their amino acid sequence. Two remained clear solutions while the others turned into a milky suspension, an opaque gel, a translucent gel, and crystalline needle-like fibers.

Next the researchers polymerized the peptide assemblies by adding the enzyme tyrosinase and incubating them, which oxidizes and polymerizes them into different melanin-like polymers. The two transparent solutions became light brown in coloration, the milky suspension became beige, while the other three took on brown-black colors, indicating that the peptides polymerized to different extents.

Transmission electron microscopy and atomic force microscopy revealed that the sequence of peptides also affected the nanoscale morphology, crystallinity, and stiffness. In the journal Science, the researchers report the synthetic melanin materials’ UV-visible absorbance across a broad range of wavelengths from 300 nm to 650 nm, with some polymers absorbing more light than others.  

In terms of applications, the materials can be used to make effective sunscreens and UV-protective coatings, Ulijn says. And because the colors can mimic real skin cells, they might be of interest for making cosmetic pigments. The team plans to pursue commercializing the technology.

In a Perspectives commentary piece accompanying the article, Marco d’Ischia of the University of Naples and Philip Messersmith of the University of California, Berkeley say that the new advance could lead to the rational design of melanin-like pigments with numerous potential technological applications. “It would be intriguing to probe the tuning effects of integrating reactive amino acids, such as cysteine and arginine, into the sequence and to investigate what color would be generated by cross-coupling between different peptides or by the introduction of metal species,” they write.

Read the abstract in Science.