Eumelanin, a form of melanin typical of mammals, is a brown-black coloured pigment found in skin, hair, and eyes. It absorbs sunlight energy and transforms it into heat, acting as a natural sunscreen.
For a pigment that plays this crucial role—and many more—surprisingly little is known about its composition and biosynthesis. Dr. Jean-Phillip Lumb, associate professor in McGill’s Department of Chemistry, and his research group recently published their research in Nature Chemistry, exploring eumelanin’s composition and chemical function.
In addition to its more well-known function as a natural form of sun protection, melanin has several additional functions that are not well understood.
“Melanin is produced in certain regions of your body that are not exposed to sunlight,” Lumb said in an interview with The Tribune. “One example is in the inner ear, there are [pigmented] hair follicles. If those hair follicles get damaged and stop producing pigment, it can lead to problems with ability to balance.”
The substantia nigra, a region of the brain involved in dopamine production, also contains a type of eumelanin. “People who experience neurodegeneration have a decline in the amount of the pigment in that region,” Lumb said. “But the truth is, a defined precise role for melanin in the brain is not available—we don’t know exactly what it’s doing.”
One of the main objectives of Lumb’s research is to understand the structure of eumelanin to provide insights into its physiological roles.
Eumelanin granules are formed in pigment-producing cells called melanocytes, within football-shaped compartments called melanosomes. Inside them, a naturally occurring amino acid L-tyrosine becomes oxidized, causing it to lose electrons. This part of the process is well-documented; however, researchers still don’t fully understand what happens after oxidation. The next step is thought to be polymerization: The formation of a long ‘string’ of smaller subunits.
“What we think happens is that L-tyrosine gets oxidized [into] DHI [5,6-dihydroxyindole], and then DHI continues in this oxidation-polymerization pathway. Up to the work that we did, nobody had ever been able to isolate anything from the oxidation of DHI,” Lumb shared.
DHI is further oxidized into indole-5,6-quinone (IQ)–after which everything gets more complex. Polymerization likely accompanies this oxidation, making it hard to isolate IQ. To tackle this, Lumb’s group chemically modified DHI by adding bulky groups surrounding the periphery of the DHI molecule, making polymerization difficult. By doing so, only oxidation occurred—allowing researchers to study the process in isolation.
Lumb’s research group soon discovered that some of eumelanin’s components possess its properties. Semiquinone radical (SQ)—an intermediate between DHI and IQ—has paramagnetic properties just like eumelanin, which is very unusual for a biomolecule. Equally interesting is that green-colored IQ exhibits sun-protective properties reminiscent of those of eumelanin.
Eumelanin-derived sunscreen would be different from our traditional reflective sunscreens, such as zinc oxide, which reflect light and typically have white or light color. Absorptive sunscreens absorb radiation and transform it into something innocuous, like heat, instead of giving it off in the same form, rendering it less dangerous to the skin.
“That is exactly what eumelanin does—it is extremely good at converting light energy into heat, and that’s how it plays the role of a sunscreen.” Lumb shared. “It absorbs everything from the beginning of the ultraviolet spectrum all the way into the near infrared.”
Since IQ possesses eumelanin’s sun-protective properties, it could potentially be isolated and used as a sunscreen. However, there is one major problem: The compound is green.
This could limit its wide use as sunscreen—unless we all agree that looking like an alien from a ‘90s movie is fashionable. Either way, Lumb’s new research expands our understanding of eumelanin’s components and roles, with potential applications in medicine.
