Sunburn: How exactly does melanin shield you from UV – and what does it have to do with solar panels?

Melanin pigments in our skin protect us from UV light, and people with higher levels have more protection from the sun’s harmful rays.

But how exactly does melanin work?

Scientists have known for a while that melanin absorbs UV light and converts it into heat, but the precise mechanism by which melanin does this has – until now – been unclear.

A team of New Zealand (Aotearoa) researchers have combined a trio of techniques to figure it out.

Publishing their findings in PNAS, they’ve demonstrated that a type of melanin called ‘eumelanin’ goes through a two-step reaction to intercept the UV and prevent damage to our skin.

The whole thing happens in less than a trillionth of a second.

Senior author Professor Justin Hodgkiss, co-director of the MacDiarmid Institute and a researcher at Te Herenga Waka – Victoria University of Wellington, New Zealand, says there are two reasons that the mechanism hasn’t already been found.

“One, because that key process happens really fast. And secondly, because the material itself is so heterogeneous and so disordered, and ill-defined, it’s really hard to understand exactly what you’re looking at.”

Series of instruments on a bench bouncing an orange laser beam around
VUW’s Ultrafast Optical Spectroscopy lab. Credit: © VUW Image Services

The researchers countered this by using three different types of ultrafast optical spectroscopy.

Spectroscopy is a way of analysing molecules by how they absorb and emit light. It’s a key process for researchers to know how molecules are shaped, and how they react.

Different spectroscopic techniques yield different information about molecules.

These researchers used transient absorption spectroscopy, fluorescence spectroscopy (a technique developed in their lab in the early 2010s), and stimulated Raman spectroscopy, to find out how the eumelanin was behaving.

Two photos: left, two bottles with brown and yello liquid, right: small plastic ring with brown film
Left: dispersion of the synthetic melanin. Right: a thin film from the lab (the type of samples the researchers looked at). Credit: © VUW Image Services

When levelling these three techniques (all of which can provide resolution at the level of a few femtoseconds, or 0.000000000000001 seconds) at eumelanin from squid ink, alongside eumelanin made in the lab, the researchers were able to show how melanin absorbs UV light.

“Eumelanin’s job in nature, at least in our skin, is to convert light energy very rapidly to heat which is the safest way to dissipate it before radicals can be generated,” says Hodgkiss.

More on melanin: Explainer: Why does hair turn grey?

It turns out that the eumelanin shares the energy from UV light with neighbouring molecules called chromophores, and then undergoes a reaction called a partial proton transfer with water molecules. This stops the UV from prompting free radical molecules to form, which can be carcinogenic.

Having shown it works for melanin, the researchers are trying the technique out on other molecules.

“It’s a pretty powerful combination for lots of things,” says Hodgkiss.

It’s likely to be particularly useful for making better solar panels.

“Most of our work is looking at light-to-current processes in next generation photovoltaic materials,” says Hodgkiss.

Rectangular plastic case with green laser shining through it
Ultrafast optical spectroscopy in action – but this sample, pictured, isn’t eumelanin. Credit: © VUW Image Services

“There, we’re looking at converting light energy to current and mechanism – why it happens efficiently in some materials and not so in others.”

Solar panels operate in the same realm as melanin – but, ideally, doing exactly the opposite thing.

The materials in photovoltaics are supposed to use UV to make radicals and charged molecules.

“You’re looking for light and converting into charges,” says Hodgkiss.

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The researchers are also interested in examining another type of melanin called ‘pheomelanin,’ linked with pinkish colouring and red hair, which doesn’t have the same UV protective properties and may even cause more damage in UV light.

“The other thing that this has inspired us to do is to think about whether we can learn from nature to design either components of sunscreens that work with a similar mechanism, or even additives in paints or coatings for things that go outside where they get damaged by UV light,” says Hodgkiss.

“When you reduce it down to the mechanisms that we uncovered, can we embody that in some kind of synthetic additive?”

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