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Melanin is one of the most common bio-pigments and eumelanin forms the class of melanin that is brownish/black in color. Eumelanin is well known for its extreme photo-absorption properties across all wavelengths of UV and visible light and protects human beings from harmful solar radiation. Over the past decade research on eumelanin has been gaining impetus because of its unusual properties, such as extreme photo-absorption, efficient non-radiative decay and quenching of reactive oxygen species.

Eumelanin is, of course, central to the photo-protection ability in humans, animals and some plants. So, why has this clearly important biomolecule been so little studied until now? One of the major reasons happens to be the difficulty in ascertaining the exact molecular structure of eumelanin. In the absence of a resolved structure, the structure–function correlation of its most spectacular properties remains an enigma.

The main reason behind the absence of a resolved structure is the extreme heterogeneity in the eumelanin structure. And it is this same heterogeneity that is probably responsible for many of its fascinating properties. In a recent review published in WIREs Computational Molecular Science, Dr. Debashree Ghosh of the Indian Association for the Cultivation of Science, discusses how computational approaches have been instrumental in revealing some of these aspects.

“While most organic molecules have a characteristic wavelength around which it absorbs most strongly, eumelanin has a featureless monotonic spectrum that spans the entirety of the UV and visible [light] range. While this makes characterizing the structure extremely hard, it also makes eumelanin the perfect species to absorb sunlight. The next question one is tempted to ask is what happens once the radiation is absorbed? Heterogeneity seems to again play a pivotal role and the molecular species within eumelanin interchange between each other along excited state chemical pathways. However, the final structural heterogeneity is retained and therefore, eumelanin can absorb all of that light and quench it without too much damage to its own structure,” explained Ghosh. “This is fascinating and we are just starting to scratch the surface of this problem and many of the other properties of eumelanin still remain enigmatic to us.”

The benefit of computational studies is the ability to create a plethora of different structures of eumelanin, thereby emulating the natural heterogeneity in its structure and being able to find the properties of each substituent part. This bottom-up approach gives us a window into the structure-function relationship we are after. While combinatorial methods can be used to generate these structures, modern quantum chemical methods in computationally efficient software packages can be used to calculate the properties accurately.

“Recently we have been able to elucidate the excited state pathways for all the constituent monomers of eumelanin to show the interconversion between these species. We are in the process of extending these studies to understand the effect of nearby molecules on these pathways,” said Ghosh. “Because we have to remember that in vivo the eumelanin is probably in the presence of many more bio-molecules and still retains all its attractive features.”

With improved knowledge about the structure–function relationship in eumelanin, there are already quite a few efforts toward engineering eumelanin-based materials with desired properties, such as semiconductors and supercapacitors.

“There is need for experiment and computation to work in tandem to elucidate all the processes in eumelanin.”

Written by: Debashree Ghosh, WIRES author

Reference: Debashree Ghosh, Computational Aspects Towards Understanding the Photoprocesses in Eumelanin, WIRES Computational Molecular Science (2020). DOI: 10.1002/wcms.1505