Prawn exoskeleton – a rich source of efficient sunscreen filters

Aniket Majumdar

Commercially available sunscreen products usually contain chemically active ingredients to absorb ultraviolet (UV) radiation. But these chemicals are not eco-friendly and are inefficient UV absorbers. Besides, long-term use of such products can even lead to hazardous side-effects such as allergy and hypersensitivity of the skin. As a result, the focus is now shifting towards natural sunscreens, since they do not have such adverse effects. Notable examples include mycosporine-like amino acids with a central aromatic ring such as tryptophan, phenylalanine, and tyrosine, which are found in marine-based mucus and can absorb the entire range of UV radiation.

A good source of these amino acids is the prawn exoskeleton, which is the external shell that covers the entire body of the prawn. Previous skincare studies have already shown that proteins extracted from prawn shells enhance anti-ageing effects in the skin because of their anti-microbial and skin regeneration properties. In their latest work, researchers from the Indian Institute of Science (IISc), led by Bikramjit Basu, Professor at the Materials Research Centre, in collaboration with those from the National Institute of Technology Karnataka (NITK) Surathkal, have conducted a detailed study on the prawn exoskeleton to understand the nature of the proteins. Their results showed that amino acids present in prawn shells absorb the entire range (200-400 nm) of UV radiation and hence serve as excellent sunscreen filters.

The team collected shells of the Arabian Sea prawn (Fenneropenaeus indicus) from the market, cleaned them with hot water and dried them overnight. The chemical composition of the compounds present in the shell was examined using X-ray spectroscopic techniques, which revealed that the prawn shells are primarily composed of calcite, α-chitin and proteins. Thermal analysis of the prawn shell further confirmed that it contains 6 percent moisture, 42 percent calcite and the remaining 52 percent organic matter, which includes α-chitin and proteins. Microscopic imaging of prawn shells showed regular spherical calcites embedded into the α-chitin protein structure. The team also carried out X-ray photoelectron spectroscopic (XPS) measurements on the prawn shells to identify the organic compounds, and hence the amino acids present in them. UV-visible spectroscopy results confirmed the presence of mycosporine-like amino acids with a central aromatic ring (namely tryptophan, tyrosine, phenylalanine) in prawn exoskeleton.

The team concluded that extraction of UV-absorbing proteins from prawn shells can have potential applications as permissible eco-friendly sunscreen filters in polymer coatings, cosmetics, textiles, food, pharmaceuticals and biodegradable packaging products.

Figure: (Top Panel) Image of an Arabian Sea prawn and its exoskeleton/shell, discarded as food waste. (Bottom Panel) UV absorbance spectra of prawn shell, showing wide absorption of UV irradiation in the range of 200–400 nm

Reference:

Komalakrushna Hadagalli, Rahul Kumar, Saumen Mandal, Bikramjit Basu, Structural, compositional and spectral investigation of prawn exoskeleton nanocomposite: UV protection from mycosporine-like amino acids,  materials Chemistry and Physics,249 (2020) 123002. https://doi.org/10.1016/j.matchemphys.2020.123002

Lab Website:  http://www.mrc.iisc.ac.in/bikramjit-basu/
https://mme.nitk.ac.in/professor/saumen-mandal