Mystery of natural sunscreen solved

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Compounds found in coral transform light energy from the sun into heat energy without damaging plants

Spanish scientists have established how natural products protect plants from sun damage. The compounds could be used as active ingredients in sunscreens.

Using computational techniques on palythine - a compound found in coral - as a model compound, Diego Sampedro at the University of La Rioja, Logroño, investigated what happens to the molecule after it absorbs UV light.

Sampedro found that when UV light was shone on palythine, the molecule rapidly dissipated the light energy into heat energy without forming reactive, harmful, photoproducts. He looked at the mechanism in detail on both the protonated and neutral forms of palythine, as scientists were unsure which form was active in the coral. He found that both forms underwent a bond rotation to transform light into heat energy, but the protonated form was responsible for the main absorption of the radiation.

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Only bond rotation is involved in dissipating UV energy as heat in palythine

Palythine is a mycosporine-like amino acid (MAA), compounds present in organisms such as fungi, bacteria, algae and marine organisms. MAAs are thought to have a number of biological activities, including photoprotection. Although evidence for the latter has been circumstantial, they are being considered for use in commercial sunscreens. In order to aid their commercialisation, the mechanism of MAAs’ photoprotective effect needs to be understood.

’MAAs may be commercially important in the near future as they present the features needed for a very efficient photoprotector: strong absorption over a useful range of wavelengths, efficient conversion of light energy into heat, photostability and lack of reactive intermediates or by-products,’ explains Sampedro. ’Some of the components of commercially available sunscreens do not meet all of these criteria.’

Mike Robb of Imperial College, London, an expert in computational chemistry, praises the timeliness of the study. ’MAAs are already being studied as industrial photostabilisers. Understanding the details of the mechanism should help in the design of such species,’ he says.

Sampedro is aware that there is much scope for further work and hopes to tune the absorption wavelengths of these molecules and to investigate the effects of structural modification on photoprotective ability.

Yuandi Li