Chemists Synthesize New Hypoallergenic Sunscreen from Alkaloid Homodimer

Most of the people in this country are aware of the need to wear sunscreen. Not everyone may pay attention to this need, but most of us realize that failure to wear sunscreen before exposure to sunlight can result in darkening of the skin as well as potential skin lesions, even cancer. As an organic chemist, I’ve always had a somewhat different viewpoint on the subject, as part of my training over my fifteen year career has been to synthesize compounds that absorb light. Knowing which molecular fragments aid in light absorption and designing atomic arrangements which can maximize or minimize that absorption is central to the idea of conjugated organics, which is my specialty. Sunscreens use conjugated organics to provide a measure of protection for our skin.

Light from the sun is a mixture of colors. Put more precisely, light is a mixture of wavelengths. Red light has a relatively long wavelength, and as you progress across the rainbow (ROYGBIV) the wavelength gets shorter. As you approach the violet region and even beyond (into the ultraviolet), the wavelength gets very short. It’s a principle of chemistry that as the wavelength of light decreases, it’s energy increases. As a result, ultraviolet light can potentially be damaging; it has so much energy that it can be absorbed by skin cells. This is followed by chemical reactions between pyrimidine compounds to form dimers which then lead to skin cancer. Sunscreens offer protection in the form of a barrier of absorbing compounds that is smeared onto the skin surface. As UV light penetrates the chemical layer, it is absorbed by compounds in the sunscreen and therefore the light energy never damages the underlying skin.

The problem with this approach is that the absorption of light by the sunscreen is followed by chemical reactions, similar to the absorption of light by human skin. As the artificial sunscreen undergoes molecular rearrangement it can form chemical byproducts that are extremely irritating. Many sunscreen users are sensitive to these reaction products and so even though the sunscreen lotion itself may not be an irritant, using the sunscreen for it’s intended purpose results in rashes and damage to the underlying skin. This provides negative incentive for people to use sunscreen, with the result that more people are exposed to damaging UV light. They trade an immediate rash for an increased chance of cancer in the future, which is not a very attractive trade. As a chemist who has designed conjugated organics similar to sunscreen components for fifteen years, I’ve always been intrigued by the possibilities of a synthetic sunscreen that wouldn’t form irritating decomposition products after absorbing UV light. While I’ve never made a breakthrough in the field myself, I’ve tried to keep current, and I recently read an article from the American Chemical Society journal Organic Letters which gives me hope.

Researchers from Sweden report a new synthetic route that leads to the compound “scytonemin” in high yield. The reaction sequence is tandem, meaning that unlike normal organic preparations which occur one step at a time, multiple reactions occur within the same flask. Most organic chemists strive for simple, one step reactions so that they have time to isolate and purify the products before proceeding onwards. Failure to isolate after each major transformation means that it becomes increasingly difficult to purify the final product – “trash in” becomes “trash out”. With this new sequence, the authors report that a sequence of cascading steps happens all in the same reaction flask. The final product (an indolone homodimer) is a heavily conjugated organic molecule. As a result, it can absorb light in the UV range. Importantly, it doesn’t produce irritating decomposition products after absorbing the UV light – the excess energy is dissipated as heat, as there doesn’t seem to be a readily accessible photochemical route towards decomposition products. This product (named “scytonemin”) shows great promise as a hypoallergenic sunscreen. Hopefully, it will convince a higher percentage of people to wear sunscreen on sunny days, as there will be less skin irritation. This will lead to a lower incidence of skin cancer, which can only be a good thing.

The source of this article can be found at:

Ekebergh, A. “Oxidative coupling as a biomimetic approach to the synthesis of scytonemin”. Org. Lett., 2011, 13, 4458-4461.

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