What is it about?

Wearing sunscreen everyday decreases your risk of skin cancers and helps your skin maintain a healthier look. But how does a sunscreen work? The key ingredients are molecules known as UV-filters which absorb UV light. Octinoxate is one such molecules. It has excellent filter properties, but its action is degraded by light. In fact, under sunlight, the active form of the molecule ("trans") is converted to a less effective form, named "cis". Understanding why this process occurs can help to improve the efficacy of sunscreen lotion and creams. In this work, we studied with accurate calculations the two forms of the octinoxate molecules. We found that, in contrast to what was previously thought, the less active "cis" form is slightly more stable than the "trans" molecule. This finding explains why the UV filter loses efficacy, and may suggest possible ways to limit the degradation processes.

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Why is it important?

All previous studies predicted trans octinoxate (the commercially valuable form) to be more stable than cis octinoxate. By using accurate computational methods, here we show that the octinoxate is actually sligthly more stable than the Trans form. The greater stability of the cis form is due to the folding of the long alkyl chain, leading to a more compact form of the molecule. This long chain makes this molecule special, because it allows the filter to be easily dispersed in the lotions we commonly use. The striking difference of shape between nearly flat trans octinoxate and folded cis octinoxate suggests that this change of shape might help to dissipate the energy accumulated upon light absorption. This effect may be beneficial to the sunscreen action.


Our findings imply that the alkyl chain might be more than a spectator in the photochemical process, hence its effects should be carefully considered by researchers and, in perspective, also by sunscreen producers. As a further step to understand the peculiarity of this molecule, the cis- and trans- forms of octinoxate could be also investigated via molecular dynamics simulations to better capture the differences in their complex behaviour.

Gloria Tabacchi
university of insubria

Read the Original

This page is a summary of: A post-HF approach to the sunscreen octyl methoxycinnamate, The Journal of Chemical Physics, March 2021, American Institute of Physics,
DOI: 10.1063/5.0046118.
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