For the first time, scientists have captured how complex molecules change shape when exposed to UV light—an ultra-fast process previously impossible to observe.

An international team of researchers used European XFEL, one of the world’s most powerful X-rays, to slow down this process. Focused on 2-thiouracil a molecule related to DNA building blocks known as nucleobases the study revealed structural shifts that could explain how certain drugs work and why some molecules become dangerously reactive after UV exposure.

The research’s lead author, Markus Gühr, said that 2-thiouracil contains a sulfur atom, giving the molecules unusual, medically relevant properties. However, this same sulfur atom also makes the molecule highly reactive on UV exposure—an effect linked to an increased risk of skin cancer.

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