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Breakthrough in Molecule Analysis Using X-Ray Light

An international team of researchers has made strides in understanding the behavior of molecules when exposed to ultraviolet (UV) light. This research, involving scholars from Goethe University Frankfurt, the European XFEL in Schenefeld, and the Deutsches Elektronen-Synchrotron (DESY) in Hamburg, has revealed the rapid transformations of biologically significant molecules, highlighting a technique that captures these changes in slow motion using X-ray light.

“We focused on 2-thiouracil, a molecule that is chemically related to key DNA building blocks known as nucleobases,” stated Markus Gühr, the study’s principal author and head of DESY’s free-electron laser FLASH, as well as a Professor of Chemistry at the University of Hamburg. The unique structural feature of 2-thiouracil is its sulfur atom, which contributes to its medicinal properties. However, this molecule also exhibits increased reactivity under UV light exposure, leading to concerns regarding its potential link to heightened skin cancer risk.

To gain deeper insights into the reactions of 2-thiouracil, the researchers improved upon an established technique known as Coulomb explosion imaging. “This method utilizes intense X-ray pulses to knock electrons from the molecule,” explained Till Jahnke, Professor of Experimental Atomic and Molecular Physics at Goethe University and the study’s first author. “This process results in the molecule acquiring a positive charge, rendering it unstable and causing it to fragment almost instantaneously.” By analyzing the trajectory of the escaping molecular fragments, the team could infer details about the molecule’s internal structure.

Coulomb explosion imaging had previously shown promise primarily with simpler molecules. However, with a specially designed experimental setup at Goethe University, this study successfully integrated this method with the European XFEL’s cutting-edge X-ray laser, employing the SQS (“Small Quantum Systems”) scientific instrument. “This experiment represents a significant technical advancement, expanding the experimental capabilities at the SQS instrument,” noted Michael Meyer, head of the SQS, regarding the groundbreaking experiment.

The powerful X-ray pulses facilitated the disintegration of 2-thiouracil, allowing researchers to examine the arrangement of its constituent atoms. The researchers introduced the molecules to the X-ray laser beam using a fine gas nozzle to ensure that individual, isolated molecules were analyzed. A UV pulse was applied shortly before the X-ray pulse to excite the molecules and initiate their transformation.

“By manipulating the time difference between the two pulses, we were able to develop a type of slow-motion film capturing these rapid processes that occur within mere femtoseconds—trillionths of a second,” Jahnke explained. Ultimately, a sophisticated detection setup recorded the arrival points and timings of the various atoms within 2-thiouracil.

The study produced two notable outcomes. Firstly, it was found that UV exposure causes the otherwise planar molecule of 2-thiouracil to bend, leading to an outward protrusion of the sulfur atom. This newly formed structure maintains stability for a significant duration, enhancing the molecule’s propensity for reactivity, ultimately raising the stakes for conditions like skin cancer. “This varies considerably from typical nucleobases, which, despite similar structures, possess a sulfur atom that hinders efficient UV absorption and heat conversion,” Gühr explained.

Secondly, the research uncovered insights regarding the experimental approach itself. “Our findings indicate that it isn’t necessary to track every atom in the structure to understand the molecule and its changes. Measuring only the sulfur and oxygen atoms along with four hydrogen nuclei sufficed, allowing us to bypass the need to account for the six carbon atoms,” Jahnke highlighted. This simplification will pave the way for future research involving more intricate molecules and demonstrates the significant potential of this innovative technique in molecular analysis.

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