FRANKFURT. Many biologically important molecules change shape when stimulated by UV radiation. Although this property can also be found in some drugs, it is not yet well understood. Using an innovative technique, an international team involving researchers from Goethe University Frankfurt, the European XFEL in Schenefeld and the Deutschen Elektronen-Synchrotron DESY in Hamburg has elucidated this ultra-fast process, and made it visible in slow motion, with the help of X-ray light. The method opens up exciting new ways of analyzing many other molecules.

“We investigated the molecule 2-thiouracil, which belongs to a group of pharmaceutically active substances based on certain DNA building blocks, the nucleobases,” says the study’s last author Markus Gühr, the head of DESY’s free-electron laser FLASH and Professor of Chemistry at University of Hamburg. 2-thiouracil and its chemically related active substances have a sulfur atom, which gives the molecules its unusual, medically relevant properties. “Another special feature is that these molecules become dangerously reactive when exposed to UV radiation.” Studies indicate an increased risk of skin cancer due to this effect.

To better understand what happens during such processes, the research team used an already well-established method, bringing it to a new level by applying the technical possibilities available today. “Coulomb explosion imaging involves irradiating a molecule with intense X-ray pulses, which knock out electrons,” explains Till Jahnke, Professor of Experimental Atomic and Molecular Physics at Goethe University and the study’s first author. “Thereby, the molecule charges up positively and thus becomes unstable, so that it is torn apart within fractions of a second.” By tracking the direction in which the various fragments of the molecule – the atoms – fly apart, it is possible to derive information about the molecule’s structure.

To date, Coulomb explosion imaging had only yielded useful results for very simple molecules. Using an experimental setup specially developed at Goethe University, the research team now combined this technique with the world's most powerful X-ray laser, European XFEL using the SQS (“Small Quantum Systems”) scientific instrument of EuXFEL. “This experiment is a technical innovation in many ways and it constitutes an important expansion of the experimental possibilities available at the SQS instrument. For the first time ever, it is now possible to use these imaging techniques on a biologically and medically relevant molecule, and not just for fundamental physics research,” says Michael Meyer, head of the SQS instrument, about the successful experiment.

European XFEL’s enormously powerful X-ray pulses made it possible to fragment this molecule, and thereby to conduct an analysis of its structure. The researchers sent the molecules into the X-ray laser beam using a fine gas nozzle, which means that only single, isolated molecules are irradiated at a time. An additional UV pulse, irradiated shortly before the X-ray pulse, was used to excite the molecules.

“By varying the time interval between the two pulses, it becomes possible to obtain something like a slow motion movie of these processes, which take place at an amazing speed within 100-1000 femtoseconds, that is less than a millionth of a millionth of a second” explains Jahnke. At the end of the process, a sophisticated detector registered the impact points and times of the various atoms of 2-thiouracil.

The experiment revealed two important findings, the first of which concerns 2-thiouracil: UV radiation causes this otherwise flat molecule to bend, which in turn results in the protrusion of the sulfur atom. This state is stable for a relatively long time; it ensures that the molecule becomes very reactive and might cause skin cancer, for instance. “This is also a significant difference to ordinary nucleobases, which are structurally very similar but do not have a sulfur atom,” says Gühr. “Instead, they have a mechanism for dealing with UV radiation and ultimately converting it into harmless heat via various excitation and oscillation states.” In the case of 2-thiouracil, the sulfur atom prevents such a conversion.

“The second finding is related to the experimental technique itself,” says Jahnke. “As we have seen, we don't need to track down all the atoms by the detector to reconstruct the molecule and its structural changes. All we needed in this case was to measure the sulfur and oxygen atoms as well as the four hydrogen nuclei, and we could ignore the six carbon atoms.” This finding will significantly simplify measurements in future investigations on even more complex molecules, and clearly illustrates the vast possibilities of this innovative method.