Water protects sensitive molecules from radiation damage

X-ray experiment reveals extremely rapid exchange of energy

radiation damages on pyrimidine

While a double positive charge induced by radiation causes pyrimidine to break quickly (top), the molecule can transfer a positive charge to water in an aqueous environment (bottom) and remain intact (Credit: University of Kassel, Andreas Hans).

Water can protect small organic molecules from being damaged by X-rays. This is revealed by an experiment at PETRA III led by the University of Kassel. Instead of being torn apart by the electrical charge induced by the X-rays, the molecules are able to transfer some of that charge to the water and so remain intact, according to a detailed model produced by a team from the University of Heidelberg based on the measurements. The researchers have presented their findings in the scientific journal The Journal of Physical Chemistry Letters.

When X-rays strike a molecule, they often knock an electron out of the inner region of the molecule. This leaves behind a hole which is usually filled by an electron from the outer region of the molecule. So much energy is released in this process that another electron is knocked out of the molecule (known as the Auger effect). This leaves the molecule with two positive charges because it is now missing two electrons. Many sensitive molecules cannot survive this charge build-up and break apart due to the electrical repulsion (in what is known as a Coulomb explosion).

“However, if the molecules are dissolved in water, they often don’t rupture,” explains Andreas Hans from the University of Kassel, who led the study. “Various conjectures exist as to why this might be.” Hans’s team was able to show that the protective effect of the water sets in very quickly, preventing the rupture by removing one of the two electrical charges. Working on the P04 beamline, the researchers studied the organic molecule pyrimidine (C4H4N2) which is one of the building blocks of the nucleobases in the genome and is often used in research as a model system for fundamental biochemical analyses. They shone X-rays at the pyrimidine dissolved in water and observed the molecular fragments that were produced.

As expected, some of the molecules did not break apart. The theoretical analysis of the measured data by Nikolai Kryzhevoi’s group at the University of Heidelberg shows how this is possible: In water, the pyrimidine molecules form a weak bond with small clusters of just a few water molecules. When an electron is knocked out of the pyrimidine, the resulting hole is filled by an electron from the outer region of the molecule, as before. However, the energy released can be transferred to the water cluster which then loses an electron as a result. “In the end, the pyrimidine and the water cluster each have a single positive electrical charge. This causes them to separate from each other, but the charge is not strong enough to tear the pyrimidine molecule apart,” says co-author Florian Trinter from DESY and the Fritz-Haber-Institute in Berlin.

“It’s currently thought that about a third of radiation damage suffered by biological organisms stems from the radiation energy being directly transferred to the molecules,” Hans summarises. “About two thirds are due to indirect effects, such as the production of free radicals in water or free electrons. This study gives us a better understanding of the role played by water in radiation damage.”


(from DESY News)



Suppression of X‐ray-Induced Radiation Damage to Biomolecules in Aqueous Environments by Immediate Intermolecular Decay of Inner-Shell Vacancies; Andreas Hans, Philipp Schmidt, Catmarna Küstner-Wetekam, Florian Trinter, Sascha Deinert, Dana Bloß, Johannes H. Viehmann, Rebecca Schaf, Miriam Gerstel, Clara M. Saak, Jens Buck, Stephan Klumpp, Gregor Hartmann, Lorenz S. Cederbaum, Nikolai V. Kryzhevoi, and André Knie; The Journal of Physical Chemistry Letters, 2021; DOI: 10.1021/acs.jpclett.1c01879