Rapid decay: Researchers have for the first time followed the ultrafast decay of water molecules through high-energy radiation. This radiolysis also occurs in our body and generates aggressive hydroxyl radicals that can damage cells and DNA. The images with an X-ray laser now revealed when the decisive step of this reaction takes place. At the same time, they provide valuable information about the course of this process, as the researchers report in the scientific journal Science.
Water is a very special substance. Because it shows one density anomaly, can form more than a dozen different ice shapes and is thanks to its dipole and the ability to form Hydrogen bonds a good solvent. In the liquid state, water also forms a highly complex mixture of molecular lumpsthat change their structure and arrangement in fractions of a second. At the same time, the self-dissociation for the fact that a small part of the H2O molecules constantly decay – into a hydroxyl ion (OH–) and a hydronium ion (H3O+).
Proton transfer from an ionized to a neutral water molecule. The blue, dumbbell-shaped cloud shows the orbital from which the electron was previously struck by radiation. © DESY / Caroline Arnold
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Radiation creates an aggressive radical
The proportion of hydroxyl ions in water is usually extremely low – and that’s a good thing. Because these OH ions are extremely reactive and can cause serious damage to tissues, cells and the DNA of living organisms. However, when high-energy radiation hits water or water-rich body fluids, this dramatically increases water dissociation.
“We are all exposed to ionizing radiation in everyday life – whether through X-rays, natural radioactivity or, for example, cosmic radiation on flights,” explains co-author Robin Santra from the German Electron Synchrotron (DESY) in Hamburg. “That is why what happens in radiolysis is of fundamental importance.” However, it is only partially known how this disintegration process works.
Decay in two steps
It seems clear that the radiation-related water decay takes place in two steps: First, the radiation knocks out an electron from the H2O molecule and ionizes it to H2O+, In the second step, this unstable ion releases a proton to a neighboring water molecule. This proton transfer then creates the extremely reactive hydroxyl radical (OH) and the hydronium ion (H3O+).
But this reaction is happening so quickly that researchers have rarely been able to observe these steps individually – until now. The team around Santra and first author Zhi-Heng Loh Argonne National Laboratory has now for the first time succeeded in following this fastest part of the radiolysis. To do this, they bombarded a water sample with ultrafast pulses from the X-ray laser LCLS at the US research center SLAC and used it at the same time to take snapshots of the molecular state.
Proton transfer after less than 50 femtoseconds
The images revealed: The transition state H2O+ stops only very briefly. “In just under 50 billionths of a second, the surrounding water molecules move towards the ionized molecule and turn its most reactive side towards it until one of them is close enough to grab one of the protons in a kind of handshake,” reports Santra. “It turns into hydronium and leaves a hydroxyl radical behind.”
Accordingly, proton transfer during radiolysis typically takes only 46 femtoseconds. “Until now, nobody knew the time frame for proton transfer – now we have measured it,” says Young. The measurement data now confirm the theoretical modeling of this crucial process in the formation of the hydroxyl radicals.
First crucial insight
This gave the scientists a first decisive insight into the extremely fast dynamics of water radiolysis. However, they are still only at the beginning of a deeper overall picture. “While 50 femtoseconds are short by most standards, there are many physical processes within these 50 femtoseconds that have not yet been resolved,” explains Loh.
The researchers hope that the lifespan of H2O will be higher in the future with analyzes that have a higher resolution+-Ions narrow down even more precisely and also to grasp its molecular properties in more detail. (Science, 2020; doi: 10.1126 / science.aaz4740)
Source: DOE / Argonne National Laboratory, German Electron Synchrotron DESY
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