Double bombardment reveals detailed dynamics in the disintegration of water molecules
With the European X-ray laser European XFEL, an international research team has gained new insights into the origin of radiation damage in biological tissue. For the first time, the study reveals in detail how water molecules break apart through high-energy radiation into potentially dangerous radicals and electrically charged ions, which can then trigger harmful reactions in the organism. The team around Maria Novella Piancastelli and Renaud Guillemin from the Sorbonne in Paris, Ludger Inhester from DESY and Till Jahnke from European XFEL in Schenefeld near Hamburg present their observations and analyzes in the journal "Physical Review X".
Since water occurs in every living organism, this so-called photolysis of water, i.e. the splitting of water molecules (H 2 O) by high-energy radiation, is often at the beginning of damage. "The chain of reactions that can be triggered in the body by high-energy radiation is still not fully understood," explains Inhester. "It is already very difficult to follow the formation of individual charged ions and reactive radicals in water that are formed after the absorption of high-energy radiation."
To investigate this sequence, the researchers bombarded water vapor with the flashes of the X-ray laser. A water molecule normally disintegrates through the absorption of a single high-energy X-ray photon. "Thanks to the particularly intense pulses of the X-ray laser, it was even possible to observe water molecules that absorb not just one, but two or more X-ray photons before the fragments flew apart," reports Inhester. This gives researchers a glimpse of what is going on in the molecule after the first absorption of an X-ray photon.
"The movement of the molecule between two absorptions leaves a clear fingerprint, which means that its fragments fly apart in a very specific, unmistakable way," says Piancastelli. “Through the precise analysis of this fingerprint and detailed simulations, we were able to draw conclusions about the ultra-fast dynamics of the water molecule after the absorption of the first X-ray photon.” The team measured the directions and speeds of flight of the fragments with a so-called reaction microscope. In this way it was possible to show the breakup of the water molecule into hydrogen and oxygen ions, which only lasted a few femtoseconds (quadrillionths of a second), as if in slow motion.
The disintegration of the water molecule is therefore much more complex than initially expected. The molecule begins to stretch and stretch before finally breaking apart. The two hydrogen atoms (H), which are normally bonded to the oxygen atom (O) at an angle of 104 degrees, can build up so far that after only ten femtoseconds they are approximately 180 degrees opposite each other. As a result, the oxygen atom is not so strongly thrown away when the molecule breaks, because the impulses of the two hydrogen nuclei flying away almost balance each other and the oxygen remains largely dormant in the middle. In an aqueous environment, this free oxygen radical can then more easily lead to further potentially harmful chemical reactions.
"In our work we succeeded for the first time in taking a closer look at the dynamics of a water molecule after absorption of high-energy radiation," says Inhester, who works at the Center for Free-Electron Laser Science (CFEL), a cooperation between DESY, the University of Hamburg and the Max Planck Society. “In particular, we were able to characterize the formation of the oxygen radical and the hydrogen ions as well as the timing of this process more precisely. This disintegration process of the water molecule is an important initial step for further reaction chains that ultimately lead to radiation damage. "
The analysis supplements the picture of the radiation effect on water. A previous study, in which members of the same team were involved, had investigated in detail the formation of so-called free radicals due to less high-energy radiation in water. The processes observed here have a similar dynamic as the secondary processes in the absorption of high-energy radiation that is now being investigated. The newly gained insights address elementary questions about reaction dynamics in water, for the further investigation of which the Center for Molecular Water Science (CMWS) is currently being set up with international partners at DESY.
The new experiments on individual water molecules were among the first to take place with a new so-called reaction microscope called COLTRIMS at the SQS experiment station of the European XFEL. "The results show that we can also investigate other solvents and molecules with a more complex structure, such as ethanol or cyclic compounds, which are of great interest in chemistry and other disciplines," explains Jahnke.
Researchers from the universities of Frankfurt am Main, Freiburg, Hamburg and Kassel as well as Gothenburg, Lund and Uppsala in Sweden and Turku in Finland as well as from the Fritz Haber Institute of the Max Planck Society and the Max Planck were involved in the current study -Institute of Nuclear Physics, from the Lawrence Berkeley National Laboratory and the Kansas State University in the USA, the National Research Council and the Technical University of Milan in Italy, the Sorbonne in Paris, European XFEL and DESY.