Ultrafast, atom-resolved probes of aqueous hydrogen bond dynamics at SQS
Scientists studied the processes when water particles are exposed to X-ray light using the SQS instrument to investigate pairs of water molecules called dimers. This is the first step in understanding more complicated processes, such as the mechanisms behind radiation damage in biological systems.
Pairs of water molecules exposed to X-rays
One of the ways scientists are learning about water is by studying the elementary processes that occur when systems of water particles are exposed to X-ray light. This happens in biology, for example when exposed to X-rays in medical examinations, but also in more extreme conditions, high up in the atmosphere and in outer space. In particular, scientists conducting an experiment at European XFEL’s Small Quantum Systems (SQS) instrument are looking at pairs of water molecules, or ‘dimers’.
A water dimer is simply two molecules of water bound together by a ‘hydrogen bond’. Water consists of two hydrogen atoms and one oxygen atom. Within a water molecule, the hydrogen atoms become faintly positively charged, and the oxygen atom becomes faintly negatively charged. This means that the positively-charged hydrogen in one water molecule can become attracted to a negatively charged oxygen atom in another water molecule, forming a bond known as a hydrogen bond.
Using bright X-rays to observe how charges move
“A single molecule cannot do very much when exposed to X-ray light,” says Prof Olle Björneholm, the principal investigator studying water dimers at European XFEL. “It gets ionised and then breaks apart. But with water dimers, or systems of multiple dimers, there are many more interesting things that can happen, and these are some of the processes we want to study.”
In systems of water dimers, water molecules influence one another, causing hydrogen bonds between the molecules to grow and wane in strength. This ‘rearrangement of charge’ is what scientists are aiming to study, to understand how water as a liquid behaves under X-ray light. To do this, they are using the European XFEL’s bright X-ray flashes to watch how charges move between water dimers. The team uses a first X-ray pulse to strip a pair of electrons from one of the oxygen molecules in a dimer. They then use a second X-ray pulse to fully strip the electrons from the molecules. By then detecting all of the involved components—the water ions, the electrons—the scientists can understand how the charges in the molecule behaved in the time between the two pulses.
“We can trace if electrons and protons jumped between the two water molecules in the dimer on extremely short time scales, just a few femtoseconds after the first pulse,” continues Björneholm. A femtosecond is to one second, roughly what one second is to the lifetime of the universe. “European XFEL’s bright X-ray pulses are what gives us the ability to do this. It’s a very rare capability.”
Learning about the behaviour of water when exposed to X-rays enables scientists to understand the fundamentals of water. This is the first step in understanding more complicated processes, such as the mechanisms behind radiation damage in biological systems, which is also due to the ionisation of water particles.