Thanks to a new technique for observing chemical reactions in liquids, UNIGE and ETH are shedding light on one of the hypotheses about the chemical origins of life.
Photoionization-induced proton transfer between two urea molecules in an aqueous urea solution.. © Ludger Inhester, DESY, Hamburg
What processes led to the appearance of life on our planet? Some scientists believe that urea played a key role. After being enriched in warm puddles, this small molecule is thought to have been exposed to cosmic rays to produce malonic acid, the source of RNA and DNA. To observe how a urea solution exposed to such rays reacts, a team from the University of Geneva (UNIGE) and ETH Zurich (ETH) developed a spectroscopic method for observing chemical reactions in liquids, with extremely high temporal resolution. These results, published in the journal Nature, shed new light on the appearance of life on Earth.
The first evidence of life on Earth dates back 3.8 billion years. However, there are many hypotheses as to how this first appearance of life came about. One of them suggests that urea was at the origin. This small molecule containing carbon and nitrogen is thought to have been enriched in warm puddles. As the water gradually evaporated, the concentration of urea increased in this primordial soup. Under the ionising effect of cosmic rays, this substance would then have produced malonic acid, which may have created the building blocks of RNA and DNA.
A team from UNIGE and ETH has developed a new X-ray spectroscopy method that enables this type of chemical reaction in liquids to be observed with extremely high temporal resolution. Thanks to this method, scientists can examine how molecules change in a few femtoseconds, i.e. within a few quadrillionths of a second. This technique builds on previous work carried out by the same team.
A zoom on the original molecules
To extend their spectroscopic observations to liquids, which is the natural environment of bio-chemical processes, the researchers had to design a device capable of producing a jet of liquid with a diameter of less than one millionth of a metre in a vacuum. This was essential because if the jet were any thicker, it would absorb some of the X-rays used to measure it. "This very thin liquid film is obtained by the collision of two liquid jets at a very precise angle", explains Jean-Pierre Wolf, full professor in the Department of Applied Physics in the Physics Section of the UNIGE Faculty of Science, in whose laboratories this work was carried out.
"Thanks to the extraordinary advancements in lab-based sources, which provides exceptional short X-ray pulses, we were able to observe the ultrafast dynamics in solvated urea", says Zhong Yin, then a researcher at ETH and one of the leading experimentalists of the work.
Thanks to this new technique, researchers at UNIGE and ETH have been able to study the first stage in the long series of chemical reactions that would have led to the appearance of life. In other words, the way in which a concentrated urea solution reacts when exposed to ionising radiation. "The molecules in a concentrated urea solution come together in pairs, called dimers. We discovered that ionising radiation causes a hydrogen atom within each of these dimers to move from one urea molecule to another,"" explains Hans Jakob Wörner, full professor in the Department of Chemistry and Applied BIosciences of ETH, who co-directed the study.
150 quadrillionths of a second reactions
During this stage, one of the two molecules in the dimer is transformed into a protonated urea molecule, and the other into a urea radical. The latter is so chemically reactive that it is highly likely to react with other molecules, forming malonic acid. The scientists were also able to show that this transfer of hydrogen atoms occurs extremely rapidly, in the space of around 150 femtoseconds, or 150 quadrillionths of a second.
"It's so fast that this reaction precedes all the other reactions that could theoretically take place. This explains why concentrated urea solutions produce urea radicals rather than hosting other reactions that would produce other molecules", says the ETH researcher. In the future, the research team would like to examine the subsequent steps that lead to the formation of malonic acid.
The experimental results obtained were analysed in collaboration with researchers from the University of Hamburg and CFEL, who carried out the calculations required to interpret the data. This new method opens up new prospects for studying the origins of life and, more broadly, any chemical reactions occurring in liquids. The most promising applications will involve the development of new medicines or new materials to capture solar energy more efficiently.
