New experiment reveals how ultrafast conversion of urea progresses – key to life’s origins found

How did life begin on Earth? Specialists have long been fascinated by this question and have come up with various theories over the years. One hypothesis is that life originated in warm small ponds that are thought to have existed on Earth four billion years ago. The water in these ponds probably contained urea molecules; they were exposed to the sun’s ultraviolet radiation, which at the time would have reached the earth’s surface largely unhindered. This high-energy radiation was able to convert urea into reaction products that then formed the biomolecules that later served as the building blocks of life – or so the idea goes.

Ludger Inhester, DESY

Two adjacent urea molecules exchange protons in aqueous solution.

This theory of “warm little ponds” was one of the things that led a team of researchers from Hamburg, Zurich and Geneva to design and conduct a new experiment. A team led by Hans Jakob Werner of the Swiss Federal Institute of Technology ETH Zurich and Jean-Pierre Wolff of the University of Geneva used a special X-ray source to probe the first, extremely fast steps of light-induced urea conversion. Scientists report in the journal Nature.

For the experiment at the University of Geneva, the team fired pulses of a laser beam through a jet of highly concentrated urea solution. The laser pulses ionized some of the urea molecules, knocking off one electron from each. Immediately afterwards, the scientists sent an ultra-short pulse of soft X-rays behind them. They acted as a probe, revealing in great detail how urea responds to the ejection of an electron. The group then repeated the experiment several times, systematically varying the time interval between the ionizing laser and the X-ray pulses.

As a result, it was possible to accurately reconstruct the sequence of events – down to a few femtoseconds (quadriillionths of a second). The interpretation of the resulting spectra turned out to be particularly difficult. “This required detailed computer simulations, which we developed here at DESY over many years,” explains DESY physicist Ludger Inhester, who works at the Center for Free Electron Laser Science, a joint venture between DESY, the University of Hamburg and the Max Planck Society. .

Scientists used this setting to develop the initial steps in the conversion of urea. When a urea molecule is ionized, it loses an electron and becomes positively charged. It would like nothing better than to get rid of this positive charge, and this becomes possible when another, non-ionized urea molecule is in close proximity. “The first molecule then pushes the proton, which is the hydrogen nucleus, onto a neutral molecule,” says Inhester, who also works at Hamburg University’s Center of Excellence CUI: Advanced Imaging of Matter. “This proton transfer produces a urea radical along with a positively charged urea ion.” Both are chemically reactive and could have led to the formation of RNA molecules billions of years ago, an essential building block of early life.

The experiment conducted by the scientists not only demonstrates for the first time the rapid “proton transaction” between two urea molecules, but also reveals its duration – the entire process takes only a few hundred femtoseconds.

“What is new about our experiment is that we were able to observe extremely fast processes in a molecule that exists in an aqueous environment,” explains Inhester. “Previous experiments have only looked at such reactions in the gas phase.” The behavior of molecules suspended in a liquid such as water is of particular importance for many questions, especially when it comes to biological processes. Experiments in this type of environment are challenging not only when it comes to making measurements, but also in terms of the calculations required to interpret the measured data.

In the future, the new technique could also reveal more details about what happens when ionizing radiation hits tissue to cause radiation damage, the kind of research the new Center for Molecular Water Science (CMWS) is currently building internationally. cooperation in the DESY campus. The researchers are also toying with the idea of ​​conducting similar experiments at a much larger X-ray source, Europe’s XFEL in Hamburg. More than three kilometers long, this X-ray laser, in which DESY plays a central role, produces the world’s most powerful X-ray pulses. “This would allow us to study this proton transfer from different angles,” says Inhester, who hopes it will reveal more information about this very important process.

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