Researchers catch protons in the act of dissociation with ultrafast 'electron camera'

Scientists have observed the rapid motion of hydrogen atoms, which are essential for innumerable chemical and biological processes.

Ultrafast electron diffraction (UED) was employed by a group of researchers from Stanford University and the Department of Energy's SLAC National Accelerator Laboratory to capture the motion of hydrogen atoms inside ammonia molecules. Although some had hypothesized that they could use electron diffraction to follow hydrogen atoms, no one had yet completed the experiment successfully.

The findings, which are detailed in Physical Review Letters, take use of the advantages that come with using high-energy Megaelectronvolt (MeV) electrons to investigate hydrogen atoms and proton transfers—the movement of a single proton, which constitutes a hydrogen atom's nucleus, from one molecule to another.

It would be beneficial to understand precisely how its structure changes throughout the innumerable biological and chemical activities that are powered by proton transfers. Examples of these reactions include the production of enzymes, which aid in the catalysis of biochemical reactions, and proton pumps, which are vital to mitochondria, the cells' power plants. Proton transfers, however, occur extremely quickly—in a matter of femtoseconds, or one millionth of a billionth of a second. It's difficult to observe them in action.

One option is to target a molecule with X-rays and then utilize the dispersed X-rays to get information about the molecule's structure as it changes. Unfortunately, it's not the most sensitiv
e technique since X-rays only interact with electrons, not atomic nuclei.

MeV-UED, SLAC's ultrafast electron diffraction camera, was used by a team led by physicist Thomas Wolf to obtain the answers they needed. They made use of gas-phase ammonia, which is composed of one nitrogen atom and three hydrogen atoms. The scientists used UV light to disrupt one of the hydrogen-nitrogen bonds in ammonia. They then shot an electron beam across the material to collect the diffracted electrons.

Not only did they detect signals originating from the hydrogen's separation from the nitrogen nucleus, but they also detected the corresponding alteration in the molecule's structure. Additionally, scientists were able to distinguish between the two signals because the dispersed electrons blasted out at different angles.

"Having something that's sensitive to the electrons and something that's sensitive to the nuclei in the same experiment is extremely useful," Wolf stated. "If we can see what happens first when an atom dissociates—whether the nuclei or the electrons make the first move to separate—we can answer questions about how dissociation reactions happen."

With such knowledge, researchers might get closer to understanding the enigmatic proton transfer process, which could aid in the resolution of several chemical and biology-related queries. In structural biology, where "seeing" protons is a challenge for conventional techniques like cryo-electron microscopy and X-ray crystallography, understanding what protons are doing might be crucial.

To find out how different the results are, the team plans to repeat the experiment using X-rays at SLAC's Linac Coherent Light Source (LCLS) in the future. To really discern distinct stages of proton dissociation across time, they also intend to increase the electron beam's intensity and enhance the experiment's temporal resolution.