Scientists catch fast-moving hydrogen atoms in action, using ultrafast electron diffraction

Scientists have successfully captured the rapid movements of hydrogen atoms, which play pivotal roles in numerous biological and chemical reactions. Researchers from the SLAC National Accelerator Laboratory and Stanford University employed ultrafast electron diffraction (UED) to record the dynamic behavior of hydrogen atoms within ammonia molecules, a feat previously only theorized but never realized.

Their groundbreaking results, published in Physical Review Letters, utilized high-energy Megaelectronvolt (MeV) electrons to study hydrogen atoms and proton transfers—events where a hydrogen atom’s singular proton relocates from one molecule to another. These proton transfers are fundamental in various biological and chemical reactions, including enzyme-catalyzed processes and mitochondrial functions. However, these transfers occur incredibly swiftly, within femtoseconds (one quadrillionth of a second), making them challenging to observe.

One potential approach involves directing X-rays at a molecule and analyzing the scattered X-rays to monitor the molecule’s evolving structure. However, X-rays primarily interact with electrons, not atomic nuclei, limiting their sensitivity.

To tackle this challenge, a team led by SLAC scientist Thomas Wolf utilized MeV-UED, SLAC’s ultrafast electron diffraction camera. They employed gas-phase ammonia, composed of three hydrogen atoms bonded to a nitrogen atom, as their test subject. The researchers initiated the experiment by subjecting ammonia to ultraviolet light, causing the dissociation of one hydrogen-nitrogen bond. Subsequently, they directed a beam of electrons through the molecule and captured the resulting diffracted electrons.

Remarkably, this experiment not only detected signals from the separation of hydrogen from the nitrogen nucleus but also revealed alterations in the molecule’s structure. Furthermore, the scattered electrons traveled in distinct angles, allowing for the differentiation of these signals.

Thomas Wolf explained the significance, stating, “Having something that’s sensitive to the electrons and something that’s sensitive to the nuclei in the same experiment is extremely useful. 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.”

This breakthrough promises to shed light on the elusive mechanism of proton transfer, addressing numerous questions in the fields of chemistry and biology. Understanding the behavior of protons could have far-reaching implications in structural biology, where conventional methods like X-ray crystallography and cryo-electron microscopy struggle to observe protons.

In the future, the research group plans to conduct similar experiments using X-rays at SLAC’s Linac Coherent Light Source (LCLS) to compare the results. They also aim to enhance the intensity of the electron beam and improve the experiment’s time resolution to discern individual stages of proton dissociation over time.

Source: SLAC National Accelerator Laboratory

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