Every second, countless electrical charges move through the human body. These microscopic signals are essential for life. They drive communication between cells, enable energy production, and support metabolism. None of this would work without the careful and controlled movement of charges across cell membranes and within cells. In many ways, charge transport acts as a fundamental control system for biology.
Phosphoric acid (H3PO4) and related compounds are found almost everywhere in living systems. They are key components of DNA and RNA, part of cell membranes, and central to ATP, the molecule that stores and transfers energy in cells. These compounds play a major role in moving positive charges, also known as protons. Beyond biology, phosphoric acid is widely used in technologies such as batteries and fuel cells because of its exceptional ability to conduct protons.
How Protons Travel Through Molecules
Protons move through phosphate-containing materials in a unique way. Instead of traveling freely, they hop from one molecule to another. Hydrogen bonds act as pathways that guide this movement. This process, known as “proton-shuttling,” allows charges to move very quickly.
While scientists have long understood that proton-shuttling occurs, the exact molecular details have remained unclear. To investigate this, researchers from the Department of Molecular Physics at the Fritz Haber Institute, along with collaborators from Leipzig and the United States, focused on a key molecular structure involved in this process. Their goal was to identify how the earliest steps of proton transfer actually take place.
Studying Proton Transport at Extreme Cold
Previous research suggested that a specific negatively charged molecule could initiate the proton-shuttling process. This molecule, known as the deprotonated dimer H3PO4·H2PO4–, became the focus of the study.
To examine it in detail, scientists created the molecule in the lab and cooled it to extremely low temperatures. By placing it inside a helium nanodroplet, they reduced its temperature to just 0.37 degrees above absolute zero. At this temperature, unwanted disturbances are almost completely eliminated. This allowed the researchers to analyze its structure with high precision using infrared spectroscopy.
The experimental results were combined with quantum chemical calculations, which help predict how molecules are arranged and how they behave. Together, these approaches provided a clearer picture of the molecule than either method alone.
A Single Structure Emerges
The findings revealed an unexpected result. Theoretical models had predicted that the molecule could exist in two equally likely structures. However, the experimental data showed only one stable configuration.
This structure is relatively rigid and features three hydrogen bonds connected through a shared oxygen atom. It also presents high barriers that limit how easily protons can move within it. Similar bonding patterns have been observed in other phosphoric acid clusters, suggesting that this arrangement may be a common structural feature.
These results highlight an important point. Even advanced theoretical models can miss key details, making experimental verification essential for understanding molecular structures.
Why This Discovery Matters
This research helps explain the molecular basis of phosphoric acid’s remarkable proton conductivity, often described as “Nature’s proton highway.” By identifying a single, well-defined structure for the key anionic dimer H3PO4·H2PO4–, scientists now have a better understanding of how protons move through these systems.
The findings also provide a valuable reference point for improving quantum chemical models of phosphate-based molecules. In addition, they could guide the development of new materials with enhanced proton conductivity, which are important for technologies like fuel cells. At the same time, the work deepens our understanding of how proton transfer operates in biological systems.
Key highlights
- What they studied: Researchers focused on a pair of phosphoric acid molecules known as an ionic dimer. This small but powerful system plays a major role in moving positive charges inside living organisms and is also widely used in technologies like fuel cells. The goal was to understand what makes it so remarkably efficient at transporting charge.
- How they studied it: To capture an ultra-clear view of the molecule, the team cooled it to an extreme temperature of just 0.37 Kelvin. At this near absolute zero condition, they used infrared spectroscopy along with quantum chemical calculations to map its structure with exceptional precision.
- What they discovered: Instead of finding two possible structures as predicted by theory, the experiments revealed only one stable form. This structure features a specific hydrogen bonding arrangement that closely matches patterns seen in other phosphoric acid systems, suggesting a common structural design.
- Why it matters: The findings help explain the molecular foundation of Nature’s proton highway, the process that allows phosphoric acid to conduct protons so efficiently. This deeper understanding could guide the development of better energy materials and improve our knowledge of how charge transfer works in living systems.