Understanding nature’s proton highway

Key aspects:

• The studied system: The team studied ionic dimers of phosphoric acid, a small molecule capable of transporting positive charges in living organisms with exceptional efficiency, and thus widely used in energy technologies such as fuel cells. They tried to find out exactly how and why this molecule is so efficient at moving charges.

• The experiment: The researchers cool the phosphoric acid dimers down to just 0.37 Kelvin to be able to precisely determine their structure using IR radiation and quantum chemical calculations.

• The finding: The experimental data clearly show that only one structural species of the phosphoric acid dimer is formed, rather than the two predicted by calculations. The observed structure has a similar hydrogen bonding motif to that of other phosphoric acid-containing clusters, suggesting that a specific structural motif may be general for phosphoric acid interactions.

• Why that matters: The study sheds light on the molecular basis of Nature’s proton highway: phosphoric acid’s renowned proton conductivity. The results pave the way for designing new proton-conducting materials and deepen our understanding of proton transfer in biological systems.

How Tiny Electrical Signals Control Life

At every moment, thousands of charges move through our bodies. These tiny electrical signals are fundamental to life: signaling, energy conversion or metabolic processes all depend on the precise, regulated movement of charges across biological membranes and within cells. Charge transport is a central control mechanism.

Phosporic acid (H3PO4) and its derivatives are ubiquitous in nature, for example, found as the main component of DNA and RNA, in cell membranes and as part of the universal energy carrier ATP. These molecules have proven to be particularly important in the transport of positive charges in living systems. Phosphoric acid particularly is also of great technical importance, and widely used in certain batteries and in fuel cells, where a unique property of phosphoric acid is exploited─its exceptionally high proton conductivity.

Protons, carriers of a positive charge, travel through phosphate-containing compounds like passengers travelling on a bus: They "jump" from molecule to molecule using hydrogen bonds as their routes. This mechanism, called "proton-shuttling", allows charges to be transferred very rapidly. Although it is generally known that this mechanism exists, fundamental questions remain unanswered. In their current study, researchers from the Department of Molecular Physics at the Fritz Haber Institute, together with their collaborators from Leipzig and the USA, aimed to determine the structure of a key phosphoric acid anionic complex. In doing so, they shed light on the elementary first steps of the fascinating charge transfer process.

A Cold Look at Hot Chemistry with Cryogenic Spectroscopy

From earlier studies it is already known that possibly a specific negatively charged species of phosphoric acid may be the starting point of the proton-shuttling cascade: the deprotonated dimer H3PO4·H2PO4-. To find out more about its role, the researchers produced this molecule in the laboratory and investigated it under cryogenic conditions. They placed the molecule inside a helium nanodroplet, which cools it down to just 0.37 degrees above absolute zero and investigated its structure using infrared radiation. The extreme cooling virtually eliminates interfering factors and thus enables a highly precise resolution of the molecular structure. The experimental structure analysis was supported by quantum chemical calculations, which allow for the prediction of the molecule’s structure and behavior.

The Invisible Network: Structure and Hydrogen Bonds Found

Interestingly, the experimental data showed only partial agreement with the theoretical prediction. While the calculations predicted two possible structures that should theoretically be equally likely to occur, the experimental data clearly revealed that the deprotonated dimer of phosphoric acid adopts a unique, stable structure. This structure is relatively rigid, with high barriers for proton transfer. It involves three hydrogen bonds and a shared acceptor oxygen atom. Other studies on phosphoric acid-containing clusters reported a similar coordination, suggesting that this hydrogen-bonding motif is likely common for such systems. This result underscores the limitations of theoretical predictions and highlights the necessity of experiments for accurate structure assignment.

Why It Matters

This work provides insight into the molecular origin of phosphoric acid’s extraordinary proton conductivity, “Nature’s proton highway”. The structure analysis reveals one unique structure of the key anionic dimer H₃PO₄·H₂PO₄- with a novel hydrogen-bonding motif that may be key to understanding proton transport in phosphoric acid-based systems. The study serves as a benchmark for quantum chemical methods in modeling phosphate-containing clusters, opening new pathways for designing more efficient proton-conducting materials and understanding biological proton transfer.

Source: Fritz Haber Institute of the Max Planck Society