Infrared water messengers

Water molecules continuously form short-lived clusters that can be rapidly protonated in the liquid state. Now, computer simulations revealed how protonated water clusters interact with nearby messenger molecules, which are required to measure their geometrical structures and the chemical properties by IR spectroscopy.

We think we know water, but we really don't. Its anomalous behaviour is well known regarding its expansion when it freezes, its almost universal ability to solvate, its heat capacity and many other phenomena. We explain these anomalous with recourse to hydrogen bonding. To some extent those apparently weak and fleeting bonds do explain water, but then they too are an anomaly, weak yet strong, transient but simultaneously persistent. Now, researchers in Germany have thrown yet another molecular curve ball in their studies of clusters of water molecules.

Water molecules are forever forming short-lived networks called clusters in the liquid state. The individual molecules in such clusters are restricted in their movements, which provides another perspective on the anomalous behaviour of water. These clusters can bond to protons and have important implications for protein folding and function because such clusters can provide active functional groups in proteins. For instance, networks of internal water molecules are thought to provide proton transfer pathways in many enzymatic and photosynthetic reactions, the researchers say.

Protonated water clusters are an important model system for investigating proton hydration in aqueous solution. The simplest, and smallest, of such clusters, if it can be called that, is the hydronium cation, which is just a single water molecule with an associated proton. Next in the series is the Zundel ion, two water molecules sharing a single proton.

A team of scientists, including Gerald Mathias of Ludwig-Maximilians-Universitaet (LMU) in Munich and Dominik Marx of the Ruhr-Universität Bochum, hoped to understand how these protonated water clusters interact with their environment such that they can be reliably detected by IR spectroscopy. Unfortunately, the IR spectrum of water is too complicated to single out individual interactions and so they turned to messenger molecules in the gas phase to help them tease apart the spectral bands.

In order to measure the vibrational spectra of water clusters in the gas phase, one needs to use small molecules or one of the noble gases, such as neon or argon, as messengers. These stick to the clusters and detach due to the infra-red induced vibrations and thereby report the spectrum as 'messengers'. "These spectra may be modified by the messenger molecules, so that it is important to understand their interactions with the clusters," explains Mathias.

The researchers have now shown that the messengers have unexpected effects on the absorption bands in the spectrum of even the simple hydronium cation. However, by simulating the dynamics of the complexes formed between protonated water clusters and messenger molecules, the team was able to extract the actual spectrum for the cation from the raw data obtained in the presence of the messengers.

"The results obtained with the Zundel ion which is constantly changing its form were even more interesting," adds Mathias. "We were able to show that this structure exists in two different forms. In one, the messenger is tightly bound to the cation, in the other it is only loosely associated with it and orbits around it. In the latter case, we observed that the infrared absorption spectrum was practically identical to that calculated for the unbound Zundel cation - so that these spectra are not influenced by the presence of the messenger molecules."

This result provides a better understanding of experimental messenger spectroscopy, which is used for chemical analyses of the components of Earth's atmosphere and to detect molecules in interstellar space. The researchers hope that it will also provide new insights into the structure and function of protonated water clusters in proteins. Moreover, the results that could emerge from IR studies using messenger molecules will improve scientists' ability to interpret spectroscopic data considerably, according to Mathias.

"That is an important step towards a better understanding of the function of protonated water clusters in proteins," he explains. "Because water molecules can be found virtually everywhere, the new findings will also have a positive impact on studies devoted to atmospheric chemistry and interstellar chemistry."