Water is one of the most common molecules found upon our planet. However, whilst it has a simple structure, it has some unique, and complex, physical properties which are embedded into every aspect of life on Earth. The hydrogen bonds between the water molecules are the key to the unique properties of water. This bonding gives rise to several different types of molecular structures.
The transport of water through biological membranes has always been a water interaction that has generated a lot of interest. Internal membrane proteins called aquaporins (AQPs) transport the water at high speed via single-file water wires. A previous observation that certain materials actually generate self-assembling water channels, similar to those found in nature, has opened the possibility of using materials with this property to engineer water channels for specific applications.
Researchers from Bangor University were investigating a material that was discovered serendipitously. It consists of water and 2,2,6,6-tetramethylpiperidine (TMP) in an exact 2:1 ratio. It was found that this material self-assembles into a 3D structure that exhibits molecular water channels.
To better understand the role of water and hydrogen bonding in assembling and stabilizing the structure, single-crystal neutron diffraction was performed at 100 and 10 K at the ISIS Facility. Data collected from this analysis revealed the positional disorder of the hydrogen atoms in the water molecules. It was observed that the layout of the TMP molecules creating a water channel was similar to other materials with artificial water channels but with a larger diameter.
As well as neutron diffraction, inelastic neutron scattering (INS) spectroscopy was also used to perform analysis on the material. The researchers compared the INS spectrum of TMP with that of the TMP:H2O and TMP:D2O complexes.
The results of the analysis on the TMP water adduct has revealed that the structure consists of quartets of TMP water molecules stacked along the ‘a’ crystallographic axis, forming channels filled with pseudo-cubic octameric water clusters that are stacked face-on-face. These clusters are quite unusual to find within crystalline structures, especially in this arrangement. The presence of these clusters in this new structure can help lead to a greater understanding and methods to improve the future design of artificial water pipes. It is anticipated that this is just the first example of similar adducts that may have played an important role in the evolution of life.