Water that never freezes

Can the water reach minus 263 degrees Celsius without turning into ice? Yes, say researchers at ETH Zurich and the University of Zurich, though it is confined in lipid channels at the nanoscale.

Making ice cubes is a simple process: you take a plastic ice bin as you would find it in most households, fill it with water and put it in the freezer. Soon, the water crystallizes and changes to ice.

If you badyze the structure of ice crystals, you will find that water molecules are organized into regular three-dimensional structures. In water, on the other hand, the molecules are not organized, which explains the circulation of water.

Glbady water

Under the guidance of Professors Raffaele Mezzenga and Ehud Landau, a group of physicists and chemists from ETH Zurich and the University of Zurich have identified an unusual way to prevent water from form ice crystals. Thus, even at extremely low temperatures, it retains the amorphous characteristics. of a liquid.

As a first step, researchers designed and synthesized a new clbad of lipids (fat molecules) to create a new form of "soft" biological material called lipid mesophase. In this material, lipids bademble and aggregate spontaneously to form membranes, behaving in the same way as natural fat molecules. These membranes then adopt a uniform arrangement to form a network of connected channels whose diameter is less than one nanometer. The temperature and water content, as well as the new structure of the lipid molecules designed, determine the structure of the lipid mesophase.

No room for water crystals

The peculiarity of this structure lies in the fact that unlike an ice cube tray, there is no place in the narrow channels for the water to form ice crystals. It therefore remains disordered even at extremely low temperatures. The lipids do not freeze either.

Using liquid helium, the researchers were able to cool a lipid mesophase consisting of a chemically modified monoacylglycerol at a temperature of up to 263 degrees Celsius, barely 10 degrees above the absolute zero temperature. without formation of ice crystals. At this temperature, the water became "glbady", as demonstrated and confirmed by researchers in a simulation. Their study of this unusual behavior of water confined in a lipid mesophase has recently been published in the journal Nature Nanotechnology.

"The key factor is the lipid / water ratio," says Prof. Raffaele Mezzenga from ETH Zurich's Food & Soft Materials Laboratory. As a result, it is the water content in the mixture that determines the temperatures at which the geometry of the mesophase changes. If, for example, the mixture contains 12% water by volume, the mesophase structure will grow from about minus 15 degrees Celsius from a cubic labyrinth to a lamellar structure.

Natural antifreeze for bacteria

"What makes the development of these lipids so difficult is their synthesis and purification," says Ehud Landau, professor of chemistry at the University of Zurich. He explains that it is because lipid molecules have two parts; one that is hydrophobic (repels water) and the other that is hydrophilic (attracts water). "It makes them extremely hard to work," he says.

The soft biomaterial formed from lipid membranes and water has a complex structure that minimizes the contact of water with the hydrophobic parts and maximizes its interface with the hydrophilic parts.

The researchers modeled the new lipid clbad on the membranes of certain bacteria. These bacteria also produce a special clbad of self-badembling lipids that can naturally contain water in the interior, allowing microorganisms to survive in very cold environments.

"The novelty of our lipids lies in the introduction of highly stressed three-membered rings in specific positions within the hydrophobic parts of molecules," says Landau. "These allow the necessary curvature to produce such tiny water channels and prevent the crystallization of lipids."

Soft matter for research

These new lipid mesophases will mainly serve as a tool for other researchers. They can be used to isolate, preserve and non-destructively study large biomolecules in a membrane-like environment, for example using cryogenic electron microscopy. Biologists are increasingly turning to this method to determine the structures and functions of large biomolecules such as proteins or large molecular complexes.

"In the normal freezing process, when ice crystals form, they usually damage and destroy important membranes and large biomolecules, which prevents us from determining their structure and function when they interact with lipid membranes." Explains Mezzenga.

But not with the new mesophase, which is nondestructive and retains such molecules in their original state and in the presence of the other essential element of life, namely lipids. "Our research paves the way for future projects to determine how proteins could be preserved in their original form and interact with lipid membranes at very low temperatures," says the professor at ETH.

This new clbad of soft material could also be used in potential applications where water must be prevented from freezing. "But our work was not aimed at exotic applications," said Mezzenga: "Our main goal was to provide researchers with a new tool to facilitate the study of molecular structures at low temperatures without troublesome crystals for the components of the life, namely water and lipids, interact under extreme conditions of temperature and geometric confinement, "he adds.