The lipids form net-like branched membranes which enclose water (light blue). (Graphic: Livia Salvati Manni / ETH Zurich)
What’s so special about this structure is that – unlike in an ice-cube tray – there is no room in the narrow channels for water to form ice crystals, so it remains disordered even at extreme sub-zero temperatures. The lipids don’t freeze either.
Using liquid helium, the researchers were able to cool a lipidic mesophase consisting of a chemically modified monoacylglycerol to a temperature as low as minus 263 degrees Celsius, which is a mere 10 degrees above the absolute zero temperature, and still no ice crystals formed. At this temperature, the water became “glassy”, as the researchers were able to demonstrate and confirm in a simulation. Their study of this unusual behaviour of water when confined within a lipidic mesophase was recently published in the journal
Nature Nanotechnology.
“The key factor is the ratio of lipids to water,” explains Professor Raffaele Mezzenga from the Laboratory of Food & Soft Materials at ETH Zurich. Accordingly, 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 percent water by volume, the structure of the mesophase will transition at about minus 15 degrees Celsius from a cubic labyrinth to a lamellar structure.
Natural antifreeze for bacteria
“What makes developing these lipids so tricky is their synthesis and purification,” says Ehud Landau, Professor of Chemistry at the University of Zurich. He explains that this is because lipid molecules have two parts; one that is hydrophobic (repels water) and one that is hydrophilic (attracts water). “This makes them extremely difficult to work with,” he says.
The soft biomaterial formed from the lipid membranes and water has a complex structure that minimises the water’s contact with the hydrophobic parts and maximises its interface with the hydrophilic parts.
The researchers modelled the new class of lipids on membranes of certain bacteria. These bacteria also produce a special class of self-assembling lipids that can naturally confine water in their interior, enabling the microorganisms to survive in very cold environments.
“The novelty of our lipids is the introduction of highly strained three-membered rings into specific positions within the hydrophobic parts of the molecules”, says Landau. “These enable the necessary curvature to produce such tiny water channels and prevent lipids to crystallize.”
