How antifreeze proteins make ice crystals grow
International research team confirms twofold ability of molecules
Bacteria, plants, insects, or even fish use antifreeze proteins to protect themselves from the cold. The proteins block the growth of ice crystals. In a new study, a German–Israeli research team has confirmed that these proteins also possess an unusual second property: at low temperatures, they can promote rather than inhibit the growth of ice crystals. The study brought together researchers from Bielefeld University, the Hebrew University of Jerusalem, and the Weizmann Institute of Science in Rehovot (Israel). It is being published today (07.03.2019) in the ‘Journal of Physical Chemistry Letters’.
‘There are many other organisms in contrast, that can benefit from making water turn into ice,’ says Koop. This is the case with, for example, bacteria that trigger the formation of ice. They secrete proteins on which crystal embryos can form, or nucleate from the cold liquid water, and thereafter grow into large ice crystals. Bacteria can use this, for example, to split open the skin of a tomato.
Until now, science has viewed ice-promoting and ice-inhibiting proteins as two different types of protein. That is also indicated by their different sizes: ice-inhibiting proteins are made up of small molecules; ice-promoting proteins, of large long molecules.
However, the new experiments show that, ‘an antifreeze molecule cannot just inhibit the growth of ice, it can also trigger its growth,’ says Koop.
The scientists have tested two naturally occurring antifreeze proteins: the protein of the larvae of the mealworm beetle and the protein of an artic fish, the ocean pout. They observed the effect of the proteins on thin microfluidic chips, developed at the Weizmann Institute, that are permeated with microscopically small channels with droplet traps. They took pure distilled water and added a set concentration of the specific protein. Then they injected this protein solution into the chip. In the chip, minute drops of water were collected in the droplet traps. Then they placed the chips in a temperature-controlled cooling chamber that cooled them down to minus 40 degrees.
Some of the experiments for the study were carried out at Bielefeld University. Complementary freezing experiments and the chips used to study the water as well as the protein solutions came from the Weizmann Institute of Science in Rehovot. The antifreeze proteins of the larvae of the mealworm beetle and the artic fish were produced at the Hebrew University of Jerusalem at the Rehovot campus. Ice inhibition of the same protein solutions was also demonstrated there.
Ice-inhibiting and ice-promoting proteins are not just common in nature. Nowadays, they are also used as technical aids. For example, antifreeze proteins in varnish can help protect the varnished surfaces from frost. The proteins can also be added to ice cream to help keep it creamy. Ice-forming proteins are used in, for example, ski resorts so that artificial snow can already be produced at a temperature of minus 3 degrees without having to wait for temperatures to drop further.
Lukas Eickhoff, Katharina Dreischmeier, Assaf Zipori, Vera Sirotinskaya, Chen Adar, Naama Reicher Ido Braslavsky, Yinon Rudich, Thomas Koop: Contrasting Behavior of Antifreeze Proteins: Ice Growth Inhibitors and Ice Nucleation Promoters. Journal of Physical Chemistry Letters. http://doi.org/10.1021/acs.jpclett.8b03719, published in the printed edition on 7 March 2019.
Prof. Dr Thomas Koop, Bielefeld University
Faculty of Chemistry
Telephone: +49 521 106-6135