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How Do Antifreeze Agents Work?

Preventing ice crystals from forming in undesirable locations is a constant challenge. We add salt to icy roads, and we put antifreeze (a mixture of ethylene glycol and water) in our cars' radiators to prevent them from freezing. The cells of some organisms produce antifreeze proteins to prevent the lethal formation of ice crystals. All of these processes work because of a chemistry concept known as "freezing-point depression," i.e., the lowering of a freezing point.


The extent of freezing-point depression is based on two factors: (1) The number of dissolved particles present (a feature known as a "colligative property") and (2) The nature of the dissolved particle. Exactly how this process works -- even including how water freezes -- is not well understood.

Researchers at New York University are trying to shed light on freezing-point depression, specifically which properties of antifreeze agents cause the biggest drop in freezing point and the greatest reduction in the growth of ice crystals. They used short organic polymers called peptoids, which are similar in structure to peptides (short proteins). (See diagram.) 
peptoids.jpg

The N is "nitrogen" and O is "oxygen," while the "R" stands for some generic organic side chain. The molecule is shown in brackets to indicate that the structure can repeat over and over again. By changing the R group and/or the number of repeating units, the scientists could systematically analyze how this affected the freezing point of water and the growth rate of ice crystals.

They analyzed three different R groups: methyl (-CH3), ethyl methyl ether (-CH2CH2-O-CH3) and ethyl alcohol (-CH2CH2OH). For controls, they used glycerol and a short peptide made from repeating units of serine, a type of amino acid similar in structure to the peptoid they made using ethyl alcohol.

They found that a peptoid with 3 repeating units bearing the ethyl alcohol side chain was the most effective experimental molecule because it both lowered the freezing point of water and greatly slowed the growth rate of ice. Further, they found that this had less to do with colligative effects (i.e., the number of molecules present), and more to do with the inherent nature of the molecule itself. In other words, its specific structure was key to these abilities.

Bizarrely, if the molecule had 4 or 5 repeating units, the freezing point of water was still lowered, but the growth rate of ice was increased over that of water alone. However, if there were 6 repeating units, once again, the growth rate of ice was decreased.

This research demonstrates just how complex the seemingly simple process of water freezing actually is. Additionally, the authors hope that they have laid the foundation for further investigations into discovering the most effective antifreeze molecules.

Diagram: Peptoids via CEM

Source: Mia L. Huang, David Ehre, Qi Jiang, Chunhua Hu, Kent Kirshenbaum, and Michael D. Ward. "Biomimetic peptoid oligomers as dual-action antifreeze agents." PNAS. November 19, 2012. doi: 10.1073/pnas.1212826109

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