Slowing the growth of ice with synthetic macromolecules: beyond antifreeze(glyco) proteins

Biological antifreezes are a relatively large and diverse class of proteins (and very recently expanded to include lipopolysaccharides) which are capable of interacting with ice crystals in such a manner as to influence and, under the correct conditions, to prevent their growth. These properties allow for the survival of organisms which are either continuously or sporadically exposed to subzero temperatures which would otherwise lead to cryo-injury/death. These proteins have been found in a range of organisms, including plants, bacteria, insects and fish, and the proteins themselves have a diverse range of chemical structures ranging from the highly conserved antifreeze glycoproteins (AFGPs) to the more diverse antifreeze proteins AFPs. Their unique abilities to non-colligatively decrease the freezing point of aqueous solutions, inhibit ice recrystallisation and induce dynamic ice shaping suggest they will find many applications from cell/tissue/organ cryostorage, frozen food preservatives, texture enhancers or even as cryosurgery adjuvants. However, these applications have been limited by a lack of available material and also underlying questions regarding their mode of activity. The aim of this review article is to highlight the potential of polymeric materials to act as synthetic mimics of antifreeze(glyco) proteins, as well as to summarise the current general challenges in designing compounds capable of mimicking AF(G)Ps. This will cover the basic properties and modes of action of AF(G)Ps along with the methods commonly used to evaluate their activity. This section is essential to specifically define the 'antifreeze' terminology in terms of these proteins' unique function and to distinguish them from conventional antifreezes. A detailed evaluation of the processes involved in AF(G)P activity is beyond the scope of this review, but the reader will be pointed towards relevant literature. This will then be placed in the context of modern polymer science, with a focus on the ability of synthetic polymers to display some type of specific antifreeze activity, which will be summarised. Finally, the potential applications of these materials will be highlighted and future avenues for their research and the challenges faced in achieving these goals suggested.