Abstract
The objective of this study is to enhance the inhibition of ice growth in the aqueous solution of a polypeptide, which is inspired by winter flounder antifreeze protein. We carried out measurements on unidirectional freezing of the polypeptide solution. The thickness of the solution was 0.02 mm, and the concentration of polypeptide was varied from 0 to 2 mg/mL. We captured successive microscopic images of ice/solution interfaces, and measured the interface velocity from the locations of tips of the pectinate interface in the images. We also simultaneously measured the temperature by using a small thermocouple. The ice/solution interface temperature was defined by the temperature at the tips. It was found that the interface temperature was decreased with an increasing concentration of polypeptide. To try varying the activity of the polypeptide, we preheated the polypeptide solution and cooled it before carrying out the measurements. Preheating for 1–5 hours was found to cause a further decrease in the interface temperature. Furthermore, wider regions of solution and ice with inclined interfaces in the pectinate interface structure were observed, compared with the case where the solution was not preheated. Thus, the ice growth inhibition was enhanced by this preheating. To investigate the reason for this enhancement, we measured the conformation and aggregates of polypeptide in the solution. We also measured the local concentration of polypeptide. It was found that the polypeptide aggregates became larger as a result of preheating, although the polypeptide conformation was unchanged. These large aggregates caused both adsorption to the interface and the wide regions of supercooled solution in the pectinate interface structure.
Highlights
The inhibition of ice growth is an important issue in various fields, such as the maintenance of the quality of food texture in food preservation [1, 2]; the storage of cells, tissues and organs in PLOS ONE | DOI:10.1371/journal.pone.0154782 May 6, 2016Ice Growth Inhibition in Antifreeze Polypeptide Solution hospitals [1, 3]; and cryosurgery [4]
We experimentally investigate the following two factors for the polypeptide: the ice morphology and interface temperature in the unidirectional freezing of polypeptide solution (related to factor (1) mentioned in the third paragraph of this section), and whether or not the morphology and temperature change as a result of preheating the polypeptide solution (related to factors (2) and (3))
We have carried out experiments on the gradual unidirectional freezing of the dilute solutions of polypeptide in a narrow gap between two cover glasses
Summary
The inhibition of ice growth is an important issue in various fields, such as the maintenance of the quality of food texture in food preservation [1, 2]; the storage of cells, tissues and organs in PLOS ONE | DOI:10.1371/journal.pone.0154782 May 6, 2016Ice Growth Inhibition in Antifreeze Polypeptide Solution hospitals [1, 3]; and cryosurgery [4]. It should be noted that these freezing and melting points were defined to be respectively the temperatures at which a small seed crystal started to grow and shrink in the solution in an osmometer [7, 8] Some of these properties of AF(G)P solutions can be explained by the following hypotheses concerning the interaction between AF(G)P molecules and water molecules: (i) AF(G)P is adsorbed onto the specific facets of the ice crystals in a manner in which some hydrophilic residues on one or two sides of the AF(G)P are bound to these facets by the hydrogen bond, and (ii) some hydrophobic residues on the other sides of the AF(G)P form hydrophobic hydration shells. The adding of AF(G)P to water is promising for ice growth inhibition
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