Fish Antifreeze Proteins and the Creep of Polycrystalline Ice

Creep polycrystalline ice samples antifreeze glycopeptides re-accelerating or tertiary samples. Previous work grain-boundary migration consistent with this, as nucleation and growth of curves containing (AFGPs) creep that has shown obtained from the low concentrations of do not show the is found in pure ice that AFGPs inhibit in ice and the present results are tertiary creep is associated with new ice grains. IT has been reported (Knight and others, 1984, 1988) that the antifreeze glycopeptides (AFGPs) which protect Antarctic fish from freezing also inhibit recrystallization and grainboundary migration in ice, even at concentrations as low as 10-7 g g -1 (c. 10-10 M). Recrystallization is involved in the creep of polycrystalline ice and the process of creep may therefore be influenced by the presence of AFGPs. Single ice crystals deform easily by slip on the basal (0001) plane; a polycrystalline aggregate cannot deform using only this easy-glide mechanism but needs a further process such as grain-boundary migration or recrystallization. Thus, unlike single crystals, polycrystalline ice initially has a creep rate which decreases with time as deformation of those grains oriented for slip under the applied stress is inhibited by the effect of other grains oriented so that the shear stress on the slip plane is small. This is the explanation given for the initial decelerating or transient component of the typical creep curve of polycrystalline ice. It is followed by a secondary, or steady creep attributed to some recovery mechanism and at larger strains, by an accelerating or tertiary creep associated with the dynamic recrystallization of the ice. This is the explanation given (Glen , 1955) for the typical creep curve of polycrystalline ice. Thus, if AFGPs inhibit recrystallization, they should also suppress the tertiary creep and perhaps slow down the earlier stages if significant grain-boundary migration is involved in them. This short note reports the first tests to investigate this phenomenon. The proteins used were a mixture of AFGPs 1-5 isolated from Antarctic fish by A.L. DeVries. Cylindrical specimens of randomly oriented polycrystalline ice with and without the AFGPs were prepared by first loosely filling moulds with frost particles from the cold-room cooling tubes. The moulds were then filled with ice-cold water (with or without the additive) and then frozen at c. -15°C. The AFGP solutions used had a concentration of c. 10-7 g g-l. The creep-testing machine used was similar to that used by Homer and Glen (1978). The stress was supplied by weights and the strain was measured using a displacement transducer. The creep tests were all performed at -1.5°C. Three stress levels were used: 570, 860, and 1210 kPa. The resultant creep curves are plotted in Figure I in the form of strain-rate plotted logarithmically against strain. In this plot the transient primary creep is represented by the descending curve on the left of the plot. Continuing steady-state creep gives a horizontal line, while tertiary creep gives a rising curve on the right. In no case did a specimen contammg the antifreeze protein show tertiary creep. In all pure ice samples, a minimum creep rate was reached at about the same strain (2.5-3%) and was followed by a f1smg, tertiary, creep rate. For a given stress and strain, the strain-rates of the samples containing AFGPs were significantly less even during primary creep. Since it is known (Glen, 1955; Gow and Sheehy, 1987) that pure ice recrystallizes both during deformation and after deformation has ceased, and since this recrystallization