AFPs inhibit growth

Antifreeze proteins (AFPs) are ice-binding proteins found in some organisms (such as fish, insects, plants and soil bacteria) that live at the temperature of their surroundings and encounter freezing conditions. AFPs help organisms to survive below 0°C by inhibiting ice growth. AFPs are structurally diverse, each is radically different from the others in its primary,... 

Carlo simulation of ice-crystal growth to study the mechanism of inhibition of AFPs on the surface of the THF hydrate and propane hydrate. They found that most of the octahedron surfaces of the THF hydrate were covered with AFP molecules, which could reduce the growth rate of the THF hydrate only allowing plate growth perpendicular to that surface. It is thus necessary to look for other experimental evidences to clarify the common features of AFPs on the inhibition of the clathrate hydrate formation. 

The inhibition effects of type-III AFP and trehalose, two cryoprotecting materials produced in animals, on type-I CO2 clathrate-hydrates were examined. For comparison with the results of a previous study in which the lateral growth rates of COi-hydrate film were dependent on temperature, pressure and NaCl concentration, the solution droplet was observed in a high pressure vessel filled with CO2. Type-III AFP was found to increase the induction period and to reduce the lateral growth rate of C02-hydrate films. It worked well at low concentrations, indicating that AFP works as a kinetic inhibitor. It was also indicated that AFP would weaken the memory effect of C02-hydrate formation. Trehalose had similar inhibition effects on both the induction period and the lateral growth rate, but it had little apparent concentration-dependence on them. Since trehalose also causes the equilibrium conditions of the CO2 hydrate to shift to lower temperatures, it works not only as a thermodynamic inhibitor but also as a kinetic inhibitor, especially as an anti-agglomerant. 

Although AF(G)Ps and LDHIs are distinct, they both inhibit the growth of crystals. Neither AFGP nor PVP are reported to significantly affect ice nucleation, and similary, we have shown that AFPs and PVP did not affect homogeneous nucleation of THF hydrate. It is not known if these two types of inhibitors can adsorb to other hydrophilic surfaces, however silica is an ubiquitous impurity and common to both these systems. Thus, it is of interest to determine the effects of these inhibitors on heterogeneous nucleation of ice/clathrate hydrate. 

One has to distinguish our studies here from the more popular studies on the adsorption of AFPs or LDHIs on ice or gas hydrate since the former focuses on the effect of AFP/LDHI on heterogeneous nucleation of ice/gas hydrate and the later focuses on the effect on the effects on the ice/hydrate growth. Thus, the studies represented in this study deals with impurity surfaces but not ice/hydrate surface. The authors think that this is an important aspect for the understanding of good ice/hydrate inhibition since the nucleation process is heterogeneous and must involve impurity surfaces. 

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