ANTIFREEZE GLYCOPROTEINS - INFLUENCE OF POLYMER LENGTH AND ICE CRYSTAL HABIT ON ACTIVITY

The effect of the ice crystalline habit and the length of the polymer on the ability of the antifreeze glycoproteins (AFGP) from polar fish [Pagothenia borchgrevinki] to depress the freezing temperature of water was investigated. The low-MW components of the glycoproteins, AFGP 6-8, are inactive when a solution of such a sample is nucleated at -6.degree. C. A solution of large AFGP (1-4) is fully functional under the same conditions. The low-MW components differ from the high-MW components in that they contain some Pro replacing the Ala in the Ala-Ala-Thr .cntdot. disaccharide polymer unit. In the present experiments, antifreeze activity was examined in the presence of 2 different forms of ice crystal growth habits, and homodimers of AFGP 6 and 8 were prepared to investigate the function of polymer length and type on antifreeze activity at different degrees of supercooling. The ice crystal growth habit and the introduction of Pro into the polymer unit may be responsible for the loss of activity at deep supercooling (-6.degree. C) of AFGP 6-8. The loss in the ability of AFGP to depress the freezing temperature of water at deep supercooling is not solely due to polymer length, as carbodiimide-linked dimers of AFGP 6 do not function under these freezing conditions. A model of antifreezing action based on Langmuirian adsorption of AFGP on the ice surface and direct competition between water and AFGP molecules for the incorporation sites in the ice crystal lattice is presented.