Antifreeze glycoproteins from polar fish blood.

Abstract

Existing experimental evidence strongly suggests that the mechanism of activity of the Antarctic AFGP molecules is the inhibition of ice growth by competitive adsorption onto the growth sites of ice. The data further suggest the blocking of the formation of large critical nuclei for ice growth. Experiments showing that the longer polymers (AFGP 1-5) have different growth-prevention properties with different types of ice than the shorter polymers (AFGP 6-8) provide additional evidence that crystal size and habits are linked to function. Four main observations have been used in AFGP studies: (a) The ice crystal habit (size) affects the activity, (b) AFGP is on the surface of ice crystals, as shown by surface second harmonic generation (SSHG), (c) the presence of AFGP lowers the surface energy at the ice-solution interface, and (d) kinetic calculations of the inhibition of ice-crystal growth fit adsorption isotherms. In particular, AFGP 4 fits a Langmuirian adsorption curve. On the detailed mechanistic side, there is a need to quantify the competitive rates of water-molecule attachment to ice as well as the rates of adsorption-desorption of AFGP on the ice surface. Toward this end, experiments are currently being conducted to examine the differential growth rates of ice crystals freely growing into a solution of AFGP. Direct observation of the adsorption and desorption of AFGP on the ice surface is possible in principle using the SSHG technique. A nonperturbative probe method that can differentiate adsorbed AFGP from the solution phase molecules is needed. Finally, a mechanistic description of molecular function is never complete unless the detailed molecular binding to the surface is elucidated. This task has not been accomplished yet, and thus there is still a question as to which molecular group is actually adsorbed onto the surface and for how long. Theoretically, as we have seen, the departure from Langmuirian adsorption is pronounced for small AFGP molecules under certain conditions. More refinements of the ideas of intermolecular or intramolecular cooperative interaction of AFGP on the ice surface are needed.