Several types of natural molecules interact specifically with ice crystals. Small antifreeze proteins (AFPs) adsorb to particular facets of ice crystals, thus inhibiting their growth, whereas larger ice-nucleating proteins (INPs) can trigger the formation of new ice crystals at temperatures much higher than the homogeneous ice nucleation temperature of pure water. It has been proposed that both types of proteins interact similarly with ice and that, in principle, they may be able to exhibit both functions. Here we investigated two naturally occurring antifreeze proteins, one from fish, type-III AFP, and one from beetles, TmAFP. We show that in addition to ice growth inhibition, both can also trigger ice nucleation above the homogeneous freezing temperature, providing unambiguous experimental proof for their contrasting behavior. Our analysis suggests that the predominant difference between AFPs and INPs is their molecular size, which is a very good predictor of their ice nucleation temperature.
Bibliographical noteFunding Information:
We thank C. Budke for the provision of original IRI data for type-III AFP, S. Ben Bassat for thermal hysteresis measurements of type-III AFP, H. P. Dette for helpful comments, and the anonymous referees for helpful comments and for pointing us to the statements on ice nucleation in the paper by Modig et al. We are also grateful for funding by the German Research Foundation (DFG) through the research unit FOR 1525 (INUIT) for T.K. (KO 2944/2-2) and a DFG Mercator Fellowship for Y.R. Y.R. also acknowledges support from the Estate of David Levinson, a research grant from the David and Fela Shapell Family Foundation INCPM Fund for Preclinical Studies and the DeBotton center for Marine Science, and I.B. acknowledges support from the Israel Science Foundation (ISF grant no. 930/16).
© 2019 American Chemical Society.