Crystal structure of an antifreeze polypeptide and its mechanistic implications.

The X-ray crystallographic structure of an antifreeze polypeptide from the fish winter flounder, has been determined at 2.5 A by an analysis of the Patterson function. This is the first report of a polypeptide of this size that is a single alpha-helix. A proposed mechanism of antifreeze binding to ice surfaces is given which requires: first, that the dipole moment from the helical structure dictates the preferential alignment of the peptide to the c-axis of ice nuclei; second, amphiphilicity of the helix; and third, torsional freedom of the side chains to facilitate hydrogen bonding to ice surfaces.