Artificial antifreeze polypeptides: α-helical peptides with KAAK motifs have antifreeze and ice crystal morphology modifying properties

Antifreeze polypeptides from fish are generally thought to inhibit ice crystal growth by specific adsorption onto ice surfaces and preventing addition of water molecules to the ice lattice. Recent studies have suggested that this adsorption results from hydrogen bonding through the side chains of polar amino acids as well as hydrophobic interactions between the non-polar domains on the ice-binding side of antifreeze polypeptides and the clathrate-like surfaces of ice. In order to better understand the activity of one of the antifreeze polypeptide families, namely the α-helical type I antifreeze polypeptides, four α-helical peptides having sequences not directly analogous to those of known antifreeze polypeptides and containing only positively charged and non-polar side chains were synthesized. Two peptides with regularly spaced lysine residues, GAA KAA KAAAAAAA KAA KAAAAAAA KAA KAAGGY-NH 2 and GAA LKAA KAAAAAA LKAA KAAAAAA LKAA KAAGGY-NH 2, showed antifreeze activity, albeit weaker than seen in natural antifreeze polypeptides, by the criteria of freezing point depression (thermal hysteresis) and ice crystal modification to a hexagonal trapezohedron. Peptides with irregular spacing of Lys residues were completely inactive. Up to now, lysine residues have not been generally associated with antifreeze activity, though they have been implicated in some antifreeze polypeptides. This work also shows that lysine residues in themselves, when properly positioned on an α-helical polyalanine scaffold, have all the requisite properties needed for such an activity.