Interaction among the twelve-residue segment of antifreeze protein type I, or its mutants, water and a hexagonal ice crystal

We have carried out a molecular dynamics analysis on a mixture of supercooled water, a hexagonal ice crystal and segments of winter flounder antifreeze protein. The segment consists of nine alanine residues, two threonine residues and one asparagine residue. Mutant segments, in which the threonine residues are replaced with valine residues, or serine residues, are also used. It is found that the threonine residue near the asparagine residue of the original segment is located in the vicinity of the prism face of the ice crystal. This is due to the hydrogen bond between the hydrophilic sites of these residues and water molecules, and the hydrogen bond between these water molecules and the water molecules on the ice surface. The valine and serine residues in the mutant segments do not approach the prism face of the ice crystal compared with the threonine residue near the asparagine residue. The motion of five segments, closely located side by side, is not remarkable. This is because of the gathering of water molecules caused by hydrophobic hydration, not only around alanine residues but also around the methyl sites of threonine residues.