Molecular structure of a hyperactive antifreeze protein adsorbed to ice

Antifreeze proteins (AFPs) are a unique class of proteins that bind to ice crystal surfaces and arrest their growth. The working mechanism ofAFPs is not well understood because, as of yet, it was not possible to perform molecular-scale studies of AFPs adsorbed to the surface of ice.Here, we study the structural properties of an AFP from the insectRhagium mordax(RmAFP) adsorbed to ice with surface specific heterodyne-detected vibrational sum-frequency generation spectroscopy and molecular dynamic simulations. We find thatRmAFP, unlike other proteins,retains its hydrating water molecules upon adsorption to the ice surface. This hydration water has an orientation and hydrogen-bond structuredifferent from the ice surface, thereby inhibiting the insertion of water layers in between the protein and the ice surface.