The structure of water in protein crystals

Protein crystals grown from aqueous media invariably contain substantial amounts of the crystallizing solution, and the structure of the protein molecules and surrounding solvent can be determined from X-ray diffraction data. Electron density maps of crystalline proteins based on such data show networks of water peaks in the solvent space within approximately 6A of the protein surface. In general, the peaks are highest for water molecules within hydrogen bonding distance of acceptor or donor atoms on the protein. With increasing distance from the protein, the water peaks decrease in height, and beyond approximately 6A they merge into the structureless solvent continuum, typical of most crystalline proteins. A few water sites are often found within the protein molecule. These water molecules appear to be trapped within the protein and presumably are more tightly bound than water external to the protein.