Protein hydration: Investigation of globular protein crystal structures.

The positions of water molecules have been analyzed in high quality protein X-ray crystal structures. About 70% of these water molecules are in contact with protein atoms at the protein surface and constitute the first hydration layer. About 20% of them are close to the first hydration layer but are not in contact with protein atoms and constitute the second hydration layer. The rest of the water molecules are either buried in the protein core or close to hetero-atoms (inorganic ions and small organic molecules). Upper layers (third, fourth, etc.) are not observed in the dataset of protein crystal structures examined here. Water molecules of both layers are not, in general, surrounded by a tetrahedral arrangement of atoms, as it should be expected on the basis of the electronic structure of water. Usually there are less than four atoms around water molecules and even when there are four atoms, the stereochemistry is often distorted. Water molecules are more mobile than protein atoms, more in the second hydration layer than in the first.