When proteins are completely hydrated in crystals

In the crystalline state, protein surface patches that do not form crystal packing contacts are exposed to the solvent and one or more layers of hydration water molecules can be observed. It is well known that these water molecules cannot be observed at very low resolution, when the scarcity of experimental information precludes the observation of several parts of the protein molecule, like for example side-chains at the protein surface. On the contrary, more details are observable at high resolution. Here it is shown that it is necessary to reach a resolution of about 1.5–1.6 Å to observe a continuous hydration layer at the protein surface. This contrasts previous estimations, which were more tolerant and according to which a resolution of 2.5 Å was sufficient to describe at the atomic level the structure of the hydration layer. These results should prove useful in guiding a more rigorous selection of structural data to study protein hydration and in interpreting new crystal structures.