Multiple hydration layers in cubic insulin crystals.

Cubic insulin crystals contain approximately 30-A-diameter channels filled with aqueous solvent, providing a useful system in which to analyze hydration structure at a variety of distances from protein surfaces. Beginning with an atomic model for the protein and ordered water molecules, the density distribution in the solvent volume of the phasing model was iteratively refined to improve the fit of calculated structure factors with x-ray diffraction data. The free R value, which compares calculated structure factors with a subset of observed structure factors deliberately omitted from the refinement, was used to provide an objective confirmation of the effectiveness of the refinement procedure. Electron density maps of the solvent, computed using the solvent-refined phases and complete low-resolution diffraction data, reveal multiple hydration layers around the protein.