Quasi-Laue neutron-diffraction study of the water arrangement in crystals of triclinic hen egg-white lysozyme

Triclinic crystals of lysozyme, hydrogen–deuterium exchanged in deuterated solvent, have been studied using neutron quasi-Laue techniques and a newly developed cylinder image-plate detector. The wavelength range employed was from 2.7 to 3.5 Å, which gave 9426 significant reflections [F ≥ 2σ(F)] to a resolution limit of 1.7 Å. The deuteration states of the H atoms in the protein molecule were identified, followed by an extensive analysis of the water structure surrounding the protein. The final R factor was 20.4% (R_free = 22.1%). In total, the 244 observed water molecules form approximately one layer of water around the protein with far fewer water molecules located further away. Water molecules covering the apolar patches make tangential layers at 4–5 Å from the surface or form C—HO contacts, and several water-molecule sites can be identified in the apolar cavities. Many of the water molecules are apparently orientationally disordered, and only 115 out of the 244 water molecules sit in mean single orientations. Comparison of these results with quasi-elastic neutron scattering observations of the water dynamics leads to a picture of the water molecules forming an extended constantly fluctuating network covering the protein surface.