Fig. 13.10 (k) The spatial arrangement of the molecules in the crystal lattice shows a dense packing of the protein molecules (here thrombin). However, large, at first glance "empty" channels exist in the crystal packing between the molecules. These channels are occupied by a large number of water molecules. Because of their strong thermal motion and the resulting disorder, they are not detected in the electron density. However, small ligand molecules can diffuse through these channels if the crystals are soaked with a solution of these ligands.