Morphology and the strength of intermolecular contacts in protein crystals

The strengths of intermolecular contacts (macrobonds) and the areas occupied by each contact on the molecular surface were estimated in four polymorphic modifications of lysozyme crystals based on the bond strengths between individual atomic pairs belonging to the molecules in contact. It has been shown that the periodic bond chains of these macrobonds account for the morphology of protein crystals. The Coulombic contribution to the macrobond strength has also been estimated. Making use of the contact strengths and taking into account bond hydration, crystal-water interfacial energies were also estimated for different crystal faces. The areas of all contacts are mapped on the molecular surface, making use of a polar-coordinate representation of the contact. Comparing the locations of the intermolecular contacts in the different polymorphic crystal modifications, it is shown that these contacts can form a wide variety of patches on the molecular surface. The patches are located practically everywhere on the surface except for the inside of a concave active site. It is also shown that the contacts, which frequently involve water molecules, are formed by specific intermolecular hydrogen bonds on a background of non-specific attractive electrostatic interactions. Typical values of the macrobond strength are compared with the strength of association in other protein-complex systems.