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Hydrogen bonds, salt bridges and van der Waals also provide attractive forces between molecules. Hydrogen bonds between protein and ligand can be more favourable than between protein and solvent[Fersht, 1987], and although they are not found in large numbers in all interfaces, they make an important contribution to the binding energy of association. Hydrogen bonds via water molecules trapped in the interface are also commonly observed. Hydrogen bonds and salt-bridges are thought to confer specificity to interactions due to their dependence on the precise location of participating atoms[Fersht, 1984,Fersht, 1987].
Van der Waals interactions occur between all neighbouring atoms in structures and interfaces, but are not significantly different to those made by the same atoms with solvent. Thus a requirement exists for the atoms in the interface to be at least as densly packed as in bulk solvent; implying shape complementarity as discussed in the following section.