Tight packing of protein cores and interfaces. Relation to conservative amino acid sequences and stability of protein-protein interaction.

The tightly packed protein interiors and interfaces are taken to be essentially solids. The tight packing, and the r -6 dependence of the energy of van der Waals' interactions may account for the highly conservative core regions of homologous proteins. The hydrophobic free energies used to calculate confirmational stability and the association constants for protein-protein interactions are not adequate, since the free energies are obtained from liquid-liquid transfer of model compounds. An additional term is required, the enthalpy of fusion. This provides an additional approximately 7 kcal mol -1 for the stabilization of the trypsin-trypsin inhibitor complex.