Repulsive interactions between polar and apolar atoms in globular proteins

A significant number of tetrahedral carbon-backbone nitrogen pairs in ferrocytochrome c have interatomic distances that are more than 0.4 Å shorter than the sum of the atomic van der Waals' radii. The non-bonded repulsions of these pairs destabilize the native structure by as much as 50 to 60 kcal/mol. A detailed examination of the close contacts suggests that these result from packing forces associated with the formation of secondary and tertiary structure in the protein.