Structural invariants in protein folding.

An analysis of 15 protein structures indicates: First, the loss of accessible surface area by monomeric proteins on folding-proportional to hydrophobic energy-is a simple function of molecular weight; second, the proportion of polar groups forming intramolecular hydrogen bonds is constant; and third, protein interiors are closely packed, each residue occupying the same volume as it does in crystals of amino acids.