Close packing of amino acid residues in globular proteins: specific volume and site binding of water molecules

The av. sp. vol. of proteins in soln. calcd. with respect to (1) van der Waals vol. of an av. amino acid residue, (2) crystal vol. of an av. amino acid residue, and (3) packing d. of an av. amino acid residue was lower than the av. exptl. value. This suggests an even higher than expected packing d. of the amino acid residues. This deviation may be due to H2O mols. filling niches on the surface of the protein or holes in the interior. The no. of these bound H2O mols. evaluated for several proteins appears to be in reasonable agreement with that inferred from the refinement of some x-ray structures.