Role of medium-range interactions in proteins.

The energies of oligopeptide segments of lysozyme are minimized with respect to the dihedral angles of the central residue. As the length of the oligopeptide segment increases, up to a nonapeptide, the low-energy conformation becomes that observed in the x-ray structure in most cases. This finding suggests that, while short-range interactions appear to play the dominant role in determining the conformation of an amino-acid residue in a protein, the additional interactions required to stabilize the conformation uniquely may be only of medium range, i.e., those within a nonapeptide, and longer-range interactions may be of considerably less importance.