Stability of α-Helical Structure of Poly(L-aianine)

Abstract

The intramolecular interaction energy and the structure of interactions in poly(L-alanine) were calculated in the region near the αR-helix with the semiempirical potential functions of nonbonded, electrostatic, rotational, and hydrogen-bonding interactions. In the conformation of the αR-helix of poly(L-alanine) the nonbonded interaction makes the most dominant contribution to the total energy of the conformation and the hydrogen-bonding energy is almost its 1/7. However there exists between a peptide pair separated by three peptide units a large interaction energy which comes predominantly from the hydrogen-bonding interaction. On the contrary the nonbonded interaction exists mostly between nearest neighbor peptide units. On deforming the conformation slightly from the αR-helix, this hydrogen-bonding interaction disappears, although the other kinds of interaction remain almost unchanged. Spacially specific hydrogen-bonding fastens the polypeptide chain as a clamp only when the peptide chain takes certain special conformations. Thus it can be concluded that the hydrogen bond is very important for the stability of the α-helix, and it plays an especially essential role for the cooperative helix—coil transition. It is also emphasized that the structure of interaction energy is important for the study of the protein conformation.