Since the alpha helix is the most commonly occurring type of secondary structure in proteins, and as there is quite a variation in the frequency of occurrence of the various amino acid residues in alpha helices, mutations within alpha helices are of considerable importance. Alanine is the most stabilizing residue whereas glycine destabilizes the helix relative to alanine by 0.7-0.8 kcal/mol and proline by 3-4 kcal/mol.Therefore a long stretch of alpha helix substituted by alanine gives a more stable protein. The alignment of dipoles of the polypeptide backbone parallel to the axis of an alpha helices causes a net dipole moment with its positive pole at the amino terminus and negative pole at the carboxy terminus.

Alpha helix dipole diagram

Helix formation parameters for each amino acids