Proline does not paicipate in the formation of alpha helix. The stability of an alpha helix arises primarily from hydrogen bonds formed between the oxygen of the peptide bond carbonyl and the hydrogen atom of the peptide bond nitrogen of the fouh residue down the polypeptide chain. since the peptide bond nitrogen of proline lacks a hydrogen atom,it is incapable of forming a hydrogen bond with carbonyl oxygen. Glycine induces bends within a helix, because of its small size. Reference: Harper illustrated Biochemistry 30th Edition page no. 35,38