Conformational studies of sequential polypeptides of L-Alanine and β-Alanine

In order to investigate the effect of the β-alanine residue on the conformational stability of α-helical poly(L-alanine), studies have been made on the conformations of sequential copolypeptides having the following repeating sequences: (L-Ala-β-Ala) n , (L-Ala-L-Ala-β-Ala) n , (L-Ala-β-Ala-L-Ala-L-Ala) n and (L-Ala-L-Ala-X) n , where X is D,L-β-amino-isobutyric acid residue. Conformations of these polypeptides were measured both in the solution and solid states by means of optical rotatory dispersion, infrared and far-infrared spectroscopies and X-ray diffractions. All the copolypeptides studied here did not give the α-helix. It may be, therefore, concluded that the β-alanine residue is not incorporated in but impairs the α-helix of poly(L-alanine), since the hydrogen bond periodicity in the α-helical chain is disturbed by the introduction of such a β-amino acid as β-alanine.