Studies on Long Chain Alkyl Amides of Water-soluble Poly-α-amino Acids. II. Infrared Spectra of Poly-Dl-alanine Dodecylamide

Abstract

Abstract The infrared spectra of poly-Dl-alanine and poly-Dl-alanine dodecylamide were measured on film cast from various solutions, and the conformation of the polymers were elucidated on the basis of the location and intensity of absorptions, mainly in the amide I and III bands. The polymer showed an absorption at 1661 cm−1 in the amide I band if it was in the random coil form, but it showed two absorptions at 1630 and 1690 cm−1, if it was in the antiparallel β-form. While the amide III band was located at 1244 cm−1 irrespective of the polymer conformation, the band had an additional absorption at 1222 cm−1 for the β-conformation. From the above correlation of the spectra with the polymer conformation, it was found that poly-Dl-alanine and PADA prepared with high [A]/[I] ratios are in the random coil form but that PADA of [A]/[I] ratios lower than 30 is composed of both the random coil and the antiparallel β-forms when it is regenerated from formic acid solution. PADA contained a greater amount of the β-form as the [A]/[I] value or the degree of polymerization was lower. The [A]/[I] 5 preparation was mostly in the β-form. In the ionized PADA obtained from aqueous hydrochloric acid solution, the portion of the random coil form increased relative to that in the unionized PADA derived from formic acid solution, and the effect of the ionization was enhanced for the lower [A]/[I] preparations. Poly-Dl-alanine and PADA in formic acid or aqueous hydrochloric acid solution underwent a partial transformation from the random coil to the β-form during the aging of concentrated solution, which process accompanied gelation. The solution of the lowest [A]/[I] preparation did not gel. In the state regenerated from gel, poly-Dl-alanine and PADA, except for the unionized lower [A]/[I] preparations, had an increased amount of the β-form as compared with that from an initial fluid solution. In the lower [A]/[I] preparations obtained from formic acid solution, the reverse transformation apparently occured, when the films cast before and after the gelation were compared.