Etude par Spectroscopie Infrarouge de la Transformation Hélice—Chaine Statistique des Polypeptides. I. Etude Comparée de l'Interaction de la Poly-L-alanine et d'un Amide Modèle avec l'Acide Trifluoroacétique

Infrared spectra of poly-L -alanine in trifluoroacetic acid-chloroform mixtures have been investigated and compared with those of a model amide (N-methylacetamide). The purpose of this work is to determine the nature of peptide-acid specific interactions responsible for the helix-random coil transition of polymer chains. Analysis is made in using amide (A, I, II, III) and acid (νC?O, νOH) vibrations which are specially sensitive to molecular interactions. We examined a model compound to determine the spectral characteristics of the different complexes or species formed between amide and acid. At a low acid concentration, hydrogen-bonded complexes: ? (NH) C?O…?HOOCCF₃ (1) are evidenced but no association between amide NH and acid CO groups (complexes A) is observed. For higher acid concentrations complexes (I) are progressively changed into ions pairs and free ions, which result from amide protonation by acid, according to the exothermic equilibrium (I)?? (NH)COH⁺, ^?OOCCF₃(II). Amidium and carboxylate bands are localized between 1680–1705 cm^?1 and 1620–1625 cm^?1, respectively. If the cation band is always clearly seen, the anion band is only observed for the most acidic solutions. For the polymer, a gradual complexation of type (I) is observed for all acid concentrations. From our results, the assumption of an (A) type interaction seems very unlikely but cannot be excluded. Moreover, proton transfer—similar to that observed with a model amide—is never evidenced since, in particular, the amidium band characteristic of protonation is never seen. In contrast to previous investigations, we conclude that the helix-random coil transition of polypeptides is not due to the protonation of the peptide functions. This transition does suggest a strong interaction by hydrogen bonds between polymer and acid molecules.