Etudes spectroscopiques de transitions “helice‐pelote” de polypeptides en solution acide

Infrared spectra of various polypeptides (poly‐L‐alanine, or PLA; poly‐L‐methionine or PLM; poly (γ ‐ benzyl‐L‐glutamate), or PBLG) in CF3COOH‐CHCl3 solutions have been investigated in regions characteristic of the peptide group (amide A, I and II) and of the acid solvent (vOH, vC=O). These spectra have been analyzed by comparison with the infrared spectrum of a model amide (N‐methylacetamide). This analysis shows that by addition of acid the simple amide and the polypeptides exhibit at first a similar behavior by forming complexes by hydrogen bonding, i.e., >C=O…HOOCCF3. At higher acid concentrations, unlike the model amide, polypeptides are not protonated. Furthermore, the evolution of the spectra as a function of complexation clearly indicates the helix‐random coil transition of these polymers and agrees well with optical rotatory dispersion measurements. Thus, steep transitions have been obtained for PBLG and PLM in a small range of acid concentration or of temperature. To the contrary, no discontinuity in the evolution of PLA spectra has been seen. This observation compares well with the slow transformation observed by optical rotatory dispersion (ORD). Finally, it is possible to compare at the level of the repeat unit the strength of the acid perturbation which triggers the transformation of the three polypeptides. This points out the important part played by the sidechains as stabilization forces of the helix and the cooperative aspect of the transitions.