Conformational studies of poly(alanine) in dichloroacetic acid by nuclear magnetic resonance.

Proton nuclear magnetic resonance spectra were studied for monodisperse L-alanine oligopeptides (the dimer, trimer, tetramer, and nonamer) containing an n-butylamide group at the C-terminal residue and for the random and block copolymers of D- and L-alanines having sharp molecular weight distributions in dichloroacetic acid (DCA). It was found that the NH signals of poly(L-alanine)s and some copoly(D,L-alanine)s were split into three peaks in DCA, suggesting that these NH peaks reflect the conformation of the polymers. These NH peaks were assigned to the terminal helix (helix-coil junction) and the random-coil and the inner helix, respectively, from the lowest field. On the basis of the above assignment, the microconformations of polycalanine)s in DCA were examined quantitatively. Moreover, it was concluded from the specific nature of the solvent that both the effects of the hydrophobic side chains of the polymers and the acidity of the solvent are most important for the formation of the helical conformation of poly(L-alanine).