Infrared and Raman study in the solid state of fully protected, monodispersed homooligopeptides of L ‐valine, L ‐isoleucine, and L ‐phenylalanine

An ir‐absorption and Raman‐scattering study, in the solid state, has been carried out on monodispersed, N‐ and C‐protected homooligopeptides (number of residues, n , from 2 to 7) of L ‐valine, L ‐isoleucine, and L ‐phenylalanine. The amide I, II, III, V, and v NH regions have been examined. Some deuterated (ND) samples have been examined to complete the assignments. L ‐Phenylalanine dipeptide displays spectral characteristics compatible with the parallel β‐structure; L ‐isoleucine and L ‐valine dipeptides are probably in a distorted structure. A mixture of parallel and antiparallel extended chains cannot be excluded for the peptides with n = 3. In the amide I region the spectra of peptides with n ≥ 4 show the existence of the β‐conformation. The problem of chain orientation within the pleated‐sheet structure is discussed on the basis of a recent theoretical treatment of vibrational interactions of the amide I mode.