Infrared spectra and conformation of monodisperse Oligo‐ γ ‐Benzyl‐ L ‐Glutamates in solution

The ir spectra of σ–nitrophenylthio‐tetra‐ and hexa‐γ‐benzyl‐ L ‐glutamate ethylamides were measured at different concentrations in chloroform and ethylene dichloride. The molar extinction coefficients of two bands, each for the amide I and A modes, were observed as indicating the content of β structure and the fraction of hydrogen bonds; they were analyzed for elucidating the hydrogen‐bonded state of peptide residues in the σ and β conformations of oligopeptides. While the content of β structure of the tetra‐peptide increases with increasing concentration, the hexapeptide is in the β conformation above the critical concentration only. The fraction of hydrogen bonds remains finite even at infinite dilution or below the critical concentration, indicating the intramolecular hydrogen‐bonding in the σ—form. As the fundamental structure of folded forms having only intramolecular hydrogen bonds, the 2 7 ribbon is most likely. With increasing concentration or above the critical concentration, the extended forms are stabilized by the intermolecular hydrogen bonds between residues of the β‐form. The β‐form is present only when intermolecular hydrogen bonds link two residues in an antiparallel way. Possible structures of the oligopeptides in the σ– and β conformations in the two solvents are described briefly.