CONFORMATIONAL STUDIES OF PEPTIDE SYSTEMS - ROTATIONAL STATES OF NH-CH FRAGMENT OF ALANINE DIPEPTIDES BY NUCLEAR MAGNETIC RESONANCE

The IR and NMR-H1 spectra of alanine dipeptides and their N-Me derivatives have been investigated. Measurement of the integral intensities of the NH stretching vibrations in dilute solutions of CCl4 and of a (1:9) CHCl3 + CCl4 mixture showed that in these solvent approximately 70% of the alanine dipeptide molecules are in an intramolecular hydrogen-bonded folded form. It was found from analysis of the vicinal proton spin-spin coupling constant of the CONHCH fragment that the stable conformer with respect to this bond is that with cis-arrangement of the NH and CH hyrogens. The conformation of the 7-membered hydrogen-bonded ring of the dipeptides has been elucidated. An empirically found stereochemical dependence of the constant 3 JNHCH upon the dihedral angle θ of the fragment has served as basis for discussing the possible conformations of the extended form of the dipeptides in polar (including aqueous) solvents. © 1969.