Conformational studies of peptide systems : The rotational states of the NHCH fragment of alanine dipeptides by nuclear magnetic resonance

The IR and NMR-H 1 spectra of alanine dipeptides and their N-Me derivatives have been investigated. Measurement of the integral intensities of the NH stretching vibrations in dilute solutions of CCl 4 and of a (1:9) CHCl 3 + CCl 4 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 CONHCH 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 J NHCH 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.