Etude, par spectroscopie infra‐rouge, de la conformation de quelques composés peptidiques modèles

Abstract

Abstract Some experimental data are given on the infrared spectra between 3300 and 3500 cm −1 of dilute solutions in carbon tetrachloride of three types of model compounds: CH 3− CONH‐CH(R 1 )‐CONH(R 2 ), (I); CH 3 ‐CON(CH 3 )‐CH(R 1 )‐CONH(R 2 ), (II) and CH 3 ‐CONH‐CH(R 1 )‐CON(R 2 ) 2 , (III). In studying the N‐H stretching bands, it was found that there are two types of intramolecular hydrogen bonds in these molecules; these result in two different cyclized conformations, C 5 and C 7 , which contain respectively, five and seven atoms in the ring. By using model substances I, II, and III, in which the nitrogen atoms are unequally substituted, it is possible to identify the N‐H stretching bands which are to be ascribed to the N‐H oscillators included in the two different chelated conformations. It is found also that the stretching frequency of a free N‐H oscillator depends upon the substituent on the nitrogen atom. Thus, it is possible to observe, with some of the model compounds I, four different absorption bands located at 3340, 3420, 3440, and 3460 cm −1 . The first two are ascribed to the N‐H oscillators included in the Hbonds which lock the C 7 and C 5 conformations; the last two correspond to free N‐H which differ with the substituent on the nitrogen atom.