CONFORMATIONAL INVESTIGATION OF ALPHA,BETA-DEHYDROPEPTIDES - IX - N-ACETYL-(E)-ALPHA,BETA-DEHYDROBUTYRINE N'-METHYLAMIDE - STEREOELECTRONICPROPERTIES FROM INFRARED AND THEORETICAL-STUDIES

The Fourier transform infrared spectra of Ac-(E)-Delta Abu-NHMe were a nalyzed to determine the predominant solution conformation(s) of this (E)-alpha,beta-dehydropeptide-related compound and the electron densit y perturbation in its amide groups. The measurements were performed in dichloromethane and acetonitrile in the region of mode v(S) (N-H), am ide I, amide II and v(S) (C-alpha = C-beta). The equilibrium geometric al parameters, calculated by a method based on the density functional theory with the B3LYP functional and the 6-31G basis set, were used t o support spectroscopic interpretation and gain some deeper insight in to the molecule. The experimental and theoretical data were compared w ith those of three previously described molecules: isomeric Ac-(Z)-Del ta Abu-NHMe, Ac-Delta Ala-NHMe, which is deprived of any P-substituent , and saturated species Ac-Abu-NHMe. The titled compound assumes two c onformational states in equilibrium in the DCM solution. One conformer is extended almost fully and like Ac-Delta Ala-NHMe is Cg hydrogen-bo nded. The other adopts a warped C-5 structure similar to that of Ac-(Z )-Delta Abu-NHMe. The C-5 hydrogen bond, unlike the H-bond in Ac-Delta Ala-NHMe, is disrupted by acetonitrile. The resonance within the N-te rminal amide groups in either of the (E)-Delta Abu conformers is not a s well developed as the resonance in Ac-Abu-NHMe. However, these N-ter minal groups, compared with the other unsaturated compounds, constitut e better resonance systems in each conformationally related couple: th e C-5 hydrogen-bonded Ac-(E)-Delta Abu-NHMe/Ac-Delta Ala-NHMe and the warped C-5 Ac-(E)-Delta AbuNHMe/Ac-(Z)-Delta Abu-NHMe. The resonance w ithin the C-terminal groups of the latter couple apparently is similar , but less developed than the resonance in Ac-Abu-NHMe. The electron d istribution within the C-terminal group of the hydrogen-bonded C-5(E)- Delta Abu conformer apparently is determined mainly by the electron in flux from the C-alpha=C-beta double bond.