Conformational investigation of α, β-dehydropeptides. VIII.*N-Acetyl-α, β-dehydroamino acid N'-methylamides: conformation and electron density perturbation from infrared and theoretical studies

The Fourier transform infrared spectra are analyzed in the regions of v_s(N-H), amide I, amide II and v_s(C^α=C^β) bands for a series of Ac-ΔXaa-NHMe, where ΔXaa =ΔAla, (Z)-ΔAbu, (Z)-ΔLeu, (Z)-ΔPhe and AVal, to determine the predominant solution conformation of these α, β-dehydropeptide-related molecules and the electron distribution perturbation in their amide bonds. The measurements were performed in dichloromethane (DCM). To confirm and rationalize the assignments, the spectra of the respective series of saturated Ac-Xaa-NHMe, recorded in DCM, and the spectra of these two series of unsaturated and saturated compounds, recorded in acetonitrile, were examined. To help interpret the spectroscopic results, the equilibrium geometrical parameters for some selected amides were used. These were optimized with ab initio methods in the 6-31G** basis set. Each of the dehydroamides studied adopted a C₅ structure, which in Ac-ΔAla-NHMe is fully extended and accompanied by the strong C₅ hydrogen bond. Interaction with the C^α=C^β bond lessened the amidic resonance within each of the flanking amide groups. The N-terminal C=O bond was noticeably shorter, both amide bonds were longer than the corresponding bonds in the saturated entities and the N-terminal amide system was distorted. Ac-ΔAla-NHMe constituted an exception. Its C-terminal amide bond was shorter than the standard one and both amide systems were prototypically planar.