Conformational investigation of α,β-dehydropeptides. XIII. Conformational properties of N-acetyl-α,β-dehydrovaline N',N'-dimethylamide.

The crystal structure of Ac-ΔVal-NMe2 (ΔVal = α,β-dehydrovaline) was determined by X-ray crystallography. The found angles φ = -60° and ψ = 125° correspond exactly to the respective values of the (i + 1)th residue in idealised β-turn II/VIa. Ab initio/DFT studies revealed that the molecule adopts the angle ψ restricted only to about |130°| and very readily attains the angle φ = about -50°. This is in line with its solid-state conformation. Taken together, these data suggest that the ΔVal residue combined with a C-terminal tertiary amide is a good candidate at the (i + 1)th position in a type II/VIa β-turn.