Intra- and intermolecular forces dependent main chain conformations of esters of α,β-dehydroamino acids

Esters of dehydroamino acids occur in nature. To investigate their conformational properties, the low-temperature structures of Ac-ΔAla-OMe, Ac-ΔVal-OMe, Z-(Z)-ΔAbu-OMe, and Z-(Z)-ΔAbu-NHMe were studied by single-crystal X-ray diffraction. The ΔAla ester prefers the fully extended conformation C5. Both the ΔVal and (Z)-ΔAbu esters assume the conformation β, whereas the amide analogue of the latter prefers the conformation α. For the conformations found, DFT calculations using B3LYP/6-311++G(d,p) with the SCRF-PCM and M062X/6-311++G(d,p) with the SCRF-SMD method were applied to mimicking chloroform and water environment. The tendency of the ΔVal and (Z)-ΔAbu esters towards the conformation β, and their amide analogues towards the conformation α, with increase of the polarity of environment was found. The analysis of both intra- and intermolecular interactions including hydrogen bonds, carbonyl dipole attraction, and π-electron conjugation, enabled to understand and elucidate the conformational preferences of studied compounds. The studies show how the molecular structure, and in consequence, the conformation adopted by molecules is influenced by the different intra- and intermolecular forces.