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-Delta Ala-OMe, Ac-Delta Val-OMe, Z-(Z)-Delta Abu-OMe, and Z-(Z)-Delta Abu-NHMe were studied by single-crystal X-ray diffraction. The Delta Ala ester prefers the fully extended conformation C5. Both the Delta Val and (Z)-Delta Abu esters assume the conformation beta, whereas the amide analogue of the latter prefers the conformation alpha. 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 Delta Val and (Z)-Delta Abu esters towards the conformation beta, and their amide analogues towards the conformation alpha, with increase of the polarity of environment was found. The analysis of both intra- and intermolecular interactions including hydrogen bonds, carbonyl dipole attraction, and pi-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. (C) 2013 Elsevier B.V. All rights reserved.