Conformational flexibility of the dehydroalanine derivatives: molecular and electronic structure of (Z)-N-acetyldehydrophenylalanine

The molecular structure of (Z)-N-acetyldehydrophenylalanine (1-phenyl-2-acetylaminoprop-2-enoic acid) has been determined from three-dimensional X-ray data. C11H11NO3·2H2O is monoclinic, space group P21/a, with Z = 4 in a cell of dimensions a = 18.260(5), b = 6.050(2), c = 11.411(4) Å, β = 105.96(3)°. The molecules exhibit a non-planar conformation as far as the φ{symbol} torsion angle is concerned and they are linked by hydrogen bonds through two water molecules. Conformational flexibility is discussed on the basis of theoretical conformational analysis by taking into account the possibility of deviation from the planarity within the side chain. Theoretical results lead to exclusion any planar conformation for the title molecule. The vapour-phase He(I) photoelectron spectrum is also presented. The assignments are proposed by comparison with simpler related molecular systems and supported by INDO/S calculations. The effects of non-planarity on the electronic structure are discussed. © 1981.