Two E isomers of alpha,beta-dehydro-phenylalanine, Ac-(E)-Delta Phe-NHMe (1a) and Ac-(E)-Delta Phe-NMe2 (2a), have been synthesized and their low temperature structures determined by single-crystal X-ray diffraction. A systematic theoretical analysis was performed on these molecules and their Z isomers (1b and 2b). The phi,psi potential energy surfaces were calculated at the MP2/6-31+G(d,p) and B3LYP/6-31+G(d,p) levels in the gas phase and at the B3LYP/6-31+G(d,p) level in the chloroform and water solutions with the SCRF-PCM method. All minima were fully optimized by the MP2 and DFT methods, and their relative stabilities were analyzed in terms of pi-conjugation, internal H-bonds, and dipole-dipole interactions between carbonyl groups. The results indicate that all the studied compounds can adopt the conformation H (phi,psi approximate to +/- 40 degrees, -/+ 120 degrees) which is atypical for standard amino acids residues. A different arrangement of the side chain in the E and Z isomers causes them to have different conformational preferences. In the presence of a polar solvent both Z isomers of APhe (1b and 2b) are found to adopt the 3(10)-helical conformation (left- and right-handed are equally likely). On the other hand, this conformation is not accessible or highly energetic for E isomers of Delta Phe (1a and 2a). Those isomers have an intrinsic inclination to have an extended conformation. The conformational space of the Z isomers is much more restricted than that of the E derivative both in the gas phase and in solution. In the gas phase the E isomers of APhe have lower energies than the Z ones, but in the aqueous solution the energy order is reversed.