Quantum mechanics methods have been applied to investigate the effects of the selective orientation of the side chains on the conformational features of cyclopropane analogues of phenylalanine. For this purpose, the conformational preferences of the N-acetyl-N'-methylamide derivative of 1-aminocyclopropanecarboxylic acid (Ac(3)c) have been compared to those of two stereoisomers of 1-amino-2-phenylcyclopropanecarboxylic acid: (2S,3S)c(3)Phe and (2S,3R)c(3)Phe. Geometry and vibration frequencies were calculated by the HF, B3LYP, or MP2 methods using the 6-31G(d) basis set, whereas the energy was further evaluated using, in some cases, more sophisticated methods and larger basis sets. Solvent effects were modeled by the self-consistent polarization continuum model with the HF/6-311G(d,p) method. The stereochemistry of the phenyl side chain largely influences the specific backbone...side chain interactions, which in turn affect the stability of the different conformations. Furthermore, results have been compared with recently published experimental data.