The model cyclopeptide Ac-Lys-Asp-NHMe was used to test Lys as a possible substitute for Xaa in peptide fragment Xaa-Asp whose conformational mobility would be constrained by lactamization of the Lys and Asp side chains. By means of theoretical conformational analysis, such a lactam was shown to be capable of fixing several conformations of the peptide. Among them, 32 conformations correspond to 8 low-energy regions of the linear peptide Ac-Ala-Asp-NHMe, which was chosen as a model for the peptide fragment Xaa-Asp. In this case, the conformational possibilities of the Xaa residue were constrained to two regions of the φ, ψ-map, (A + G) and C according to Zimmermann-Sheraga notation.