The conformational preferences of dipeptides from alpha-aminoisobutyric acid (Aib), 1-aminocyclopropane-1-carboxylic acid (Ac(3)c), and 1-aminocyclobutane-1-carboxylic acid (Ac(4)c) residues have been determined by ab initio quantum mechanical calculations. The results obtained evidence for the intrinsic helix-forming tendency of the Aib residue. Furthermore, the conformational preferences of Ac(3)c and Ac(4)c dipeptides are reported and the differences explained in terms of the size of their cyclic side chains, Finally, the large number of minima characterized for these conformationally restricted residues is explained as a function of the deformability of the molecular geometries.