Poly-proline fragments form the distinctive PPII-helix in a high percentage in proteins and peptides, which plays an important role in signal transduction and protein complex assembly. But why and how this helical structure works is still unknown. In this work, we applied the density functional calculations to investigate the different preferences of prolines on C-gamma-endo and C-gamma-exo conformers, in which non covalent interactions play important roles. As a result, C-gamma-exo conformers are favored for short-oligomers with stronger n -> pi* interaction formed by the backbone, while C gamma-endo conformers are favored for longer oligomers with stronger dipole interactions between side groups. (C) 2016 Elsevier B.V. All rights reserved.