Electronic factors favouring the cis conformation in proline peptidic bonds

a quantum chemical study based on b3lyp, 6-31+g* computations offered the possibility of comparing three dipeptides containing as a central residue proline, n ethyl alanine and an alanine respectively in their cis and trans conformations. the presence of the electron donating alkyl substituent on the nitrogen atom is an important factor favouring the cis conformation. the stabilisation of the cis conformer is interpreted as being due to electrostatic and induction interactions between the carbonyl group and the substituent on the nitrogen atom.