Steric effect exerted by the proline residue on the antecedent alanine residue

Abstract
Five model tetrapeptides: Ala-Ala-Ala-Ala, Pro-Ala-Ala-Ala, Ala-Pro-Ala-Ala, Ala-Ala-Pro-Ala and Ala-Ala-Ala-Pro, were synthesized and measured in D2O by 13C-NMR spectroscopy. The spectra analysis led to the conclusion that for each model (irrespective of pD) in conformational equilibrium, the predominant conformation is the one in which side methyl of alanine preceding proline residue eclipses alanine carbonyl group. The influence of pD changes on cis-trans isomerism of Ala.sbd.Pro amide bond was also investigated.