Indexed on: 15 Aug '13Published on: 15 Aug '13Published in: Biochemistry
Compared to generic peptide bonds, the peptidyl-prolyl bond shows a strong propensity for the cis conformer. The presence of a sequence-contiguous aromatic (Aro) residue can further stabilize the cis conformer, as observed for the Aro-Pro motif. The cis propensity of the reverse sequence motif, Pro-Aro, is not so well understood, especially the effect of N-capping the Pro-Aro motif with different amino acid residues. From a comparative nuclear magnetic resonance study of two peptide series with the general sequences Ac-Xaa-Pro-Tyr-NH2 and Ac-Xaa-Pro-Ala-NH2, we present a relative thermodynamic scale that reflects how the nature of the Xaa side chain influences the cis propensity of the Xaa-Pro-Tyr motif, with Gly, Pro, and Ala at position Xaa giving the greatest enhancement of the cis-peptidyl-prolyl population. We also show that CH···π interaction between Xaa and Tyr is responsible for the enhanced cis population. However, the mere presence of the CH···π interaction does not guarantee that the peptidyl-prolyl bond will have a higher cis content in Xaa-Pro-Tyr than in Xaa-Pro-Ala. Xaa-dependent intramolecular interactions present in Xaa-trans-Pro-Tyr can nullify favorable CH···π interactions in Xaa-cis-Pro-Tyr. The relative cis-peptidyl-prolyl stabilizing propensities of Xaa (Xaa-Pro-Tyr) in proteins and in our peptide series show strong linear correlation except when Xaa is aromatic. We also explore the Xaa-Pro-Gly-Tyr sequence motif and show that mediated by a Pro-Tyr CH···π interaction, the cis-peptidyl-prolyl bond in the motif is stabilized when Xaa is Pro.