Synthesis and conformational analysis of Aib-containing peptide modelling for N-glycosylation site in N-glycoprotein.

A tetrapetide containing an Aib residue, Boc-Asn-Aib-Thr-Aib-OMe, was synthesized as a peptide model for the N-glycosylation site in N-glycoproteins. Backbone conformation of the peptide and possible intramolecular interaction between the Asn and Thr side chains were elucidated by means of n.m.r. spectroscopy. Temperature dependence of NH proton chemical shift and NOE experiments showed that Boc-Asn-Aib-Thr-Aib-OMe has a tendency to form a beta-turn structure with a hydrogen bond involving Thr and Aib4 NH groups. Incorporation of Aib residues in the peptide model promotes folding of the peptide backbone. With folded backbone conformation, carboxyamide protons of the Asn residue are not involved in hydrogen bond network, while the OH group of the Thr residue is a candidate for a hydrogen bond in DMSO-d6 solution.