Aza-peptides. III. Experimental structural analysis of aza-alanine and aza-asparagine-containing peptides

To determine the structural perturbations induced by the C alpha H-->N alpha exchange in aza-peptides, we have examined by 1H NMR and IR spectroscopy various derivatives of the aza-analogues of alanine, aspartic acid and asparagine in different organic solvents with increasing polarity. Their general formulas are: R1-AzXaa-NR2R3, R1-Pro-AzXaa-NR2R3 and R1-AzXaa-Pro-NR2R3 (where AzXaa denotes the aza-analogue of the amino acid residue Xaa = Ala, Asp, Asn; R1 = Boc, Z; R2, R3 = H, Me, iPr). The aza-analogue of an amino acid residue appears to be a strong beta-turn-inducing motif, and the AzAsn carboxamide side-chain is capable of interacting, as a proton donor, with the preceding peptide carbonyl group.