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

To determine the structural perturbations induced by the CαH→Nα exchange in aza-peptides, we have examined by H 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: R'-AzXaa-NR²R³, R'-Pro-AzXaa-NR²R³ and R-AzXaa-Pro-NR²R³ (where AzXaa denotes the aza-analogue of the amino acid residue Xaa = Ala, Asp, Asn; R = Boc, Z; R², R³= H, Me, iPr). The aza-analogue of an amino acid residue appears to be a strong p-turn-inducing motif, and the AzAsn carboxamide side-chain is capable of interacting, as a proton donor, with the preceding peptide carbonyl group.