In peptides, amide carbonyl and NH groups act respectively as Lewis base and Brønsted acid sites that interact within the chain and with the environment through hydrogen bonds. Surrogates that maintain the register of the peptide sequence yet enhance the Lewis basicity and Brønsted acidity of the amino amide residues are being studied to favour secondary structures such as turn conformations, to enhance metabolic stability and to improve receptor selectivity and potency (Figure 2). For example, azapeptides are peptide analogs that employ a semicarbazide as an amino amide surrogate in which the backbone α-CH is replaced by nitrogen. Through electronic interactions, the semicarbazide favours backbone β-turn geometry due to a combination of urea planarity and hydrazine nitrogen lone pair – lone pair repulsion. Side-chains may also be preserved on aza-residues in optimal orientations due to adaptive chirality about the α-nitrogen. Linear azapeptides have become drugs, due in part to enhanced stability and protease resistance. In 2009, our laboratory introduced a submonomer method that advanced azapeptide synthesis by enabling addition of diverse side chains onto a common intermediate. For example, access to aza-proparglyglycine (azaPra) residues empowered azapeptide library construction by orthogonal chemistry in which the triple bond is modified selectively in the presence of the backbone and side chain functional groups. In addition, by employing amino-sulfamides as amino amide surrogates, we synthesize azasulfurylpeptides that have been shown by X-ray analysis to replicate the transition state of amide hydrolysis and to induce γ-turn conformations. Azasulfurylpeptides are being used in applications as enzyme inhibitors and allosteric modulators.
Figure 2. Azapeptide and azasulfurylpeptides induce turns by electronic interactions