Synthesis and Conformational Study of Model Peptides Containing N-Substituted 3-Aminoazetidine-3-carboxylic Acids

Model peptides containing N-substituted 3-aminoazetidine-3-carboxylic acids have been synthesized to investigate the conformational features of these C-tetrasubstituted amino acids. The target peptides were designed and prepared by classical synthetic methods in solution, exploiting the convenient N-substituted 3-azidoazetidine-3-carboxylic acids as precursors. The conformational preferences of the newly synthesized peptides were investigated in solution by IR and NMR spectroscopy. It was observed that the 3-aminoazetidine-3-carboxylic acid moiety is likely a -turn inducer. In addition, an interesting main-chain-to-side-chain hydrogen bond that forms a six-membered pseudo-cycle was detected. It connects the nitrogen (acceptor) of the azetidine ring to the amide NH (donor) of the immediately following residue. This unexpected hydrogen bond increases the number of conformational options offered by N-substituted 3-aminoazetidine-3-carboxylic acids when designing foldamers with new and predictable 3D structures.