Abstract:
Designing protein secondary structure mimetics using non-natural amino acids has significant
importance in understanding protein folding, as well as from the perspective of medicinal chemistry. Various backbone-modified synthetic amino acids have been explored over the years to design functional foldamers. These include acyclic beta- and gamma-amino acids, carbocyclic beta- and gamma-amino acids, sugar-modified beta- and gamma-amino acids, and dialkyl beta- and gamma-amino acids. In contrast to saturated gamma-amino acids, alpha, betaunsaturated gamma-amino acids have been frequently found in many biologically active peptide natural products. Our interest lies in understanding the reactivity and conformational properties of these gamma-amino acids, which consist of carbon-carbon double bonds in the backbone. In this work, we have demonstrated the stereoselective synthesis of amino acids with conjugative multiple double bonds in the backbone using the Wittig reaction. We thoroughly investigated the stereochemistry and substrate scope of the Wittig reaction in the synthesis of conjugative multiple double bonds and successfully isolated amino acids with multiple conjugated double bonds with complete E-selectivity in excellent yields. Furthermore, we explored these amino acids for the design of helix-turn-helix type foldamers. We synthesized a series of helix-turn-helix structures and studied their conformations in single crystals and solution. By combining alpha, gamma and alpha, beta-unsaturated amino acids, we demonstrated the helix-turn-helix motifs in short sequences consisting of seven residues. Additionally, we examined the reactivity of these alpha, beta-unsaturated amino acids in peptides and monomers. Interestingly, we observed stereoselective alpha-addition of thioacids in peptides, while in monomer amides, we observed both beta- and alpha-addition products. Thus, we proved that the stereoselective alpha-addition products are possible in peptides but not in monomer building blocks. We also explored aromatic amino acids to design alpha, gamma-hybrid peptide foldamers and studied their structural analogy with hybrid peptides composed of alpha, beta-unsaturated amino acids with cis double bonds. The alpha, gammahybrid peptides composed of alpha and aromatic gamma-amino acids adopt helical structures with alternating H-bond directionality, a feature rarely observed in protein structures. In summary, our work demonstrates the synthesis, reactivity, and conformations of peptides composed of unsaturated amino acids.
