Efficient Access to Enantiopure γ4-Amino Acids with Proteinogenic Side-Chains and Structural Investigation of γ4-Asn and γ4-Ser in Hybrid Peptide Helices

Recent Advances in Amphipathic Peptidomimetics as Antimicrobial Agents to Combat Drug Resistance

The development of antimicrobial drugs with novel structures and clear mechanisms of action that are active against drug-resistant bacteria has become an urgent need of safeguarding human health due to the rise of bacterial drug resistance. The discovery of AMPs and the development of amphipathic peptidomimetics have lay the foundation for novel antimicrobial agents to combat drug resistance due to their overall strong antimicrobial activities and unique membrane-active mechanisms. To break the limitation of AMPs, researchers have invested in great endeavors through various approaches in the past years. This review summarized the recent advances including the development of antibacterial small molecule peptidomimetics and peptide-mimic cationic oligomers/polymers, as well as mechanism-of-action studies. As this exciting interdisciplinary field is continuously expanding and growing, we hope this review will benefit researchers in the rational design of novel antimicrobial peptidomimetics in the future.

The influence of backbone fluorination on the helicity of α/γ-hybrid peptides

Peptides that are composed of an alternating pattern of α- and γ-amino acids are potentially valuable as metabolism-resistant bioactive agents. For optimal function, some kind of conformational restriction is usually required to either stabilize the dominant 12-helix, or else to divert the peptide away from this conformation in a controlled way. Herein, we explore stereoselective fluorination as a method for controlling the conformations of α/γ-hybrid peptides. We show through a combination of X-ray, NMR and CD analyses that fluorination can either stabilize or disrupt the 12-helix, depending on the fluorine stereochemistry. These findings could inform the ongoing development of diverse functional hybrid peptides.

Stereoselective synthesis of backbone extended π-conjugated amino esters

Utilization of the Wittig reaction to synthesize conjugative multiple double bonds is rare. We examined the utility of the Wittig reaction to construct conjugative two and three carbon-carbon double bonds on the N-protected amino acid backbone. The ethyl esters of N-Boc amino acids with multiple carbon-carbon double bonds in the backbone were isolated in excellent yields with exceptional E-selectivity of the double bonds. The allylic alcohols of α,β-unsaturated γ-amino esters were selectively synthesized from the DIBAL-H and BF₃·OEt₂. The allylic alcohols were transformed into aldehydes using IBX oxidation. Using this protocol, we synthesized ethyl esters of N-Boc-(E,E)-α,β,γ,δ-unsaturated ε-amino acids with various side-chain functionalities and ethyl esters of N-Boc-(E,E,E)-α,β,γ,δ,ε,ζ-unsaturated η-amino acids with excellent yields. We speculated the exceptional E-selectivity is probably due to the stabilization of the planar transition state of the Wittig reaction with the double bond p-orbitals. No racemization was observed in the synthesis of amino acids. The reported process may serve as an excellent route to synthesize multiple conjugative carbon-carbon double bonds.

Structural Investigation of Hybrid Peptide Foldamers Composed of α-Dipeptide Equivalent β-Oxy-δ5 -amino Acids

Due to their equivalent lengths, δ-amino acids can serve as surrogates of α-dipeptides. However, δ-amino acids with proteinogenic side chains have not been well studied because of synthetic difficulties and because of their insolubility in organic solvents. Recently we reported the spontaneous supramolecular gelation of δ-peptides composed of β(O)-δ⁵ -amino acids. Here, we report the incorporation of β(O)-δ⁵ -amino acids as guests into the host α-helix, α,γ-hybrid peptide 12-helix and their single-crystal conformations. In addition, we studied the solution conformations of hybrid peptides composed of 1:1 alternating α and β(O)-δ⁵ -amino acids. In contrast to the control α-helix structures, the crystal structure of peptides with β(O)-δ⁵ -amino acids exhibit α-helical conformations consisting of both 13- and 10-membered H-bonds. The α,δ-hybrid peptide adopted mixed 13/11-helix conformation in solution with alternating H-bond directionality. Crystal-structure analysis revealed that the α,γ⁴ -hybrid peptide accommodated the guest β(O)-δ⁵ -amino acid without significant deviation to the overall helix folding. The results reported here emphasize that β(O)-δ⁵ -amino acids with proteinogenic side chains can be accommodated into regular α-helix or 12-helix as guests without much deviation of the overall helix folding of the peptides.

Backbone Engineered γ-Peptide Amphitropic Gels for Immobilization of Semiconductor Quantum Dots and 2D Cell Culture

We are reporting a spontaneous supramolecular assembly of backbone engineered γ-peptide scaffold and its utility in the immobilization of semiconductor quantum dots and in cell culture. The stimulating feature of this γ-peptide scaffold is that it efficiently gelates both aqueous phosphate buffers and aromatic organic solvents. A comparative and systematic investigation reveals that the greater spontaneous self-aggregation property of γ-peptide over the α- and β-peptide analogues is mainly due to the backbone flexibility, increased hydrophobicity, and π-π stacking of γ-phenylalanine residues. The hydrogels and organogels obtained from the γ-peptide scaffold have been characterized through field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), FT-IR, circular dichroism (CD), wide-angle X-ray diffraction, and rheometric study. Additionally, the peptide hydrogel has displayed a stimuli-responsive and thixotropic signature, which leads to the injectable hydrogels. 2D cell culture studies using normal and cancer cell lines reveal the biocompatibility of γ-peptide hydrogels. Further, the immobilization of semiconductor core-shell quantum dots in the transparent γ-peptide organogels showed ordered arrangement of quantum dots along the peptide fibrillar network with retaining photophysical property. Overall, γ-peptide scaffolds may serve as potential templates for the design of new functional biomaterials.

Impact of γ-Amino Acid Residue Preorganization on α/γ-Peptide Foldamer Helicity in Aqueous Solution

α/γ-Peptide foldamers containing either γ(4)-amino acid residues or ring-constrained γ-amino acid residues have been reported to adopt 12-helical secondary structure in nonpolar solvents and in the solid state. These observations have engendered speculation that the seemingly flexible γ(4) residues have a high intrinsic helical propensity and that residue-based preorganization may not significantly stabilize the 12-helical conformation. However, the prior studies were conducted in environments that favor intramolecular H-bond formation. Here, we use 2D-NMR to compare the ability of γ(4) residues and cyclic γ residues to support 12-helix formation in more challenging environments, methanol and water. Both γ residue types support 12-helical folding in methanol, but only the cyclically constrained γ residues promote helicity in water. These results demonstrate the importance of residue-based preorganization strategies for achieving stable folding among short foldamers in aqueous solution.