Efficient access to enantiopure γ⁴-amino acids with proteinogenic side-chains and structural investigation of γ⁴- asn and γ⁴-ser in hybrid peptide helices

Hybrid peptides composed of α- and β-amino acids have recently emerged as new class of peptide foldamers. Comparatively, γ- and hybrid γ-peptides composed of γ⁴-amino acids are less studied than their β-counterparts. However, recent investigations reveal that γ⁴-amino acids have a higher propensity to fold into ordered helical structures. As amino acid side-chain functional groups play a crucial role in the biological context, the objective of this study was to investigate efficient synthesis of γ⁴-residues with functional proteinogenic side-chains and their structural analysis in hybrid-peptide sequences. Here, the efficient and enantiopure synthesis of various N- and C-terminal free-γ⁴-residues, starting from the benzyl esters (COOBzl) of N-Cbz-protected (E)-α,β-unsaturated γ-amino acids through multiple hydrogenolysis and double-bond reduction in a single-pot catalytic hydrogenation is reported. The crystal conformations of eight unprotected γ⁴-amino acids (γ⁴-Val, γ⁴-Leu, γ⁴-Ile, γ⁴-Thr(OtBu), γ⁴-Tyr, γ⁴-Asp(OtBu), γ⁴- Glu(OtBu), and γ-Aib) reveals that these amino acids adopted a helix favoring gauche conformations along the central C^γï£ ¿C^β bond. To study the behavior of γ⁴- residues with functional side chains in peptide sequences, two short hybrid γ-peptides P1 (Ac-Aib-γ⁴-Asn-Aib-γ⁴-Leu- Aib-γ⁴-Leu-CONH₂) and P2 (Ac-Aib- γ⁴-Ser-Aib-γ⁴-Val-Aib-γ⁴-Val- CONH₂) were designed, synthesized on solid phase, and their 12-helical conformation in single crystals were studied. Remarkably, the γ⁴-Asn residue in P1 facilitates the tetrameric helical aggregations through interhelical H bonding between the side-chain amide groups. Furthermore, the hydroxyl side-chain of γ⁴-Ser in P2 is involved in the interhelical H bonding with the backbone amide group. In addition, the analysis of 87 γ⁴-residues in peptide single-crystals reveal that the γ⁴-residues in 12-helices are more ordered as compared with the 10/12- and 12/14-helices.