An unnatural tripeptide structure containing intramolecular double H-bonds mimics a turn hairpin conformation

A series of unnatural tripeptides, each consisting of two aromatic γ-amino acid residues and an ϖ-amino acid residue, are designed to probe their folding into hairpin conformations. The ϖ-amino acid residues, with aliphatic or aromatic spacers of different sizes, serve as the loop of the hairpins. Studies based on one-dimensional (1D) 1H NMR performed at different concentrations, solvent polarity, and temperature, along with 2D-NMR studies, demonstrated that the doubly H-bonded aromatic γ-amino acid residues play important roles in driving these tripeptides into the hairpin conformation. The loop based on 5-aminovaleric acid, which offers a four-carbon (CH2)4 spacer, enhanced the stability of the corresponding hairpin, while loops having a shorter, a longer and a more rigid spacer disfavored the formation of the hairpins. Results from computational studies are in good agreement with the experimental observations. Furthermore, the crystal structure of peptide 1b revealed the expected hairpin conformation in the solid state. This turn motif, which contains H-bonded aromatic γ-amino acid residues as the core unit and an ϖ-amino acid residue serving as the loop, provides a new platform that can be used to obtain a variety of turn conformations by incorporating diverse amino acids into the loops.

Major Factors for the Persistent Folding of Hybrid α, β, γ-Hybrid Peptides Into Hairpins

Factors responsible for the persistent adoption of hairpin conformations by hybrid oligopeptides, each having a central β/α dipeptide segment flanked by aromatic γ-amino acid (γAr) residues, are probed. Our recent studies revealed that tetrapeptide 1 and 2, having central dipeptide segments consisting of β-alanine (β-Ala) and glycine (Gly), and L-β-homophenylalanine (L-β-homoPhe) and Gly residues, respectively, that are flanked by γAr residues, fold into well-defined, expanded β-turns with doubly H-bonded γAr residues. Replacing the γAr residues of 1 and 2 with L-Val and L-Leu residues results in tetrapetides 1 ' and 2 ' that fail to fold into defined conformations, which confirms the decisive role played by the H-bonded γAr residues in the promoting folding of 1 and 2. Attaching L-Val and L-Leu residues to the termini of 1 affords hexapeptide 1a. With an additional H-bond between its L-Val and L-Leu residues, peptide 1a folds into a hairpin with higher stability than that of 1, indicating that the expanded β-turn can nucleate and stabilize β-hairpin with longer β-strands. Attaching L-Val and L-Leu residues to the termini of 2 affords hexapeptide 2a. Substituting the L-β-homoPhe residue of 2a with a D-β-homoPhe residue gives hexapeptide 2b. Surprisingly, hexapeptide 2a fold into a hairpin showing the similar stability as those of tetrapeptides 1 and 2. Hexapeptide 2b, with its combination of a D-β-homoPhe residue and the L-Val/L-Leu pair, fold into a hairpin that is significantly more stable than the other hybrid peptides, demonstrating that a combination of hetero-chirality between the β-amino acid residue of the dipeptide loop and the α-amino acid residues of the β-strands enhances the stability of the resultant β-hairpin.