Promotion of Folding in Hybrid Peptides through Unconstrained γ Residues: Structural Characterization of Helices in (αγγ)nand (αγα)nSequences

Modulating the Structural Properties of α,γ-Hybrid Peptides by α-Amino Acid Residues: Uniform 12-Helix Versus "Mixed" 12/10-Helix

The most important natural α- and 3₁₀ -helices are stabilized by unidirectional intramolecular hydrogen bonds along the helical cylinder. In contrast, we report here on 12/10-helical conformations with alternately changing hydrogen-bond directionality in sequences of α,γ-hybrid peptides P1-P5 [P1: Boc-Ala-Aic-Ala-Aic-COOH; P2: Boc-Leu-Aic-Leu-Aic-OEt; P3: Boc-Leu-Aic-Leu-Aic-Leu-Aic-Aib-OMe; P4: Boc-Ala-Aic-Ala-Aic-Ala-Aic-Ala-OMe; P5: Boc-Leu-Aic-Leu-Aic-Leu-Aic-Leu-Aic-Aib-OMe; Aic=4-aminoisocaproic acid, Aib=2-aminoisobutyric acid] composed of natural α-amino acids and the achiral γ^4,4 -dimethyl substituted γ-amino acid Aic in solution and in single crystals. The helical conformations are stabilized by alternating i→i+3 and i→i-1 intramolecular hydrogen bonds. The experimental data are supported by ab initio MO calculations. Surprisingly, replacing the natural α-amino acids of the sequence by the achiral dialkyl amino acid Ac₆ c [P6: Boc-Ac₆ c-Aic-Ac₆ c-Aic-Ac₆ c-Aic-Ac₆ c-Aic-Ac₆ c-CONHMe; Ac₆ c = 1-aminocyclohexane-1-carboxylic acid] led to a 12-helix with unidirectional hydrogen bonds showing an entirely different backbone conformation. The results presented here emphasize the influence of the structure of the α-amino acid residues in dictating the helix types in α,γ-hybrid peptide foldamers and demonstrate the consequences for folding of small structural variations in the monomers.

Heterogeneous H-bonding in a foldamer helix

Structural characterization of new α/γ-peptide foldamers containing the cyclically constrained γ-amino acid I is described. Crystallographic and 2D NMR analysis shows that γ residue I promotes the formation of a 12/10-helical secondary structure in α/γ-peptides. This helix contains two different types of internal H-bond, and the data show that the 12-atom C═O(i) → H-N(i+3) H-bond is more favorable than the 10-atom C═O(i) → H-N(i-1) H-bond. Several foldamer helices featuring topologically distinct H-bonds have been discovered, but our findings are the first to show that such H-bonds may differ in their favorability.