Hydrogen Bonded Squaramide-Based Foldable Module Induces Both β- and α‑Turns in Hairpin Structures of α‑Peptides in Water

A novel tertiary squaramido-based reverse-turn module SQ is reported, and its conformational properties are evaluated. This module is easily incorporated into a α-peptide sequence by conventional solid-phase peptide synthesis. The structure characterization of the hybrid squaramido-peptide 4 is described, showing that the turn segment induces the formation of hairpin structures in water through the formation of both αSQ- and βSQ-turns.