Left-Handed Helical Preference in an Achiral Peptide Chain Is Induced by an l‑Amino Acid in an N‑Terminal Type II β‑Turn

Oligomers of the achiral amino acid Aib adopt helical conformations in which the screw-sense may be controlled by a single N-terminal residue. Using crystallographic and NMR techniques, we show that the left- or right-handed sense of helical induction arises from the nature of the β-turn at the N terminus: the tertiary amino acid l-Val induces a left-handed type II β-turn in both the solid state and in solution, while the corresponding quaternary amino acid l-α-methylvaline induces a right-handed type III β-turn.