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.