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.