Left-Handed Helical Preference in an Achiral Peptide Chain Is Induced by an L-Amino Acid in an N-Terminal Type II beta-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 beta-turn at the N terminus: the tertiary amino acid L-Val induces a left-handed type II beta-turn in both the solid state and in solution, while the corresponding quaternary amino acid L-alpha-methylvaline induces a right-handed type III beta-turn.