Induction of unexpected left-handed helicity by an N-terminal L -amino acid in an otherwise achiral peptide chain

Peptide helices containing L-amino acids are typically right-handed. Exceptions are peptide helices containing the achiral amino acids 2-aminoisobutyric acid and glycine with a single chiral amino acid at the Nterminus. These helices are left-handed when the N-terminal residue is a common tertiary proteinogenic amino acid, such as L-valine (see picture, left), but right-handed when the N-terminal residue is the quaternary amino acid L-α-methylvaline (right).