Crystallographic characterization of the 9/11-helix: a left-handed helix for 1 : 1 α/β-peptides containing l-α-amino acid residues

Abstract

Atomic-resolution structural data for the 9/11-helix in α/β-peptides with alternating residue types (1 : 1 α/β-peptides) remain unavailable, despite its unique feature of adopting left-handed conformations with L-α-amino acid residues. We report a series of crystal structures for the 9/11-helix using racemic crystallography. cis-2-Amino-trans-4-methylcyclohexancecarboxylic acid (cis,trans-mACHC) is preorganized to promote 9/11-helical folding. 1 : 1 α/β-peptides that consist of (1S,2R,4S)-cis,trans-mACHC and L-α-alanine displayed left-handed 9/11-helical conformations in solution and in crystal state.