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

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. <i>cis</i>-2-Amino-<i>trans</i>-4-methylcyclohexancecarboxylic acid (<i>cis</i>,<i>trans</i>-mACHC) is preorganized to promote 9/11-helical folding. 1 : 1 α/β-peptides that consist of (1<i>S</i>,2<i>R</i>,4<i>S</i>)-<i>cis</i>,<i>trans</i>-mACHC and L-α-alanine displayed left-handed 9/11-helical conformations in solution and in crystal state.