Helical screw sense of peptide molecules. X-Ray diffraction structures of two oligopeptides with a single chiral centre

Abstract

A crystal-state structural analysis of Z-L-Dap(pBrBz)-(Aib)₂-NHMe and Z-L-Dab(pBrBz)-(Aib)₂-NHMe has been performed by X-ray diffraction. Both peptides are folded into incipient 3₁₀ helices stabilized by two consecutive intramolecular N–H ⋯ OC hydrogen bonds of the C₁₀ type (β-bends). While Z-L-Dap(pBrBz)-(Aib)₂-NHMe gives rise to a left-handed helix, two independent molecules of opposite helical screw sense are observed in the crystals of Z-L-Dab(pBrBz)-(Aib)₂-NHMe. The latter compound represents the first example of screw sense indifference shown in the crystal state by a peptide containing a single chiral C^α-trisubstituted α-amino acid residue.