Helical Screw Sense of Peptide Molecules - X-ray-diffraction Structures of 2 Oligopeptides With A Single Chiral Center

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 3(10) helices stabilized by two consecutive intramolecular N-H ... O=C hydrogen bonds of the C-10 type (beta-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(alpha)-trisubstituted alpha-amino acid residue.