Peptide design: Crystal structure of a helical peptide module attached to a potentially nonhelical amino terminal segment

The peptide Boc-Gly-Dpg-Gly-Val-Ala-Leu-Aib-Val-Ala-Leu-OMe has been designed to examine the structural consequences of placing a short segment with a low helix propensity at the amino terminus of a helical heptapeptide module. The Gly-Dpg-Gly segment is a potential connecting element in the synthetic construction of a helix-linker-helix motif. Crystal parameters for the peptide are P2₁, a = 8.651(3) Å, b = 46.826(13) Å, c = 16.245 Å, β = 90.13(3)*, Z = 4; 2 independent molecules/asymmetric unit. The structure reveals almost identical conformations for the two independent molecules. The backbone is completely helical for residues 2–9, with one 4 → 1 hydrogen bond and six 5 → 1 hydrogen bonds. The α,α-di-n-propylglycine residue adopts a helical conformation. Gly(1) adopts an extended conformation resulting in a nonhelical N-terminus, with the Boc group swinging away from the helix. The lateral association of helices in the b axis direction is unusual in that the helix axes are directed up or down (parallel or antiparallel) by pairs: ↓↓↑↑↓↓, etc. © 1996 John Wiley & Sons, Inc.