Peptide Design: Crystal Structure of a Helical Peptide Module Attached to a Potentially Nonhelical Amino Terminal Segment

The pept ide Boc- Gly- Dpg- Gly- Val-A la- Leu-A ib- Val-A la- Leu- 0 Me 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 Gl-v-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, beta = 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 alpha,alpha-di-n-prop-vlglycine residue adopts a helical conjormation. Gly( I ) 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