SYNTHESIS, AND CRYSTAL AND MOLECULAR-STRUCTURE OF THE 3(10)-HELICAL ALPHA,BETA-DEHYDRO PENTAPEPTIDE BOC-LEU-PHE-ALA-DELTA-PHE-LEU-OME

alpha,beta-Dehydro amino acid residues are known to constrain the peptide backbone to the beta-bend conformation. A pentapeptide containing only one alpha,beta-dehydrophenylalanine (Delta Phe) residue has been synthesized and crystallized, and its solid state conformation has been determined The pentapeptide Boc-Leu-Phe-Ala-Delta Phe-Leu-OMe (C39H55N5O(8), M(w) = 721.9) M las crystallized from aqueous methanol. Monoclinic space group was P2(1), a = 10.290(2)degrees b = 17.149(2)degrees c = 12.179(2) Angstrom, beta = 96.64(1)degrees with two molecules in the unit cell. The x-ray (M0K(infinity)lambda = 0.7107A) intensity data were collected using a CAD4 diffractometer. The crystal structure was determined by direct methods and refined using least-squares technique. R = 4.4% and R(w) = 5.4% for 4403 reflections having \F-0\ greater than or equal to 3 sigma(\F-0\). A II the peptide links are trans and the pentapeptide molecule assumes 3(10)-helical conformation. The mean phi,psi values, averaged over the first four residues, are -64.4 degrees, -22.4 degrees respectively. There are three 4 --> 1 intramolecular hydrogen bonds, characteristic of 3(10)-helix. In the crystal the peptide helices interact through two head-to-tail, N-H-O intermolecular hydrogen bonds. The peptide molecules related by 2(1) screw symmetry form a skewed assembly of helices. (C) 1995 John Wiley & Sons, Inc.