title: Synthesis, and crystal and molecular structure of the 3(10)-helical alpha,beta-dehydro pentapeptide Boc-Leu-Phe-Ala-delta Phe-Leu-OMe.
abstract: 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 (C39H55N5O8, Mw = 721.9) was crystallized from aqueous methanol. Monoclinic space group was P2(1), a = 10.290(2) degrees, b = 17.149(2) degrees, c = 12.179(2) A, beta = 96.64(1) degrees with two molecules in the unit cell. The x-ray (MoK alpha, 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 Rw = 5.4% for 4403 reflections having magnitude of F0 > or = 3 sigma(magnitude of F0). All 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.
authors.affiliation: Department of Physics, Indian Institute of Science, Bangalore.
authors.forename: K R
authors.initials: KR
authors.lastname: Rajashankar
authors.forename: S
authors.initials: S
authors.lastname: Ramakumar
authors.forename: T K
authors.initials: TK
authors.lastname: Mal
authors.forename: R M
authors.initials: RM
authors.lastname: Jain
authors.forename: V S
authors.initials: VS
authors.lastname: Chauhan
mesh_terms: D000595:Amino Acid Sequence; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000138:chemical synthesis* / Q000737:chemistry; D017433:Protein Structure, Secondary
chemical_list: D009842:Oligopeptides; C092861:tert-butoxycarbonyl-leucyl-phenylalanyl-alanyl-alpha,beta-dehydrophenylalanyl-leucine methyl ester