title: Crystal structure of tert.-butyloxycarbonyl-L-prolyl-D-alanyl-D-alanyl-N-methylamide. Dimeric beta-sheet formation.
abstract: The crystal structure of the tripeptide t-Boc-L-Pro-D-Ala-D-Ala-NHCH3, monohydrate, (C17H30N4O5.H2O, molecular weight = 404.44) has been determined by single crystal X-ray diffraction. The crystals are monoclinic, space group P2(1), a = 9.2585(4), b = 9.3541(5), c = 12.4529(4)A, beta = 96.449(3) degrees, Z = 2. The peptide units are in the trans and the tBoc-Pro bond in the cis orientation. The first and third peptide units show significant deviations from planarity (delta omega = 5.2 degrees and delta omega = 3.7 degrees, respectively). The backbone torsion angles are: phi 1 = -60 degrees, psi 1 = 143.3 degrees, omega 1 = -174.8 degrees, phi 2 = 148.4 degrees, psi 2 = -143.1 degrees, omega 2 = -179.7 degrees, phi 3 = 151.4 degrees, psi 3 = -151.9 degrees, omega 3 = -176.3 degrees. The pyrrolidine ring of the proline residue adopts the C2-C gamma conformation. The molecular packing gives rise to an antiparallel beta-sheet structure formed of dimeric repeating units of the peptide. The surface of the dimeric beta-sheet is hydrophobic. Water molecules are found systematically at the edges of the sheets interacting with the urethane oxygen and terminal amino groups. Surface catalysis of an L-Ala to D-Ala epimerization process by water molecules adsorbed on to an incipient beta-sheet is suggested as a mechanism whereby crystals of the title peptide were obtained from a solution of tBoc-Pro-D-Ala-Ala-NHCH3.
authors.affiliation: Department of Biochemistry, McMaster University, Hamilton, Ontario, Canada.
authors.forename: V S
authors.initials: VS
authors.lastname: Ananthanarayanan
authors.forename: G
authors.initials: G
authors.lastname: Boulay
authors.forename: J S
authors.initials: JS
authors.lastname: Mary
authors.forename: E
authors.initials: E
authors.lastname: Subramanian
authors.forename: A G
authors.initials: AG
authors.lastname: Michel
mesh_terms: D000595:Amino Acid Sequence; D003460:Crystallization; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D009842:Oligopeptides / Q000737:chemistry*; D011487:Protein Conformation; D014867:Water / Q000737:chemistry; D014961:X-Ray Diffraction
chemical_list: D009842:Oligopeptides; D014867:Water; C048917:N-tert-butyloxycarbonyl-prolyl-alanyl-alanyl-methylamide