Linear Oligopeptides .231. Preferred Conformation of Homo-oligomers of Alpha-aminoisobutyric-acid - Molecular and Crystal-structure of Z-(aib)7-ome

A synthetic, terminally-blocked homo-heptapeptide from alpha-aminoisobutyric acid has been studied by single-crystal X-ray diffraction and the structure refined to R = 0.05. The compound is folded into two complete turns of a regular 3(10)-helix, stabilized by five consecutive intramolecular N-H...O = C H-bonds of the beta bend III (III') type. This structure completes the series of the homo-oligomers of alpha-aminoisobutyric acid to the octapeptide level analyzed at atomic resolution by X-ray diffraction

Non coded Cα,α‐disubstituted amino acids: X‐ray diffraction analysis of a dipeptide containing (S)‐α‐methylserine

The crystal and molecular structure of the fully protected dipeptide Boc-Val-(S)-alpha-MeSer-OMe has been determined by X-ray diffraction techniques. Crystals grown from ethyl acetate/n-pentane mixtures are tetragonal, space group I4(1), with cell parameters at 295 K of a = 15.307(2), c = 18.937(10)angstrom, V = 4437.1 angstrom3, M.W. = 332.40, Z = 8, D(m) = 0.99 g/cm3 and D(x) = 0. 995 g/cm3. The structure was solved by application of direct methods and refined to an R value of 0.028 for 1773 reflections with I greater-than-or-equal 3sigma(I) collected on a CAD-4 diffractometer. Both chiral centers have the (S) configuration. The dipeptide assumes in the solid state an S shape. The urethane moiety is in the cis conformation, while the amide bond is in the common trans conformation. The conformational angles phi1, psi1 of the Val and phi2, and psi2 of the (S)-alphaMeSer fall in the F region of the phi-psi map. The isopropyl side chain of the Val residue has the (t, g-) conformation, while the Ser side chain has a g+ conformation. The hydrogen bond donor groups are all involved in intermolecular H-bond interactions. Along the quaternary axis the dipeptide molecules are linked to each other with the formation of infinite rows

BETA-ALANINE CONTAINING PEPTIDES - GAMMA-TURNS IN CYCLOTETRAPEPTIDES

In the present paper we describe the synthesis, purification, single‐crystal x‐ray analysis, solution conformational characterization, and conformational energy calculations of the cyclic tetrapeptide cyclo‐ (β‐Ala‐L‐Pro‐β‐Ala‐L‐Val). The peptide was synthesized by classical solution methods and the cyclization of the free tetrapeptide was accomplished in good yields in diluted methylene chloride solution using N,N‐dicyclohexyl‐carbodiimide. The compound crystallizes in the monoclinic space group P21 from ethanol with two independent molecules in the unit cell. All peptide bonds are trans. The nmr molecular conformation in the acetonitrile solution as well as that derived from the molecular dynamic simulation in vacuo is quite different from those observed in the solid state and is very similar to that previously observed for the parent compound cyclo‐(β‐Ala‐L‐Pro‐β‐Ala‐L‐Pro). © 1993 John Wiley & Sons, Inc. Copyright © 1993 John Wiley & Sons, Inc