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Preferred Conformation (prefer + conformation)
Selected AbstractsPreferred Conformations of Peptides Containing tert -Leucine, a Sterically Demanding, Lipophilic ,-Amino Acid with a Quaternary Side-Chain C, AtomCHEMISTRY - A EUROPEAN JOURNAL, Issue 8 2005Fernando Formaggio Prof. Abstract Terminally protected homopeptides of tert -leucine, from the dimer to the hexamer, co-oligopeptides of tert -leucine in combination with ,-aminoisobutyric acid or glycine residues up to the hexamer level, and simple dipeptides representing known scaffolds for catalysts in asymmetric organic reactions were prepared by solution methods and fully characterized. The results of conformation analysis, performed by use of FT-IR absorption, NMR, CD, and X-ray diffraction techniques, indicate that this hydrophobic ,-amino acid with tetrasubstitution at the C, atom is structurally versatile. We show that it prefers extended or semiextended conformations, but can also be accommodated in folded structures, provided that these are biased by the presence of helicogenic residues. The current large-scale production of Tle, combined with its conformational preferences unravelled in this work, should make this bulky, hydrophobic, C, -trisubstituted ,-amino acid a regular building block of any strategy seeking to tailor peptides with improved catalytic and pharmacological properties. [source] Preferred conformations in the solid state of some ,-(p -phenylsulfinyl)- p -substituted acetophenonesACTA CRYSTALLOGRAPHICA SECTION B, Issue 1 2000Paulo R. Olivato Information on the geometrical structures of ,-(p -phenylsulfinyl)- p -substituted acetophenones X,PhC(O)CH2S(O)Ph,Y [X = OMe, Y = H (1); X = NO2, Y = OMe (2); X = OMe, Y = NO2 (3); IUPAC names: (1) 4-methoxyphenyl phenylsulfinylmethyl ketone; (2) 4-nitrophenyl 4-methoxyphenylsulfinylmethyl ketone; (3) 4-methoxyphenyl 4-nitrophenylsulfinylmethyl ketone] have been obtained from X-ray diffraction analyses. A comparison of these results with those previously obtained from X-ray diffraction and ab initio computations of ,-methylsulfinylacetophenone, PhC(O)CH2S(O)Me, indicated that (1) and (2) adopt in the crystal a cis1 conformation and (3) assumes a quasi-gauche geometry. The stabilization of these conformations in the crystal is discussed in terms of the dipole moment coupling, Coulombic and intramolecular charge transfer interactions between the oppositely charged atoms of the C=O and S=O dipoles. The p -substituted benzene ring is quasi -coplanar with the sulfinyl group for (1) and (3), but is quasi -perpendicular for (2). Conjugation and repulsion between the sulfinyl sulfur lone pair and the ,-benzene ring seem to be responsible for the observed geometries. [source] Chondrochloren A and B, New ,-Amino Styrenes from Chondromyces crocatus (Myxobacteria)EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2003Rolf Jansen Abstract In a screening for biologically active metabolites of the genus Chondromyces, two novel metabolites, chondrochloren A (1) and B (2), were isolated from several strains of C. crocatus. Compounds 1 and 2 are unique chloro-hydroxy-styryl amides of a highly modified C14 carboxylic acid, which comprises an unsaturated ketone, two hydroxy, two methoxy and three methyl groups. After assignment of the absolute configuration of both carbinol stereocenters by Mosher's method, NMR spectroscopic data combined with MM2 calculations allowed the prediction of the preferred conformation in solution. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Gold(III) Chloride-Catalyzed Diastereoselective Alkylation Reactions with Chiral Benzylic AcetatesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 7-8 2008Philipp Rubenbauer Abstract Gold(III) chloride was shown to be an efficient catalyst for the diastereoselective CC bond formation of various chiral para -methoxybenzylic acetates and different nucleophiles. All electrophilic acetates carried next to the reacting center a stereogenic carbon center bound to a functional group (FG), a methyl substituent and a proton. Selectivities were good (dr>80/20) in favor of the anti -product for FG=COOMe, NO2, CN and in favor of the syn -product for FG=SO2Et, PO(OEt)2. The reactions proceed most likely via a free carbocation, in which a face differentation is facilitated by a preferred conformation. Several arene nucleophiles were shown to be compatible with the catalysis conditions providing the corresponding substitution products in high yields (13 examples, 62,98%). Moreover, other nucleophiles (allyltrimethylsilane, trimethylsilyl cyanide, 2,2-dimethyl-3-(trimethylsilyloxy)butane, p -toluenesulfonamide, and acetylacetone) reacted with a representative chiral electrophile in a high yielding and diastereoselective fashion. [source] Probing structural requirements of fMLP receptor: On the size of the hydrophobic pocket corresponding to residue 2 of the tripeptideJOURNAL OF PEPTIDE SCIENCE, Issue 2 2002Susanna Spisani Abstract The conformationally constrained f- L -Met-Acnc- L -Phe-OMe (n = 4,9,12) tripeptides, analogues of the chemoattractant f- L -Met- L -Leu- L -Phe-OH, were synthesized in solution by classical methods and fully characterized. These compounds and the published f- L -Met-Xxx- L -Phe-OMe (Xxx = Aib and Acnc where n = 3, 5,8) analogues were compared to determine the combined effect of backbone preferred conformation and side-chain bulkiness at position 2 on the relation of 3D-structure to biological activity. A conformational study of all the analogues was performed in solution by FT-IR absorption and 1H-NMR techniques. In parallel, each peptide was tested for its ability to induce chemotaxis, superoxide anion production and lysozyme secretion from human neutrophils. The biological and conformational data are discussed in relation to the proposed model of the chemotactic receptor on neutrophils, in particular of the hydrophobic pocket accommodating residue 2 of the tripeptide. Copyright © 2002 European Peptide Society and John Wiley & Sons, Ltd. [source] New oxidized sterols from Aspergillus awamori and the endo -boat conformation adopted by the cyclohexene oxide systemMAGNETIC RESONANCE IN CHEMISTRY, Issue 1 2010Hao Gao Abstract Two new oxidized sterols 1 and 2 were obtained from the active fraction of a mangrove fungus Aspergillus awamori isolated from the soils around the mangrove plant Acrostichum speciosum. Their structures were elucidated using spectroscopic methods as 22E -7,-methoxy-5,,6,-epoxyergosta-8(14),22-dien-3,-ol (1) and 22E -3,-hydroxy-5,,6,,8,,14,-diepoxyergosta-22-en-7-one (2). The NMR data and complete assignments for both DMSO- d6 and CDCl3 were given. Their cytotoxic activity against A549 cell line was evaluated. Furthermore, the detailed conformation analysis for ring B (cyclohexene oxide system) of sterol 1 was given on the basis of NOEs. The endo -boat conformation was considered as the preferred conformation for ring B rather than half-chair conformation. Copyright © 2009 John Wiley & Sons, Ltd. [source] Inter-ring and endo anomeric effects, and hydrogen-bonded supramolecular motifs in two 2,4,6,8-tetraazabicyclo[3.3.0]octane derivativesACTA CRYSTALLOGRAPHICA SECTION C, Issue 4 2009Zhenfeng Zhang In 2,4,6,8-tetrakis(4-chlorophenyl)-2,4,6,8-tetraazabicyclo[3.3.0]octane, C28H22Cl4N4, the imidazolidine rings adopt envelope conformations, which are favoured by two equal endo anomeric effects. The molecule lies on a crystallographic twofold axis and molecules are linked into a three-dimensional framework via two C,H...Cl hydrogen bonds. In 2,4,6,8-tetrakis(4-methoxyphenyl)-2,4,6,8-tetraazabicyclo[3.3.0]octane, C32H34N4O4, one of the methyl groups is disordered over two sets of sites and the same methyl group participates in an intermolecular C,H...O hydrogen bond, which in turn causes a considerable deviation from the preferred conformation. There are two unequal inter-ring anomeric effects in the N,C,N groups. Molecules are linked into corrugated sheets by one C,H..., hydrogen bond and two independent C,H...O hydrogen bonds involving methoxy groups. [source] Vibrational infrared conformational studies of model peptides representing the semicrystalline domains of Bombyx mori silk fibroin,BIOPOLYMERS, Issue 5 2005Paola Taddei Abstract The structural organization of Bombyx mori silk fibroin was investigated by infrared (IR) spectroscopy. To this aim, (AG)15 and other model peptides of varying chain length, containing tyrosine (Y), valine (V), and serine (S) in the basic (AG)n sequence were synthesized by the solid phase method and their spectroscopic properties were determined. Both the position and the relative content of Y, V, and S residues in the (AG)n model system appeared critical in determining the preferred conformation, i.e., silk I, silk II, and unordered structures. Curve fitting analysis in the amide I range showed that the model peptides with prevailing silk II structure displayed different ,-sheet content, which was dependent on the degree of interruption of the (AG)n sequence. In this regard, the bands at about 1000 and 980 cm,1, specifically assigned to the AG sequence of the B. mori silk fibroin chain, were identified as marker of the degree of interruption of the (AG)n sequence. A stable silk I structure was observed only when the Y residue was located near the chain terminus, while a silk I , silk II conformational transition occurred when it was positioned in the central region of the peptide. Analysis of the second-derivative spectra in the amide I range allowed us to identify a band at 1639 cm,1 (4 , 1 hydrogen-bonded type II ,-turns), which is characteristic of the silk I conformation. © 2005 Wiley Periodicals, Inc. Biopolymers 78: 249,258, 2005 [source] Synthesis and conformation of cis -1,2-disubstituted cyclododeceneCHINESE JOURNAL OF CHEMISTRY, Issue 6 2004Xiang-Yu Han Abstract Eight 1,2-disubstituted cyclododecenes were synthesized from ,-alkoxycarbonyl-cyclododecanone and alkyl chloroformate. Their configuration and conformation determined by IR, NMR spectroscopy and X-ray diffraction analysis showed that the carbon-carbon double bond of all of the synthesized compounds has cis -configuration. and the ring skeleton of their preferred conformation is [lene2333] in solid, and they may adopt two different [lene2333] conformations, which exist in a dynamic equilibrium in solution. [source] DFT conformational studies of ,-maltotriose,JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 7 2008Udo Schnupf Abstract Recent DFT optimization studies on ,-maltose improved our understanding of the preferred conformations of ,-maltose. The present study extends these studies to ,-maltotriose with three ,- D -glucopyranose residues linked by two ,-[1,4] bridges, denoted herein as DP-3's. Combinations of gg, gt, and tg hydroxymethyl groups are included for both "c" and "r" hydroxyl rotamers. When the hydroxymethyl groups are for example, gg-gg-gg, and the hydroxyl groups are rotated from all clockwise, "c", to all counterclockwise, "r", the minimum energy positions of the bridging dihedral angles (,H and ,H) move from the region of conformational space of (,, ,), relative to (0°, 0°), to a new position defined by (+, +). Further, it was found previously that the relative energies of ,-maltose gg-gg-c and "r" conformations were very close to one another; however, the DP-3's relative energies between hydroxyl "c" or "r" rotamers differ by more than one kcal/mol, in favor of the "c" form, even though the lowest energy DP-3 conformations have glycosidic dihedral angles similar to those found in the ,-maltose study. Preliminary solvation studies using COSMO, a dielectric solvation method, point to important solvent contributions that reverse the energy profiles, showing an energy preference for the "r" forms. Only structures in which the rings are in the chair conformation are presented here. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] E and Z ,-C-Galactosylceramides by Julia,Lythgoe,Kocienski Chemistry: A Test of the Receptor-Binding Model for Glycolipid ImmunostimulantsCHEMBIOCHEM, Issue 7 2006Guangwu Chen Dr. Does aglycone linker conformation matter? Both E - and Z -linked ,- C -galactosylceramides have been prepared by application of the Julia,Lythgoe,Kocienski synthesis to a C -formylgalactose and sulfones derived from phytosphingosine. Assays of cytokine levels induced by GalCers in mice suggest that the E geometry better resembles the preferred conformations of the flexible, saturated C -glycoside and the O -glycoside when bound to the CD1d molecule of antigen-presenting cells. [source] A New, Efficient and Stereoselective Synthesis of Tricyclic and Tetracyclic Compounds by Samarium Diiodide Induced Cyclisations of Naphthyl-Substituted Arylketones,An Easy Access to Steroid-Like SkeletonsCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2007Francesca Aulenta Dr. Abstract In this report, we present the application of samarium diiodide induced cyclisations of naphthyl-substituted ketones towards an easy and stereoselective access to tri- and tetracyclic-functionalised compounds. Typical naphthalene derivatives were studied to investigate the scope and limitations of this novel cyclisation process. The model substrates studied demonstrate that the samarium ketyl cyclisations are essentially restricted to the formation of six-membered rings. The diastereoselectivity of these reactions is strongly influenced by the connection of the alkyl side chain to the naphthalene core. ,-Naphth-1-yl-substituted ketones furnished cyclisation products, such as 17 or 22,26, as single diastereomers, whereas ,-naphth-2-yl-substituted precursors gave mixtures of diastereomers,as demonstrated by the conversion of model compound 10 into tricyclic products 18,a/18,b, or that of cyclohexanone derivative 33 into tetracyclic diastereomers 34,a/34,b. Cyclic ketones as ketyl precursors furnished steroid-like tetracyclic skeletons; however, due to the cis/cis fusion of rings B/C and C/D these products have an "unnatural" bowl-like shape. Several of the cyclisation products have been identified by X-ray analyses, which not only proved the constitutions, but also the relative configurations and the preferred conformations. Steroid analogue 23 was subjected to subsequent transformations, which demonstrate that the styrene-like double bond of such compounds can be used for further structural diversification. First attempts to synthesise related azasteroids by incorporating nitrogen atoms into the ketone moiety are also reported. Thus, pyrrolidine derivatives 44 and 47 as well as piperidine derivatives 50 and 52 were subjected to samarium diiodide induced cyclisations. The expected tetracyclic products 48, 49,a/49,b, 51 and 53,a/53,b were obtained in moderate to good yields. The stereoselectivities observed follow the rules already established for the all-carbon precursors. The resulting products, bearing a nitrogen atom in ring D, are interesting azasteroid analogues with "unnatural" configuration. [source] Cyclic ,-Tetra- and Pentapeptides: Synthesis through On-Resin Cyclization and Conformational Studies by X-Ray, NMR and CD Spectroscopy and Theoretical CalculationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2005Frank Büttner Dr. Abstract The solution-phase synthesis of the simplest cyclic ,-tetrapeptide, cyclo(,-Ala)4 (4), as well as the solid-phase syntheses through side chain anchoring and on-resin cyclization of the cyclic ,3 -tetrapeptide cyclo(-,3hPhe-,3hLeu-,3hLys-,3hGln-) (14) and the first cyclic ,3 -pentapeptide cyclo(-,3hVal-,3hPhe-,3hLeu-,3hLys-,3hLys-) (19) are reported. Extensive computational as well as spectroscopic studies, including X-ray and NMR spectroscopy, were undertaken to determine the preferred conformations of these unnatural oligomers in solution and in the solid state. cyclo(,-Ala)4 (4) with no chiral side chains is shown to exist as a mixture of rapidly interchanging conformers in solution, whereas inclusion of chiral side chains in the cyclo-,3 -tetrapeptide causes stabilization of one dominating conformer. The cyclic ,3 -pentapeptide on the other hand shows larger conformational freedom. The X-ray structure of achiral cyclo(,-Ala)4 (4) displays a Ci -symmetrical 16-membered ring with adjacent CO and N-H atoms pointing pair wise up and down with respect to the ring plane. CD spectroscopic examinations of all cyclic ,-peptides were undertaken and revealed results valuable as starting point for further structural investigations of these entities. [source] | ||||||||||||||||||||||