EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 31 2009
Ulrich Siemeling
Abstract The stable, crystalline N-heterocyclic diaminocarbene fc[N(CH2tBu)-C-N(CH2tBu)] (2d, fc = 1,1,-ferrocenediyl) was prepared by deprotonation of its formamidinium precursor fc[N(CH2tBu)-CH-N(CH2tBu)][BF4] (1d) and used for the preparation of the 16 valence electron complexes [Mo(2d)(CO)4], [RhCl(2d)(cod)] (cod = 1,5-cyclooctadiene) and [RhCl(2d)(CO)2]. 1d, 2d and [RhCl(2d)(cod)] were structurally characterised by single-crystal X-ray diffraction studies. The electrochemical properties of 2d, its 2-adamantyl analogue 2c, its complex [RhCl(2d)(CO)2] and of the precursors 1d and 1,1,-bis(neopentylamino)ferrocene were investigated by electrochemistry. The carbenes are easily oxidised to the corresponding radical cation, whose persistent nature is unprecedented in the chemistry of N-heterocyclic carbenes. The spin density is located at the Fe atom and the carbene C atom according to the results of EPR spectroscopic studies and DFT calculations.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Modular Routes Towards New N,O-Bidentate Ligands Containing an Electronically Delocalised ,-Enaminone Chelating Backbone

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 24 2008
Udo Beckmann
Abstract Polyketones are synthesised by a transition-metal-catalysed copolymerisation of olefins and carbon monoxide. Nickel complexes with N,O-chelating ligands turned out to be promising catalysts in that field. In this work a series of new N,O ligands with an electronically delocalised ,-enaminone backbone were synthesised and fully characterised. The ligand design was inspired by the ligand found in the most efficient nickel catalyst for polyketone synthesis and developed to a highly modular LEGO® -like arsenal of reactions to versatile substituted ,-enaminone ligands. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

Electrophosphorescent Polyfluorenes Containing Osmium Complexes in the Conjugated Backbone,

ADVANCED FUNCTIONAL MATERIALS, Issue 9 2008
Chen-Han Chien
Abstract Electrophosphorescent copolymers have been synthesized by covalent bonding of a red-emitting osmium complex Os(bpftz), which contains two 3-trifluoromethyl-5-(4- tert -butyl-2-pyridyl)triazolate (bpftz) cyclometalated ligands, into the backbone of a bipolar polyfluorene (PF) copolymer. Employing these copolymers, a highly efficient red polymer light-emitting diode has been realised that has an external quantum efficiency of 18.0%, a maximum brightness of 38,000,cd,m,2, and an emission centered at 618,nm. In addition, after incorporating appropriate amounts of green-emitting benzothiadiazole (BT) and the aforementioned Os(bpftz) into the bipolar PF, an efficient white-light electroluminescent polymer is obtained that displays simultaneous blue, green, and red emissions. [source]

Stability of Hoogsteen -Type Triplexes , Electrostatic Attraction between Duplex Backbone and Triplex-Forming Oligonucleotide (TFO) Using an Intercalating Conjugate

HELVETICA CHIMICA ACTA, Issue 5 2008
Daniel Globisch
Abstract Syntheses are described for two novel twisted intercalating nucleic acid (TINA) monomers where the intercalator comprises a benzene ring linked to a naphthalimide moiety via an ethynediyl bridge. The intercalators Y and Z have a 2-(dimethylamino)ethyl and a methyl residue on the naphthalimide moiety, respectively. When used as triplex-forming oligonucleotides (TFOs), the novel naphthalimide TINAs show extraordinary high thermal stability in Hoogsteen -type triplexes and duplexes with high discrimination of mismatch strands. DNA Strands containing the intercalator Y show higher thermal triplex stability than DNA strands containing the intercalator Z. This observation can be explained by the ionic interaction of the protonated dimethylamino group under physiological conditions, targeting the negatively charged phosphate backbone of the duplex. This interaction leads to an extra binding mode between the TFO and the duplex, in agreement with molecular-modeling studies. We believe that this is the first example of an intercalator linking the TFO to the phosphate backbone of the duplex by an ionic interaction, which is a promising tool to achieve a higher triplex stability. [source]

Oligonucleotide Analogues with Integrated Bases and Backbone.

HELVETICA CHIMICA ACTA, Issue 5 2007
Part 1
Abstract The self-complementary (Z)-configured U*[ce]A(*) dinucleotide analogues 6, 8, 10, 12, 14, and 16, and the A*[ce]U(*) dimers 19, 21, 23, 25, 27, and 29 were prepared by partial hydrogenation of the corresponding ethynylene linked dimers. Photolysis of 14 led to the (E)-alkene 17. These dinucleotide analogues associate in CDCl3 solution, as evidenced by NMR and CD spectroscopy. The thermodynamic parameters of the duplexation were determined by van't Hoff analysis. The (Z)-configured U*[ce]A(*) dimers 14 and 16 form cyclic duplexes connected by Watson,Crick H-bonds, the (E)-configured U*[ce]A dimer 17 forms linear duplexes, and the U*[ce]A(*) allyl alcohols 6, 8, 10, and 12 form mixtures of linear and cyclic duplexes. The C(6/I)-unsubstituted A*[ce]U allyl alcohols 19 and 23 form linear duplexes, whereas the C(6/I)-substituted A*[ce]U* allyl alcohols 21 and 25, and the C(5,/I)-deoxy A*[ce]U(*) dimers 27 and 29 also form minor amounts of cyclic duplexes. The influence of intra- and intermolecular H-bonding of the allyl alcohols and the influence of the base sequence upon the formation of cyclic duplexes are discussed. [source]

Oligonucleotide Analogues with Integrated Bases and Backbone.

HELVETICA CHIMICA ACTA, Issue 4 2007
Part 1
Abstract The self-complementary tetrameric propargyl triols 8, 14, 18, and 21 were synthesized to investigate the duplex formation of self-complementary, ethynylene-linked UUAA, AAUU, UAUA, and AUAU analogues with integrated bases and backbone (ONIBs). The linear synthesis is based on repetitive Sonogashira couplings and C -desilylations (34,72% yield), starting from the monomeric propargyl alcohols 9 and 15 and the iodinated nucleosides 3, 7, 11, and 13. Strongly persistent intramolecular H-bonds from the propargylic OH groups to N(3) of the adenosine units prevent the gg -type orientation of the ethynyl groups at C(5,). As such, an orientation is required for the formation of cyclic duplexes, this H-bond prevents the formation of duplexes connected by all four base pairs. However, the central units of the UAUA and AAUU analogues 18 and 14 associate in CDCl3/(D6)DMSO 10,:,1 to form a cyclic duplex characterized by reverse Hoogsteen base pairing. The UUAA tetramer 8 forms a cyclic UU homoduplex, while the AUAU tetramer 21 forms only linear associates. Duplex formation of the O -silylated UUAA and AAUU tetramers is no longer prevented. The self-complementary UUAA tetramer 22 forms Watson,Crick - and Hoogsteen -type base-paired cyclic duplexes more readily than the sequence-isomeric AAUU tetramer 23, further illustrating the sequence selectivity of duplex formation. [source]

Oligonucleotide Analogues with Integrated Bases and Backbone.

HELVETICA CHIMICA ACTA, Issue 12 2006

Abstract The self-complementary UA and AU dinucleotide analogues 41,45, 47, 48, and 51,60 were prepared by Sonogashira coupling of 6-iodouridines with C(5,) -ethynylated adenosines and of 8-iodoadenosines with C(5,) -ethynylated uridines. The dinucleotide analogues associate in CDCl3 solution. The C(6/I) -unsubstituted AU dimers 51 and 54 prefer an anti -oriented uracilyl group and form stretched linear duplexes. The UA propargyl alcohols 41 and 43,45 possess a persistent intramolecular O(5,/I)H,,,N(3/I) H-bond and, thus, a syn -oriented adeninyl and a gt - or tg -oriented ethynyl moiety; they form corrugated linear duplexes. All other dimers form cyclic duplexes characterized by syn -oriented nucleobases. The preferred orientation of the ethynyl moiety (the C(4,),C(5,) torsion angle) defines a conformation between gg and one where the ethynyl group eclipses O(4,/I). The UA dimers 42, 47, and 48 form Watson,Crick H-bonds, the AU dimers 56 and 58,60 H-bonds of the Watson,Crick -type, the AU dimers 53 and 55 reverse- Hoogsteen, and 57Hoogsteen H-bonds. The pairing mode depends on the substituent of C(5,/I) (H, OSiiPr3; OH) and on the H-bonds of HOC(5,/I) in the AU dimers. Association constants were derived from the concentration-dependent chemical shift for HN(3) of the uracilyl moiety; they vary from 45,104,M,1 for linear duplexes to 197,2307,M,1 for cyclic duplexes. The thermodynamic parameters were determined by van't Hoff analysis of the temperature-dependence of the (concentration-dependent) chemical shift for HN(3) of the uracilyl moiety. Neglecting stacking energies, one finds an average energy of 3.5,4.0,kcal/mol per intermolecular H-bond. Base stacking is evidenced by the temperature-dependent CD spectra. The crystal structure of 54 shows two antiparallel chains of dimers connected by Watson-Crick H-bonds. The chains are bridged by a strong H-bond between the propargylic OH and OC(4) and by weak reverse A,,,A Hoogsteen H-bonds. [source]

High-Efficiency White-Light Emission from a Single Copolymer: Fluorescent Blue, Green, and Red Chromophores on a Conjugated Polymer Backbone,

ADVANCED MATERIALS, Issue 8 2007
J. Luo
The synthesis and properties of a single copolymer incorporating well-separated blue, green, and red chromophores on a single conjugated polymer backbone are reported. This copolymer is shown to have CIE coordinates of (0.35,0.34) and a luminance efficiency of 6.2,cd,A,1. The color coordinates of the resulting white-light emission remained extremely stable over a wide range of driving voltages. [source]

Mechanism of the Asymmetric Sulfoxidation in the Esomeprazole Process: Effects of the Imidazole Backbone for the Enantioselection

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 6 2009
Muthu Seenivasaperumal
Abstract The asymmetric sulfoxidation reaction of imidazole-based prochiral sulfides was studied to explore the mechanistic details of the highly efficient esomeprazole process, which is one of the few industrial scale catalytic asymmetric procedures. The synthetic studies revealed that the smallest subunit governing the selectivity in the esomeprazole process is an imidazole ring. Thus, by using the esomeprazole procedure methyl imidazole sulfide could be oxidized as efficiently as its several functionalized derivatives, including pyrmetazol. However, alkylation of the imidazole nitrogen led to a major drop of the enantioselectivity. Our atmospheric pressure chemical ionization-mass spectrometry (APCI/MS) studies indicate that addition of small amounts of water to the reaction mixture facilitates the formation of mononuclear titanium species, which are the active catalytic intermediates of the selective oxidation reaction. One of the most important features of the esomeprazole procedure is that amine additives increase the enantioselectivity of the oxidation process. The NMR studies of the presumed reaction intermediates show that under catalytic conditions the amines are able to coordinate to titanium and dissociate the coordinated imidazole substrate. The density functional theory (DFT) modelling studies provided new insights in the mechanism of the asymmetric induction. It was found that the oxidation requires a lower activation energy if the imidazole sulfide precursor does not coordinate to titanium. Two possible reaction paths were explored for this out of sphere oxidation mechanism. The most important interaction governing the enantioselection is hydrogen bonding between the NH of the imidazole ring and the chiral tartrate ligand on titanium. Furthermore, the oxidation reaction imposes an important structural constraint to the TS structure involving a linear arrangement of the peroxide oxygens and the sulfur atom. This constraint and the N coordination of imidazole leads to a very strained structure for the inner sphere mechanism of the oxidation, which leads to a much higher activation barrier than the corresponding out of sphere process, and therefore it is unlikely. [source]

Synthesis of Diastereomeric 1,4-Diphosphine Ligands Bearing Imidazolidin-2-one Backbone and Their Application in Rh(I)-Catalyzed Asymmetric Hydrogenation of Functionalized Olefins

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4 2005
Jian Zhang
Abstract The diastereomeric 1,4-diphosphine ligands, (S,S,S,S)- 1a, (R,S,S,R)- 1b and (R,S,S,S)- 1c, with the imidazolidin-2-one backbone were synthesized, and utilized for an investigation of the effects of backbone chirality on the enantioselectivity in the Rh(I)-catalyzed hydrogenation of various functionalized olefinic substrates. It was found that the catalytic efficiencies are largely dependent on the configurations of the ,-carbons to phosphine. Thus, the Rh complex of the pseudo- C2 -symmetrical diphosphine, (R,S,S,S)- 1c, showed excellent enantioselectivities (93.0,98.6% ees) in the hydrogenations of a broad spectrum of substrates, and especially in the hydrogenations of methyl ,-(N -acetyamino)-,-arylacrylates (95.3,97.0% ees). However, the enantioselectivities obtained with the C2 -symmetrical (R,S,S,R)- 1b were largely dependent on the substrate (19.8,97.3% ees). The Rh complex of ligand 1a having the (S,S,S,S)-configuration showed the lowest catalytic efficiency for all of the substrates examined (0,84.8% ees). [source]

New angle-dependent potential energy function for backbone,backbone hydrogen bond in protein,protein interactions

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 5 2010
Hwanho Choi
Abstract Backbone,backbone hydrogen bonds (BBHBs) are one of the most abundant interactions at the interface of protein,protein complex. Here, we propose an angle-dependent potential energy function for BBHB based on density functional theory (DFT) calculations and the operation of a genetic algorithm to find the optimal parameters in the potential energy function. The angular part of the energy funtion is assumed to be the product of the power series of sine and cosine functions with respect to the two angles associated with BBHB. Two radial functions are taken into account in this study: Morse and Leonard-Jones 12-10 potential functions. Of these two functions under consideration, the former is found to be more accurate than the latter in terms of predicting the binding energies obtained from DFT calculations. The new HB potential function also compares well with the knowledge-based potential derived by applying Boltzmann statistics for a variety of protein,protein complexes in protein data bank. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010 [source]

Free Radical Graft Copolymerization of Methyl Methacrylate onto Polyolefin Backbone: Kinetics Modeling through Model Compounds Approach

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 13-14 2009
Thierry Badel
Abstract The grafting of poly(methyl methacrylate) (PMMA) from poly[ethylene- co -(1-octene)] in the molten state by in situ radical polymerization of methyl methacrylate (MMA) was investigated through a kinetic modeling using a model compound approach. We resorted to pentadecane (C15H32) and 2,6,10,15,19,23-hexamethyltetracosane (Squalane, C30H62) as models for both the copolymer ethylene and octene moieties. The attention was focused on the simulation of MMA conversion and PMMA average polymerization degree according to temperature, reaction time, and initiator ratio required for the polymerization of MMA in the presence of alkoxyl radicals and alkanes. [source]

White-Light Emission from a Single Polymer with Singlet and Triplet Chromophores on the Backbone

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 24 2006
Hongyu Zhen
Abstract Summary: A strategy to generate an efficient white-light emission has been developed by mixing fluorescence and phosphorescence emission from a single polymer. Fluorene is used as the blue-emissive component, benzothiadiazole (BT) and the iridium complex [(btp)2Ir(tmd)] are incorporated into a polyfluorene backbone, respectively, as green- and red-emissive chromophores by Suzuki polycondensation. By changing the contents of BT and [(btp)2Ir(tmd)] in the polymer, the electroluminescence spectrum from a single polymer can be adjusted to achieve white-light emission. A white polymeric light-emitting diode (WPLED) with a structure of ITO/PEDOT:PSS/PVK/PFIrR1G03/CsF/Al shows a maximum external quantum efficiency of 3.7% and the maximum luminous efficiency of 3.9 cd,·,A,1 at the current density of 1.6 mA,·,cm,2 with the CIE coordinates of (0.33, 0.34). The maximum luminance of 4,180 cd,·,m,2 is achieved at the current density of 268 mA,·,cm,2 with the CIE coordinates of (0.31, 0.32). The white-light emissions from such polymers are stable in the white-light region at all applied voltages, and the electroluminescence efficiencies decline slightly with the increasing current density, thus indicating that the approach of incorporating singlet and triplet species into the polymer backbone is promising for WPLEDs. Structure of PFIrR1G04 and the EL spectra of its devices under various voltages. Device structure: ITO/PEDOT:PSS/PVK/polymer/CsF/Al. [source]

Gel-to-Sol and Sol-to-Gel Transitions Utilizing the Interaction of , -Cyclodextrin with Dodecyl Side Chains Attached to a Poly(acrylic acid) Backbone

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 10 2005
Itsuro Tomatsu
Abstract Summary: By utilizing the interaction of , -cyclodextrin (, -CD) with dodecyl side chains in polymers of x mol-% dodecyl-modified poly(acrylic acid) (p(AA/C12(x))), systems that undergo gel-to-sol and sol-to-gel transitions were successfully constructed. Rheological experiments indicated that addition of , -CD to the hydrogel of p(AA/C12(5)) caused a drastic decrease in the viscosity, while addition of oligo(, -CD) to the solution of p(AA/C12(2)) led to a remarkable increase in the viscosity. Photographs for a gel-to-sol transition upon addition of , -CD to 5.0 g,·,L,1 p(AA/C12(5)). [source]

Polyfluorophores on a DNA Backbone: Sensors of Small Molecules in the Vapor Phase,

ANGEWANDTE CHEMIE, Issue 39 2010
Dr. Florent Samain
Immer die richtige Antwort: An Polyethylenglycol-Polystyrol-Kügelchen konjugierte Oligodesoxyfluoroside (ODFs) wurden als Fluoreszenzsensoren für organische Dämpfe genutzt (siehe Bild). Die ODFs , DNA-artige Oligomere, in denen Fluorophor-, Spacer- und Quencher-Einheiten die DNA-Basen ersetzen , waren ausreichend elektronisch verschieden, um deutlich unterschiedlich auf chemisch verschiedene flüchtige organische Verbindungen zu reagieren. [source]

Enantioselective Gold Catalysis: Opportunities Provided by Monodentate Phosphoramidite Ligands with an Acyclic TADDOL Backbone,

ANGEWANDTE CHEMIE, Issue 11 2010
Henrik Teller
Besser ohne Ring: Wird die Isopropylidenacetal-Einheit von den bekannten TADDOL-Liganden entfernt, nimmt die Leistung der daraus hergestellten Phosphoramiditliganden in der asymmetrischen Goldkatalyse zu (siehe Schema; Ts=Toluol-4-sulfonyl). Kristallographisch wurden eine Bindetasche mit dreizähliger Symmetrie und Wechselwirkungen durch den Raum zwischen den Arenringen des Liganden und dem Goldzentrum ermittelt. [source]

Control of the Helicity of Poly(phenylacetylene)s: From the Conformation of the Pendant to the Chirality of the Backbone,

ANGEWANDTE CHEMIE, Issue 8 2010
Iria Louzao Dr.
Auswahl der Gangrichtung: Das Konformerengleichgewicht der Substituenten eines Poly(phenylacetylens), und mit diesem die Helizität des Polymers, lässt sich reversibel verändern. Durch Komplexierung mit Metallkationen wie Ba2+ oder einen Wechsel in der Lösungsmittelpolarität kann die gewünschte Gangrichtung ausgewählt werden. Die mechanistische Grundlage dieses Phänomens wird im Detail erklärt. [source]

Consequences of Isostructural Main-Chain Modifications for the Design of Antimicrobial Foldamers: Helical Mimics of Host-Defense Peptides Based on a Heterogeneous Amide/Urea Backbone,

ANGEWANDTE CHEMIE, Issue 2 2010
Paul Claudon
Wie Zwillinge: Oligoharnstoffe und ,-Peptide sind isostere und quasi-isostrukturelle helicale Foldamere mit besonderen Eigenschaften bei der biomolekularen Erkennung. Die Kombination dieser beiden Rückgrate in Harnstoff-Amid-Hybriden (siehe Bild) ergab wirksamere antimikrobielle helicale Foldamere mit verminderter Cytotoxizität. [source]

Highly Efficient "Grafting onto" a Polypeptide Backbone Using Click Chemistry,

ANGEWANDTE CHEMIE, Issue 49 2009
Amanda
Klick und passt: Um dicht gepfropfte Poly(,-propargyl- L -glutamat)- g -poly(ethylenglycol)-Polypeptide zu erhalten, wurde zuerst durch Ringöffnungspolymerisation eines N -Carboxyanhydrids eine starre Poly(,-propargyl- L -glutamat)-,-Helix aufgebaut, an die dann durch eine Klick-Reaktion Poly(ethylenglycol)(PEG)-Seitenketten verschiedener Länge (750 bis 5000,g,mol,1) angebracht wurden. Die Pfropfeffizienz erreichte dabei Werte über 96,%. [source]

ChemInform Abstract: From C2 - to D2 -Symmetric: Atropos Phosphoramidites with a D2 -Symmetric Backbone as Highly Efficient Ligands in Cu- Catalyzed Conjugate Additions.

CHEMINFORM, Issue 39 2010
Hui Zhang
Abstract The axially chiral phosphoramidites (I) afford similar yields and enantioselectivities in the transformations tested. [source]

The Synthesis and Application of Novel C2 -Symmetric Chiral N,N,O,O Bisoxazoline Ligands with a Ferrocene Backbone.

CHEMINFORM, Issue 17 2007
Genghong Hua
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source]

A Hollow Tetrahedral Cage of Hexadecagold Dianion Provides a Robust Backbone for a Tuneable Sub-Nanometer Oxidation and Reduction Agent via Endohedral Doping.

CHEMINFORM, Issue 8 2007
Michael Walter
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source]

Design and Synthesis of 1,4-Amino Alcohol Ligands with a Chiral Cyclopropane Backbone for Asymmetric Diethylzinc Addition to Aromatic Aldehydes.

CHEMINFORM, Issue 48 2006
Jiangchun Zhong
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source]

Development of Chiral (S)-Prolinol-Derived Ligands for Palladium-Catalyzed Asymmetric Allylic Alkylation: Effect of a Siloxymethyl Group on the Pyrrolidine Backbone.

CHEMINFORM, Issue 6 2005
Youichi Tanaka
Abstract For Abstract see ChemInform Abstract in Full Text. [source]

ChemInform Abstract: Sesquiterpene Hydrocarbons with Trifarane Backbone in the Liverwort Trocholejeunea sandvicensis.

CHEMINFORM, Issue 49 2001
Mesmin Mekem Sonwa
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

ChemInform Abstract: Biphenyls as Surrogates of the Steroidal Backbone.

CHEMINFORM, Issue 47 2001
Part 2.
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

The Structure of a Novel Neutral Lipid,A from the Lipopolysaccharide of Bradyrhizobium elkanii Containing Three Mannose Units in the Backbone

CHEMISTRY - A EUROPEAN JOURNAL, Issue 9 2010
Iwona Komaniecka Dr.
Abstract The chemical structure of the lipid,A of the lipopolysaccharide (LPS) from Bradyrhizobium elkanii USDA 76 (a member of the group of slow-growing rhizobia) has been established. It differed considerably from lipids,A of other Gram-negative bacteria, in that it completely lacks negatively charged groups (phosphate or uronic acid residues); the glucosamine (GlcpN) disaccharide backbone is replaced by one consisting of 2,3-dideoxy-2,3-diamino- D -glucopyranose (GlcpN3N) and it contains two long-chain fatty acids, which is unusual among rhizobia. The GlcpN3N disaccharide was further substituted by three D -mannopyranose (D -Manp) residues, together forming a pentasaccharide. To establish the structural details of this molecule, 1D and 2D,NMR spectroscopy, chemical composition analyses and high-resolution mass spectrometry methods (electrospray ionisation Fourier-transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) and tandem mass spectrometry (MS/MS)) were applied. By using 1D and 2D,NMR spectroscopy experiments, it was confirmed that one D -Manp was linked to C-1 of the reducing GlcpN3N and an ,-(1,6)-linked D -Manp disaccharide was located at C-4, of the non-reducing GlcpN3N (,-linkage). Fatty acid analysis identified 12:0(3-OH) and 14:0(3-OH), which were amide-linked to GlcpN3N. Other lipid,A constituents were long (,-1)-hydroxylated fatty acids with 26,33 carbon atoms, as well as their oxo forms (28:0(27-oxo) and 30:0(29-oxo)). The 28:0(27-OH) was the most abundant acyl residue. As confirmed by high-resolution mass spectrometry techniques, these long-chain fatty acids created two acyloxyacyl residues with the 3-hydroxy fatty acids. Thus, lipid,A from B. elkanii comprised six acyl residues. It was also shown that one of the acyloxyacyl residues could be further acylated by 3-hydroxybutyric acid (linked to the (,-1)-hydroxy group). [source]

Synthesis of ansa- [n]Silacyclopentadienyl,Cycloheptatrienyl,Chromium Complexes (n = 1, 2): Novel Precursors for Polymers Bearing Chromium in the Backbone

CHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2006
Alexandra Bartole-Scott
Abstract Reaction of [(,5 -C5H4Li)(,7 -C7H6Li)Cr],tmeda with a variety of dialkyl(dichloro)silanes in aliphatic solvents afforded the corresponding [1]silatrochrocenophanes. Structural characterization by X-ray diffraction analysis of the [1]silatrochrocenophanes bearing Me2Si, (iPr)2Si, and silacyclobutane bridges revealed tilt angles , of 15.56(12)°, 15.8(1)°, and 16.33(17)°, respectively. Analogously, a [2]silatrochrocenophane (6) was prepared in excellent yield by reaction of [(,5 -C5H4Li)(,7 -C7H6Li)Cr],tmeda with 1,2-dichloro-1,1,2,2-tetramethyldisilane. This complex also was characterized structurally and exhibited a tilt angle , of 2.60(15)°. The [1]silatrochrocenophane bearing the Me2Si bridge underwent facile and regioselective carbon,silicon bond cleavage with [Pt(PEt3)4] to give a very high yield of an oxidative addition product. The ring-opening polymerization of these novel [1]silatrochrocenophanes afforded ring-opened chromium-based polymers. [source]

Oligonucleotides with Sugars Other Than Ribo- and 2,-Deoxyribofuranose in the Backbone: the Solution Structures Determined by NMR in the Context of the ,Etiology of Nucleic Acids' Project of Albert Eschenmoser

CHEMISTRY & BIODIVERSITY, Issue 9 2010
Marc-Olivier Ebert
[source]

Modeling and Selection of Flexible Proteins for Structure-Based Drug Design: Backbone and Side Chain Movements in p38 MAPK

CHEMMEDCHEM, Issue 2 2008
Jyothi Subramanian
Abstract Receptor rearrangement upon ligand binding (induced fit) is a major stumbling block in docking and virtual screening. Even though numerous studies have stressed the importance of including protein flexibility in ligand docking, currently available methods provide only a partial solution to the problem. Most of these methods, being computer intensive, are often impractical to use in actual drug discovery settings. We had earlier shown that ligand-induced receptor side-chain conformational changes could be modeled statistically using data on known receptor,ligand complexes. In this paper, we show that a similar approach can be used to model more complex changes like backbone flips and loop movements. We have used p38 MAPK as a test case and have shown that a few simple structural features of ligands are sufficient to predict the induced variation in receptor conformations. Rigorous validation, both by internal resampling methods and on an external test set, corroborates this finding and demonstrates the robustness of the models. We have also compared our results with those from an earlier molecular dynamics simulation study on DFG loop conformations of p38 MAPK, and found that the results matched in the two cases. Our statistical approach enables one to predict the final ligand-induced conformation of the active site of a protein, based on a few ligand properties, prior to docking the ligand. We can do this without having to trace the step-by-step process by which this state is arrived at (as in molecular dynamics simulations), thereby drastically reducing computational effort. [source]