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Carbonyl Oxygen Atom (carbonyl + oxygen_atom)
Selected AbstractsIndium Triiodide Catalyzed Direct Hydroallylation of EstersEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 18 2010Yoshihiro Nishimoto Abstract The InI3 -catalyzed hydroallylation of esters by using hydro- and allysilanes under mild conditions has been accomplished. Many significant groups such as alkenyl, alkynyl, cyano, and nitro ones survive under these conditions. This reaction system provided routes to both homoallylic alcohols and ethers, in which either elimination of the alkoxy moiety or of the carbonyl oxygen atom could be freely selected by changing the substituents on the alkoxy moiety and on the hydrosilane. In addition, the hydroallylation of lactones took place without ring cleavage to produce the desired cyclic ethers in high yields. [source] Lewis Acid Induced [2+2] Cycloadditions of Silyl Enol Ethers with ,,,-Unsaturated Esters: A DFT AnalysisEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 18 2005Manuel Arnó Abstract The Lewis acid (LA) induced cycloaddition of trimethysilyl vinyl ether with methyl acrylate has been studied by DFT methods at the B3LYP/6-31G* level. In the absence of an LA, a [4+2] cycloaddition between the silyl enol ether and methyl acrylate in the s-cis conformation takes place through an asynchronous, concerted bond-formation process. This cycloaddition presents a large activation enthalpy of 21.1 kcal,mol,1. Coordination of the LA AlCl3 to the carbonyl oxygen atom of methyl acrylate yields a change of molecular mechanism from a concerted to a two-step mechanism and produces a drastic reduction of the activation energy. This stepwise mechanism is initialized by the nucleophilic attack of the enol ether at the ,-position of methyl acrylate in a Michael-type addition. The very low activation energy (7.1 kcal,mol,1)associated with this nucleophilic attack can be related to the increase of the electrophilicity of the LA-coordinated ,,,-unsaturated ester, which favors the cycloaddition through a polar process. The subsequent ring-closure allows the formation of the corresponding [2+2] and [4+2] cycloadducts. While the [4+2] cycloadduct is formed by kinetic control, the [2+2] cycloadducts are formed by thermodynamic control. The energetic results provide an explanation for the conversion of [4+2] cycloadducts into the thermodynamically more stable [2+2] ones. The cis/trans ratio found for the catalytic [2+2] process is in agreement with the experimental outcome. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Expression and characterization of active site mutants of hevamine, a chitinase from the rubber tree Hevea brasiliensisFEBS JOURNAL, Issue 3 2002Evert Bokma Hevamine is a chitinase from the rubber tree Hevea brasiliensis. Its active site contains Asp125, Glu127, and Tyr183, which interact with the ,1 sugar residue of the substrate. To investigate their role in catalysis, we have successfully expressed wild-type enzyme and mutants of these residues as inclusion bodies in Escherichia coli. After refolding and purification they were characterized by both structural and enzyme kinetic studies. Mutation of Tyr183 to phenylalanine produced an enzyme with a lower kcat and a slightly higher Km than the wild-type enzyme. Mutating Asp125 and Glu127 to alanine gave mutants with ,,2% residual activity. In contrast, the Asp125Asn mutant retained substantial activity, with an approximately twofold lower kcat and an approximately twofold higher Km than the wild-type enzyme. More interestingly, it showed activity to higher pH values than the other variants. The X-ray structure of the Asp125Ala/Glu127Ala double mutant soaked with chitotetraose shows that, compared with wild-type hevamine, the carbonyl oxygen atom of the N -acetyl group of the ,1 sugar residue has rotated away from the C1 atom of that residue. The combined structural and kinetic data show that Asp125,and Tyr183 contribute to catalysis by positioning the,carbonyl oxygen of the N -acetyl group near to the C1 atom. This allows the stabilization of a positively charged transient intermediate, in agreement with a previous proposal that the enzyme makes use of substrate-assisted catalysis. [source] Influence of "Alternative" C-terminal amino acids on the formation of [b3 + 17 + Cat]+ products from metal cationized synthetic tetrapeptides,JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 5 2004V. Anbalagan Abstract The aim of this study was to investigate the dissociation patterns, and in particular the relative abundance of [b3 + 17 + Cat]+, for peptides with C-termini designed to allow transfer of the ,OH required to generate the product ion, but not necessarily as the most favored pathway. Working with the hypothesis that formation of a five-membered ring intermediate, including intramolecular nucleophilic attack by a carbonyl oxygen atom, is an important mechanistic step, several model peptides with general sequence AcFGGX were synthesized, metal cationized by electrospray ionization and subjected to collision-induced dissociation (CID). The amino acid at position X was one that either required a larger ring intermediate (,-alanine, ,-aminobutyric acid and ,-amino- n -caproic acid to generate six-, seven- or nine- membered rings, respectively) to transfer ,OH, lacked a structural element required for nucleophilic attack (aminoethanol) or prohibited cyclization because of the inclusion of a rigid ring (p - and m -aminobenzoic acid). For Ag+, Li+ and Na+ cationized peptides, our results show that amino acids requiring the adoption of larger ring intermediates suppressed the formation of [b3 + 17 + Cat]+, while amino acids that prohibit cyclization eliminated the reaction pathway completely. Formation of [b3 , 1 + Cat]+ from the alkali metal cationized versions was not a favorable process upon suppression or elimination of the [b3 + 17 + Cat]+ pathway: the loss of H2O to form [M , H2O + Cat]+ was instead the dominant dissociation reaction observed. Multiple-stage dissociation experiments suggest that [M , H2O + Cat]+ is not [b4 , 1 + Cat]+ arising from the loss of H2O from the C-terminus, but may instead be a species that forms via a mechanism involving the elimination of an oxygen atom from an amide group. Copyright © 2004 John Wiley & Sons, Ltd. [source] Templated Synthesis of Copper(II) Azacyclam Complexes Using Urea as a Locking Fragment and Their Metal-Enhanced Binding Tendencies towards AnionsCHEMISTRY - A EUROPEAN JOURNAL, Issue 42 2009Massimo Boiocchi Dr. Abstract Copper(II) azacyclam complexes 32+ and 42+ were obtained through a metal-templated procedure involving the pertinent open-chain tetramine, formaldehyde and a phenylurea derivative as a locking fragment. Both metal complexes can establish interactions with anions through the metal centre and the amide NH group. Equilibrium studies in DMSO by a spectrophotometric titration technique were carried out to assess the affinity of 32+ and 42+ towards anions. While the NH group of an amide model compound and the metal centre of the plain CuII(azacyclam)2+ complex do not interact at all with anions, 32+ and 42+ establish strong interactions with oxo anions, profiting from a pronounced cooperative effect. In particular, 1),they form stable 1:1 and 1:2 complexes with H2PO4, ions in a stepwise mode with both hydrogen-bonding and metal,ligand interactions, and 2),in the presence of CH3COO,, they undergo deprotonation of the amido NH group and thus profit from axial coordination of the partially negatively charged carbonyl oxygen atom in a scorpionate binding mode. [source] Diorganotin(IV) Derivatives of Substituted Benzohydroxamic Acids with High Antitumor ActivityCHEMISTRY - A EUROPEAN JOURNAL, Issue 6 2004Qingshan Li Abstract A series of diorganotin(IV) and dichlorotin(IV) derivatives of 4-X-benzohydroxamic acids, [HL1 (X = Cl) or HL2 (X = OCH3)] formulated as [R2SnL2] (R = Me, Et, nBu, Ph or Cl; L = L1 or L2), along with their corresponding mixed-ligand complexes [R2Sn(L1)(L2)] have been prepared and characterized by FT-IR, 1H, 13C, and 119Sn NMR spectroscopy, mass spectrometry, elemental analysis, and melting points. In addition, single-crystal X-ray diffraction analyses were carried out for [Me2SnL2] (L = L1 or L2), which show coordination structures intermediate between distorted octahedra and bicapped tetrahedra. The hydroxamate ligands are asymmetrically coordinated by the oxygen atoms, the carbonyl oxygen atom is further away from the metal center than the other oxygen atom. The complexes are stable monomeric species; most of them are soluble not only in chlorohydrocarbon solvents, but also in alcohols and hydroalcoholic solutions. In polar solvents, the mixed-ligand complexes gradually decompose into the corresponding single-ligand complex couples. The complexes exhibit in vitro antitumor activities (against a series of human tumor cell lines) which, in some cases, are identical to, or even higher than, that of cisplatin. For the dialkyltin complexes, the activity increases with the length of the carbon chain of the alkyl ligand and is higher in the case of the chloro-substituted benzohydroxamato ligand. The [nBu2Sn(L1)2] complex displays a high in vivo activity against H22 liver and BGC-823 gastric tumors, and has a relatively low toxicity. [source] Capillary electrophoretic and computational study of the complexation of valinomycin with rubidium cationELECTROPHORESIS, Issue 5 2009Sille Ehala Abstract This study is focused on the characterization of interactions of valinomycin (Val), a macrocyclic dodecadepsipeptide antibiotic ionophore, with rubidium cation, Rb+. Capillary affinity electrophoresis was employed for the experimental evaluation of the strength of the Val,Rb+ complex. The study involved the measurement of the change of effective electrophoretic mobility of Val at increasing concentration of Rb+ cation in the BGE. From the dependence of Val effective electrophoretic mobility on the Rb+ cation concentration in the BGE (methanolic solution of 100,mM Tris, 50,mM acetic acid, 0,1,mM RbCl), the apparent binding (stability) constant (Kb) of the Val,Rb+ complex in methanol was evaluated as log,Kb=4.63±0.27. According to the quantum mechanical density functional theory calculations employed to predict the most probable structure of Val,Rb+ complex, Val is stabilized by strong non-covalent bond interactions of Rb+ with six ester carbonyl oxygen atoms so that the position of the "central" Rb+ cation in the Val cage is symmetric. [source] EuIII -Doping of Lamellar Bilayer and Amorphous Mono-Amide Cross-Linked Alkyl/Siloxane HybridsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 18 2010Silvia C. Nunes Abstract Two structurally different but chemically similar families of alkyl/siloxane mono-amidosil hosts, represented by m-A(x) [where x = 14 or 8 represents the number of CH2 groups of the pendant alkyl chains directly bonded to the carbonyl group of the amide cross-link] have been doped with a wide range of concentrations of Eu(CF3SO3)3. Mono-amidosils m-A(x)nEu(CF3SO3)3 with n,,,10 (where n is the molar ratio of carbonyl groups per Eu3+ ion) have been analyzed. The m-A(8)nEu(CF3SO3)3 mono-amidosils are transparent and amorphous films, in which the alkyl chains adopt gauche conformations. In contrast, the m-A(14)nEu(CF3SO3)3 mono-amidosils are solid powders; here the lamellar bilayer hierarchical structure of m-A(14) coexists with a new lamellar phase in which the Eu3+ ions are bonded to carbonyl oxygen atoms of the amide groups. At n = 10 the hydrogen-bonded associations formed are highly ordered and considerably stronger than those found in the less concentrated hybrids and in the nondoped matrices. "Free" and weakly coordinated triflate ions occur in all the mono-amidosil samples. The hybrids are white light emitters (maximum quantum yield: 0.08,±,0.01), presenting a broad emission band in the blue/purplish-blue spectral region (ascribed to the hybrid host) superimposed on the 5D0,7F0,4 Eu3+ intra-4f6 transitions. Two Eu3+ local coordination sites (named A and B) have been discerned in both systems. Site A is attributed to weakly coordinated Eu3+/CF3SO3, ion pairs, whereas site B involves Eu3+ coordination to the oxygen atoms of the C=O groups, of the CF3SO3, ions and of the water molecules. For site B, the long-range order of the hybrid host induces distinct features in the energy of the 5D0,7F0,4 transitions, the 5D0 lifetime and the degree of covalency of the Eu3+,first-ligand bonds. [source] Extended Car,Parrinello molecular dynamics and electronic g -tensors study of benzosemiquinone radical anion,MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2005James R. Asher Abstract Car-Parrinello molecular dynamics simulations of benzoquinone and benzosemiquinone radical anion in both aqueous solution and the gas phase have been carried out at ambient conditions. Hydrogen bonding is considerably more extensive to the anionic than to the neutral aqueous system. In addition to the conventional hydrogen bonding to the carbonyl oxygen atoms, T-stacked hydrogen bonding to the , -system is statistically and energetically significant for the semiquinone anion but not for the neutral quinone. EPR g -tensors have been calculated at DFT level for snapshots taken at regular intervals from the gas-phase and solution semiquinone anion trajectories. Different criteria for extraction of semiquinone/water clusters from the solution trajectory give insight into the effects of different interactions on the g -tensor, as does correlation of the g -tensor with statistically significant hydrogen-bond configurations identified along the trajectories. Comparison of gas-phase and solution results indicates opposite directions of direct electronic and indirect structural influences of hydrogen bonding on g -tensors. Short-time oscillations in gx along the trajectory are due mainly to CO bond vibrations. Copyright © 2005 John Wiley & Sons, Ltd. [source] Theoretical and experimental study of the complexation of valinomycin with ammonium cationBIOPOLYMERS, Issue 12 2008í Dybal Abstract The interactions of valinomycin, macrocyclic depsipeptide antibiotic ionophore, with ammonium cation NH4+ have been investigated. Using quantum mechanical density functional theory (DFT) calculations, the most probable structure of the valinomycin-NH4+ complex species was predicted. In this complex, the ammonium cation is bound partly by three strong hydrogen bonds to three ester carbonyl oxygen atoms of valinomycin and partly by somewhat weaker hydrogen bonds to the remaining three ester carbonyl groups of the valinomycin ligand. The strength of the valinomycin-NH4+ complex was evaluated experimentally by capillary affinity electrophoresis. From the dependence of valinomycin effective electrophoretic mobility on the ammonium ion concentration in the background electrolyte, the apparent binding (association, stability) constant (Kb) of the valinomycin-NH4+ complex in methanol was evaluated as log Kb = 1.52 ± 0.22. © 2008 Wiley Periodicals, Inc. Biopolymers 89: 1055,1060, 2008. This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com [source] Isomeric Squaraine-Based [2]Pseudorotaxanes and [2]Rotaxanes: Synthesis, Optical Properties, and Their Tubular Structures in the Solid StateCHEMISTRY - A EUROPEAN JOURNAL, Issue 28 2010Min Xue Abstract On the basis of formation of [2]pseudorotaxane complexes between triptycene-derived tetralactam macrocycles 1,a and 1,b and squaraine dyes, construction of squaraine-based [2]rotaxanes through clipping reactions were studied in detail. As a result, when two symmetrical squaraines 2,d and 2,e were utilized as templates, two pairs of isomeric [2]rotaxanes 3,a,b and 4,a,b as diastereomers were obtained, owing to the two possible linking modes of triptycene derivatives. It was also found, interestingly, that when a nonsymmetrical dye 2,g was involved, there existed simultaneously three isomers of [2]rotaxanes in one reaction due to the different directions of the guest threading. The 1H,NMR and 2D NOESY NMR spectra were used to distinguish the isomers, and the yield of [2]rotaxane 5,a with the benzyl group in the wider rim of the host 1,a was found to be higher than that of another isomer 5,b with an opposite direction of the guest, which indicated the partial selection of the threading direction. The X-ray structures of 3,b and 4,a showed that, except for the standard hydrogen bonds between the amide protons of the hosts and the carbonyl oxygen atoms of the guests, multiple ,,,,, stacking and CH,,,, interactions between triptycene subunits and aromatic rings of the guests also participated in the complexation. Crystallographic studies also revealed that the [2]rotaxane molecules 3,b and 4,a further self-assembled into tubular structures in the solid state with the squaraine dyes inside the channels. In the case of 4,a, all the nonsymmetrical macrocyclic molecules pointed in one direction, which suggests the formation of oriented tubular structures. Moreover, it was also found that the squaraines encapsulated in the triptycene-derived macrocycles were protected from chemical attack, and subsequently have potential applications in imaging probes and other biomedical areas. [source] | ||||||||||