摘要:

Biosynthetic incorporation experiments were performed with carbon-13 labelled precursors including sodium [1-13C]-, [2-13C], and [1,2-13C2]-acetate as well as [methyl-13C]methionine into antibiotic F-244 (1) in growing cultures ofFusariumsp. ATCC 20788. After conversion to the methyl ester2, analysis by NMR showed that the carbon skeleton of1derives from seven intact acetate units; the remaining four carbons are from methionine. Hence, the pathway is similar to that reported for1inScopulariopsis. The biogenesis of the hydrogen atoms in1was also investigated. Incorporation of sodium [1-13C,18O2]acetate gives2, which exhibits18O-induced isotope shifts at C-1 and C-3. The labelling pattern is consistent with formation of the β-lactone ring by nucleophilic attack of a C-3 hydroxyl group in the nascent polyketide precursor onto the C-1 carbonyl.Keywords: biosynthesis, polyketide, F-244, β-lactone,Fusarium.

ISSN:0008-4042    

DOI:10.1139/v95-001

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The complex [Au2(μ-dppm)(C≡Ct-Bu)2],1, dppm=Ph2PCH2PPh2, is formed by reaction of [AuC≡Ct-Bu] with dppm. Compound1, as an acetone solvate, crystallizes in the monoclinic space groupP21/cwith cell dimensionsa = 14.047(1),b = 16.124(1),c = 17.140(3) Å, β = 103.17(2)° and four formula units per cell. The crystal and molecular structures of this salt were solved by X-ray diffraction techniques using 2634 counter data withI > 2σ(I). Refinement of 278 variables by full-matrix least squares converged at an agreement factorR = 0.023. Thermolysis of complex1in refluxing toluene led to intermolecular elimination oft-BuC≡CH, by combination of at-BuC≡C ligand of one molecule of1with a proton from the CH2P2group of a second, and formation of the stable tetragold(I) complex [Au4(μ3-Ph2PCHPPh2)(μ-dppm)(C≡Ct-Bu)3],2, which was characterized by its spectroscopic properties.Keywords: acetylide complexes, gold complexes, X-ray crystallography, nmr spectroscopy, bis(diphenylphosphino)methane.

ISSN:0008-4042    

DOI:10.1139/v95-002

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The concentration of glucose in aqueous solutions was monitored, using a blood glucose meter, as small weighed cyclodextrin portions were added. Equilibrium constants for the formation of inclusion complexes were calculated. A binding constant determined by this method has been verified by a technique involving competition of the guest with methyl orange indicator dye. ΔH0values for the reactions were estimated using calorimetry. These values along with ΔG0provided estimates of the standard entropies. The values calculated compare favorably with those from a previously determined empirical equation. The possibility of hydrogen bond formation is suggested by the large negative enthalpies as well as by molecular mechanics calculations.Keywords: α-, β-cyclodextrin, glucose, binding constants.

ISSN:0008-4042    

DOI:10.1139/v95-003

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The Schiff base condensation reaction of 1,2-diaminoethane with a series of 2-acetylcycloalkanones (from cyclopentanone to cyclooctanone) has been investigated and the products characterized by two-dimensional nuclear magnetic resonance. The site of attack of the amino groups, i.e., ring ketone or acetyl ketone, is determined primarily by ring size. 2-Acetylcyclohexanone yields two products in ca. 9:1 ratio, the major product where the two amino groups attack at the ring ketones of two different cyclohexanone molecules, and the minor product where one amino group attacks one ring carbonyl of one cyclohexanone while the second amino group attacks the acetyl group of another. 2-Acetylcyclopentanone yields all three possible products with the major product involving attack at the acetyl groups of two different cyclopentanones. The corresponding reactions for 2-acetylcycloheptanone and 2-acetylcyclooctanone each give a single product corresponding to attack at the acetyl groups of two different cycloalkanones. Similar product distributions are observed for the reactions of the different 2-acetylcycloalkanones with 1,4-diaminobutane.Keywords: Schiff base reactions, diketones, 2D NMR.

ISSN:0008-4042    

DOI:10.1139/v95-004

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Molecular modeling of interactions betweenD-talopyranose andD-talofuranose conformers and Pb2+and Hg2+cations was done in the gaseous state using the AM1 method, both with "naked" species and with some water molecules added. A complete conformational analysis of the freeD-talose molecule was carried out using a Monte Carlo method (BATCHMIN program). Lower minima were identified, and the different orientations of the hydroxyl groups given. The calculated equilibrium distribution of anomers in the gaseous phase differs from that obtained experimentally in aqueous solution. This could be related to the predominance of intramolecular hydrogen bonding for the isolated molecule and intermolecular hydrogen bonding in aqueous media. In the presence of a cation, the analysis was carried out using the AM1 method for every conformation of the ligand. In the gaseous phase, Angyal's predictions are not respected. The lowest energy is represented by a flexible β-pyranosic form and some complexes are tetra or pentacoordinated. For the furanosic species, the αF forms bearing acis-cis-cissequence of hydroxyl groups are the less stable. Amongst the complexes of carbohydrates with metal cations in aqueous solutions, Pb2+forms complexes and Hg2+does not. The calculations show that, for the isolated complexes, the reverse should be true. The lesser stability in water of Hg2+complexes with respect to Pb2+could be interpreted in terms of (1) a more unfavourable entropic hydration effect; (2) a less favourable difference of translational entropy. A complete answer requires the dynamical study of the system in water by a Monte Carlo method.Keywords: complexes with divalent cations,D-talopyranose andD-talofuranose conformers.

ISSN:0008-4042    

DOI:10.1139/v95-005

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The specific capsular antigen ofStreptococcuspneumoniaeserotype 35B was shown by a combination of 2D NMR methods and mass spectrometric and classical carbohydrate chemical techniques to be a high molecular weight polymer containingD-galactose,D-glucose, 2-acetamido-2-deoxy-D-galactose, and ribitol (2:1:1:1). The pentasaccharide repeating unit is polymerized through phosphate diester linkages to give the structure,Seventy percent of the β-D-Galfresidues glycosidically linked to the ribitol units carry anO-acetyl substituent.Keywords:Streptococcuspneumoniae, capsular antigen, polysaccharide structure.

ISSN:0008-4042    

DOI:10.1139/v95-006

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The transfer of oxygen from transition metal oxidants to sulfur (MO4− + R2S → MO3− + R2SO) has been studied using the oxidation of alkylthio or arylthioacetic acids (RSCH2CO2H) by perruthenate as models. These reactions exhibit a first-order dependence on the concentrations of both the oxidant and the reductant, and are directly proportional to the basicity of the medium. The Hammet ρ value is negative (ρ = −0.66), a better correlation being obtained when σ, rather than σ+, substituent constants are used. The entropies of activation for the reactions are large and negative, indicating that the transition state is highly structured. A reaction mechanism involving an activating expansion of the ruthenium coordination shell through incorporation of a hydroxide ion is proposed. Oxygen transfer is then initiated by reaction of a nonbonding pair of sulfur electrons with either a vacant rutheniumd-orbital or a Ru=O π*-orbital.Keywords: perruthenate, sulfides, oxygen transfer, oxidation.

ISSN:0008-4042    

DOI:10.1139/v95-007

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The participation of ion-paired Lewis acid/Lewis base in the dehydrohalogenations of 1,2-dibromo-1,1 -diarylalkanes catalyzed by Fe0and by Fe(II) and Fe(III) bromides was investigated. The reactivities of the catalyst employed were correlated with their chemicalhardness. The influence ofparasubstituents on the phenyl rings of the substrates and the results obtained in Lewis-acid-catalyzed and thermal dehydrobromination of the same compounds were compared. The experimental results could not be explained in terms of a Lewis-acid-catalyzed dehydrobromination.Keywords: dehydrohalogenation, bromoalkanes, Lewis acid catalysts, ion-pair mechanism.

ISSN:0008-4042    

DOI:10.1139/v95-008

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The four-coordinate anion CuI(dpym)2−(Hdpym = 3,3′,5,5′-tetramethyl-4,4′-dicarboethoxydipyrromethene) can be prepared in solution in acetone either by electrochemical reduction of the known tetrahedral complex CuII(dpym)20(E0 = −290 mV vs. SCE) or by the quantitative reaction of 2Hdpym with Cu(CH3CN)4+in the absence of O2. The latter reaction does not go to completion in solvents that bind relatively strongly to CuIor that are poor proton acceptors. Ligand exchange between CuI(dpym)2−excess Hdpym in acetone is "fast" in the1H NMR timeframe, withk1 = 1.4 × 107 L mol−1s−1at 298 K (first order in each reactant), ΔH‡1 = 3.4 ± 0.6 kJ mol−1, and ΔS‡1 = −97 ± 3 J K−1mol−1. In the absence of excess Hdpym, dissociation of CuI(dpym)2−in acetone remains negligible. Homogeneous electron exchange between CuI(dpym)2−and CuII(dpym)20in acetone falls in the "slow"1H NMR timeframe, withkex = 5.9 × 103 L mol−1 s−1, ΔH‡ex = 48.5 ± 3.0 kJ mol−1, and ΔS‡ex = −10 ± 10 J K−1, at ionic strengthI ≈ 0.007 mol L−1and 298 K, while for the same self-exchange on a Pt electrode the heterogeneous rate constantkel = 0.16±0.04 cm s−1atI ≈ 0.1 mol−1L−1and 298 K, according to AC voltammetry. These values ofKexandKelare of the order expected for CuII/Icouples in which no significant change in coordination number or geometry accompanies electron transfer.Keywords: Electron transfer, copper complexes, ligand substitution kinetics, dynamic

ISSN:0008-4042    

DOI:10.1139/v95-009

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The deprotonation constants of uridine (K1andK2), thymidine (K1andK2), cytidine (K1andK2), guanosine (K1,K2, andK3), and xanthosine (K1,K2, andK3) have been obtained in water from EMF measurements of Harned-Ehler type cells comprising H2and Ag-AgI electrodes at different temperatures. The pKvalues were substituted in the temperature equation: pK = AT−1 + B + CTandA,B, andCwere obtained by the method of least squares. Related thermodynamic quantities viz. ΔG0,TΔS0, and ΔH0were obtained from coefficientsA,B, andCof the respective nucleosides.Keywords: deprotonation energetics, uridine, thymidine, cytidine, guanosine, xanthosine.

ISSN:0008-4042    

DOI:10.1139/v95-010

出版商:NRC Research Press    

年代:1995

数据来源: NRC