摘要:

Our interests have focused on the investigation and development of biosensors that use chemically selective membranes to measure the concentration of specific species in complex media. One fundamental idea is that protein, which can bind selectively to a specific organic or biochemical species, can be incorporated into an ordered lipid or surfactant membrane assembly such that selective binding events lead to changes in the structure of the membrane (transduction) that can be measured quantitatively. The primary advantage of this method of detection is that it is applicable to interactions of enzymes, antibodies, receptors, and lectins, and it may be extended to investigations of DNA/RNA hybridization. This detection method therefore provides a sensitive generic strategy for sensor applications. The central problem to be solved is how the alteration of the structure of a membrane that is caused by binding events of protein or genetic material can give rise to an analytical signal. We have been focusing our efforts in the areas of fluorescence spectroscopy and electrochemical methods. The electrochemical methods rely on detection in changes of the permeability of membranes to ions and provide systems with very low background signal, leading to the possibility of detection of single molecular-binding events. Fluorescent systems operate on the basis that a chemically selective membrane containing a fluorescent indicator can provide an analytical signal caused by the change of the structure of the membrane due to the binding events.Keywords: fluorescence, lipid membrane, electrochemistry, receptor, biosensor.

ISSN:0008-4042    

DOI:10.1139/v95-152

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Samarium in the presence of diiodomethane selectively cyclopropanates an allenic alcohol to yield a methylenecyclopropane with modest to excellent diastereoselectivity. The effect of substituents at the carbinol carbon and the allenic carbon on the diastereoselectivity was investigated. A palladium catalyst was shown to promote the intramolecular cycloaddition of the methylenecyclopropane with an electron-deficient alkyne to yield a methylenecyclopentane. The reaction was stereospecific with retention of stereochemistry as proven by X-ray diffraction. Nickel catalysts accelerate the hydroalumination of oxabicyclo[3.2.1]alkenes, leading to bicyclic trialkylalanes. Upon treatment with a Lewis acid (diisobutylaluminum chloride), the trialkylalanes fragment to provide cycloheptenols. This reaction, followed by an enantioselective enzyme-catalyzed esterification, was used as a key step in a synthesis of the mevinic acid lactone.Keywords: methylenecyclopropane, palladium, stereocontrol, oxabicyclic, nickel.

ISSN:0008-4042    

DOI:10.1139/v95-153

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Some 2(3H)-thione-5-alkylthio-1,3,4-thiadiazoles were prepared with a view to arylating these compounds with 9-chloroacridines. Using pyridine as solvent and base, this arylation led to the 2-thione-(N)3-acridinyl-5-alkylthio-1,3,4-thiadiazoles. Molecular structures of the latter were determined either by NMR spectroscopy or by referring to X-ray crystallography of the 5-(diethylaminoethylthio)-1,3,4-thiadiazole-2-thione. Thus, with respect to the tautomeric equilibrium of the compound investigated, the thione group was detected either in the solid state or in solution. In contrast, 2-acridinylthio-5-alkylthio-1,3,4-thiadiazoles were prepared by using the sodium salt of 2-mercapto-5-alkylthio-1,3,4-thiadiazoles as starting material.Keywords: acridines, thiadiazoles, tautomerism.

ISSN:0008-4042    

DOI:10.1139/v95-154

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The minor, α(C—H) photofragmentation processes in the far-UV photolysis of gaseous propene at λ = 184.9 nm are investigated. At this wavelength the β(C—H) and α(C—C) photofragmentation processes are of major importance, accounting for more than 95% of the primary reaction process. The total quantum yield of the three α(C—H) bond cleavage processes is close to 0.03. Howeverand the sum of the twocleavages is 0.005 ± 0.003. Thuswhereas statistically one expects 1:2. Thus the energy of the bond that is breaking is important, even though the photon energy is much higher than that required to break it. The geometrical parameters, total energies, and fundamental frequencies of thecis- andtrans-propen-1-yl and propen-2-yl radicals have been determined byabinitiomethods at various levels of theory (STO-3G, 3-21G, 6-31G, 6-31G*, and 6-31G** basis sets; UHF, ROHF, and GVB methods).cis-Propen-1-yl is predicted to be 0.6–1.0 kJ mol−1lower in energy thantrans-propen-1-yl, except with the minimal STO-3G basis set. The optimizations predict that propen-2-yl is more stable than thecis- andtrans-propen-1-yl radicals by 14 ± 3 kJ mol−1.Keywords: propene, unimolecular reactions, photochemistry, α(C—H) cleavage, propenyl radicals.

ISSN:0008-4042    

DOI:10.1139/v95-155

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Densities, viscosities, and refractive indexes of the binary systems cyclohexanone +n-butanol, +sec-butanol, and + 2-methyl-1-propanol have been measured at 298.15 K and atmospheric pressure, over the whole composition range. The excess values of molar volume, viscosity, Gibbs free energy of activation of viscous flow, and internal pressure were calculated from experimental measurements. Based on the variations of the excess functions with composition, conclusions about the molecular interactions in these kinds of mixtures were obtained.Keywords: excess properties, binary mixtures, butanols, cyclohexanone.

ISSN:0008-4042    

DOI:10.1139/v95-156

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Dimeric structures2a–care confirmed for the "methine bases" obtained from 2-methylbenzo-thiazolium and -selenazolium salts and aqueous sodium hydroxide, whereas monomeric structures3are confirmed for methine bases similarly derived from 2-methylquinolinium salts. The latter monomers dimerize slowly on standing. Monomeric methine bases6–9, too hindered to dimerize, are described. 2-Acetonylidene-2,3-dihydro-3-methylbenzothiazole10aand related compounds are assigned (Z) configurations on the basis of NOE difference spectra. Molecular mechanics calculations predict a slightly higher steric energy for the observeds-cisconformer of10athan for thes-trans, indicating a stabilizinginteraction in thesconformer that need only be a few kJ mol−1in magnitude. For the quinolines13aandb, the (E) configuration/s-cisconformation is favoured. The dielectric absorption method provides a rotational energy barrier of approximately 60 kJ mol−1for acetyl group relaxation in10a. Oxygen–sulfur exchange is observed in the conversion of10ato12aby thioacetic acid.Keywords: conformation, configuration, heterocyclic compounds,interaction.

ISSN:0008-4042    

DOI:10.1139/v95-157

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Abinitiocalculations (STO-3G and 3-21G* basis sets) have been performed on2and3to determine the most favorable structure and to provide an estimate of the barrier to internal rotation of the acyl group. TheZconfiguration is preferred in2, theEconfiguration in3, with calculated barriers to internal rotation of 79.9 and 68.7 kJ/mol, respectively, at the 3-21G* level. The wave functions from the 3-21G* calculations are analyzed with the theory of atoms in molecules. The identification of bond critical points characteristic of a closed shell interaction establishes the existence of a weak bond betweenin2and betweenin3, for the preferred configurations. The energy required to break this bond as well as the loss of extended conjugation throughout the hetero ring and its side chain are responsible for the asymmetry in the barrier. These findings provide a theoretical explanation for experimental observations on this class of molecules in which one conformer is preferred to any other and only one crystal structure is identified.Keywords: nonbonded sulfur–oxygen interaction, closed shell interaction, hydrogen bonding, 3-21G* basis set, theory of atoms in molecules.

ISSN:0008-4042    

DOI:10.1139/v95-158

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Ionic conductivities of HCl, KOH, and KCl were measured in aqueous solutions of 2-butoxyethanol (BE) at 25 °C. The quantity, Λj′ = σ/xj, which is almost proportional to the molar conductivity, was extrapolated to the infinite dilutionxj → 0. σ is the conductivity andxjis the mole fraction ofj(= HCl, KOH, or KCl). The plots of0Λj′, the value of Λj′ extrapolated to infinite dilution, againstxBEshowed a change in slope atxBE = 0.0175. The previous work from this laboratory indicated that the mixing scheme changes qualitatively at the same locus,xBE = 0.0175. By mixing scheme we simply mean the way in which BE and H2O molecules mix with each other. Assuming additivity in0Λj′ in terms of constituent ions, those values for H+OH−were calculated. Plots ofthus calculated as a function ofxBEin the water-rich region, 0 <xBE < 0.0175, suggest that the hydrogen bond probability decreases in the bulk of solution, asxBEincreases.Keywords: aqueous 2-butoxyethanol, ionic conductivities, mixing schemes, hydrogen bond network.

ISSN:0008-4042    

DOI:10.1139/v95-159

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

Croconic acid, H2C5O5, is readily oxidized. This may be attested by decolorization of the solutions and by observation of oxalic acid complexes. The oxidation products are identified by13C NMR spectroscopy as oxalic and mesoxalic acids, and experimental conditions are specified. The oxidation process requires dioxygen and photons. In parallel, the electrochemical oxidation of croconic acid is studied in aqueous media and in acetonitrile. A potential–pH diagram is drawn.Keywords: croconic acid, oxalic acid, oxidation, electrochemistry, photochemistry.

ISSN:0008-4042    

DOI:10.1139/v95-160

出版商:NRC Research Press    

年代:1995

数据来源: NRC

摘要:

The acidity and keto–enol tautomerism of a series of symmetrical β-diketones (RCOCH2COR (1): R = methyl (a), phenyl (b), 3-pyridinyl (c), 4-pyridinyl (d), 3-(N-methyl)pyridinio (e), and 4-(N-methyl)pyridinio (f)) and two series of unsymmetrical β-diketones (RCOCH2COCH3(7a–7f) and RCOCH2COC6H5(8a–8f)) have been investigated in aqueous solution at 25 °C and ionic strength 0.1. Values ofwere measured spectrophotometrically, and the acidities of the enolswere obtained from the analysis of the pH dependence of the buffer catalysis for the general acid-catalyzed protonation of the enolateconjugate bases. These data in turn allowed the calculation of the acidities of the keto tautomersand the equilibrium constants for enolization (KE = [Enol]/[Keto]). In general,KEis greater for the symmetrical ketones(1) than for the corresponding R-substituted unsymmetrical ketones (7and8).KEis much more sensitive to the nature of the R substituent in these three series of β-diketones than in the corresponding series of β-keto esters and amides. Correlations betweenfor8and7combined with the known acidities of the corresponding β-keto esters and amides provide the first accurate estimates of the acidities of dimethyl malonate (pKa = 13.0)and malonamide (pKa = 12.5) in aqueous solution.Keywords: acidity, tautomerism, β-diketones.

ISSN:0008-4042    

DOI:10.1139/v95-161

出版商:NRC Research Press    

年代:1995

数据来源: NRC