摘要:

A review of our present knowledge concerning the solvation of excess electrons e−exc → e−solvproduced by photoionization or radiolysis in polar media has been attempted. Various properties of the solvated electron (proposed solvation mechanisms, structure, physicochemical characteristics) are considered. In spite of some similarities, e−solvdoes not seem to be a good prototype for solvated halide anions. The behavior of e−excin heterogeneous model systems, such as micellar solutions and molecular clusters, is described and correlated with that observed in homogeneous media.Keywords: excess electrons, homogeneous and heterogeneous polar media, photoionization, radiolysis, solvation, charge transfer to solvent, properties of the solvated electron, micellar solutions, molecular clusters, comparison with the solvation of anions.

ISSN:0008-4042    

DOI:10.1139/v96-001

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The inhibition of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase from yeast was compared for both F-244 (1) and (RS)-β -butyrolactone (5). F-244 exhibits irreversible inhibition with an IC50of 8 nM, similar to that reported for the rat liver enzyme, while the binding constant (1/K1) and inactivation rate constant (kinact) similar to values reported for the human cytoplasmic enzyme. (RS)-β-Butyrolactone (5) also irreversibly inhibits the enzyme, but with much lower efficiency (IC502 mM). The values forK1(9 mM) andkinact(0.0078 s−1) for5were determined. The results show thatkinactfor 5 and1differ by a factor of only 2.5, whileK1for5is higher by a factor of 1.8 × 105. Hence, the β-lactone ring is shown to be the sole essential structural feature in1for irreversible inactivation of HMG-CoA synthase; however, the remaining functionality enhances the binding of1to the enzyme relative to5.Keywords: HMG-CoA synthase, F-244, butyrolactone, irreversible, inhibitor.

ISSN:0008-4042    

DOI:10.1139/v96-002

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Low-temperature absorption and luminescence spectra of four organorhenium compounds were measured in the visible and near-infrared spectral regions. Effects of the temperature, formal oxidation state, and the oxo-, bromo-, and organic ligands on the metal center are reported. The emission maxima of the four compounds show a large variation between 9000 and 14 200 cm−1. Stokes shifts show a smaller variation from 3700 to 4200 cm−1. Luminescence intensities are used to qualitatively characterize nonradiative relaxation processes.Keywords: luminescence spectra, absorption spectra, organorhenium compounds.

ISSN:0008-4042    

DOI:10.1139/v96-003

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Benzocyclooctatetraene (i.e., COT,2) is the major product from the thermolysis and direct photolysis of 2,3-benzobicyclo[4.2.0]octa-2,4,7-triene (i.e., triene1). It is also a minor product from the sensitized irradiation of1. The mechanism of the reaction1 → 2was investigated using deuterium-labelled triene (i.e.,1a/b), which was prepared in a two-step procedure from a known triene mixture (1a) containing deuterium at both C-4 (43%) and C-5 (97%): the thermolysis of1aled to labelled COT2a, which upon direct irradiation (λ ≥ 310 nm) gave the deuterated triene mixture1a/bin good chemical yield (61%) but in low quantum efficiency (Φ ≈ 0.0001). The COT produced from the direct irradiation (Corex filter) of1a/bpossessed different deuterium distributions than when generated thermally or from triplet sensitization (p-dimethylaminobenzophenone), as determined by1H NMR integration. The observed deuterium-labelling patterns are in accord with a Zimmerman di-π-methane rearrangement being involved in formation of the COT from S1, of triene1, while a mechanism proceeding by initial cleavage of the cyclobutene C1—C6bond appears to operate during the thermal or sensitized (T1) generations of2.Keywords: mechanisms, rearrangements, photochemistry, di-π-methan

ISSN:0008-4042    

DOI:10.1139/v96-004

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The interactions of vanadate and its complexes of uridine, 5,6-dihydrouridine, and methyl β-D-ribofuranoside with bovine pancreatic ribonuclease A (RNase A) (EC 3.1.27.5) were studied by51V NMR spectroscopy and enzyme kinetics. From kinetic studies, it was found that neither inorganic vanadate nor the methyl β-D-ribofuranoside–vanadate complex significantly inhibited the RNase A catalyzed hydrolysis of uridine 2′,3′-cyclic monophosphate. The NMR binding studies were in full agreement with the kinetics studies and showed that neither inorganic vanadate nor the methyl β-D-ribofuranoside–vanadate complex was bound tightly by the enzyme. Approximate binding constants were (5.0 ± 1.0) × 10−7 M and (3.0 ± 0.6) × 10−6 M for the uridine–and 5,6-dihydrouridine–vanadate complexes, respectively. An induced-fit mechanism is suggested, in which the pyrimidine subsite of the active site of RNase A must be fully occupied for the enzyme to be able to tightly bind the transition state or transition state analog. Calculation of the binding energies of vanadate complexes in ribonuclease, phosphoglycerate mutase, and phosphoglucomutase revealed an excess of binding energy over the analogous phosphate derivative of about 25 kJ/mol for all enzymes, even though the binding constants themselves varied by about six orders of magnitude. This energy represents about 40% of that expected to be available for a trigonal-bipyramidal transition state and requires a reassessment of the role of vanadate as a transition state analogue for phosphate transfer.Keywords: vanadate, ribonuclease, transition state, binding constants, phosphate analogues, kinetics.

ISSN:0008-4042    

DOI:10.1139/v96-005

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

A kinetic study on the aerobic (O2(1Δg)-mediated) photooxidation of a series of dihydroxynaphthalenes was carried out. Members of this family of compounds include important environmental contaminants. The interaction with O2(1Δg), determined by both time-resolved phosphorescence and static methods, yields a quenching rate constant in the range of 105–109 M−1s−1, depending on the solvent polarity, pH, and substitution pattern of the dihydroxynaphthalene. According to experimental evidence, the mechanism of the interaction seems to be mediated by an encounter complex with a considerable charge transfer component. From a mechanistic point of view this characteristic of DHN indicates a behaviour very similar to that of simple substituted phenols and dihydroxybenzenes. Quantum yields for DHN photooxidation (Φr) indicate that these substrates are excellent candidates for O2(1Δg)-mediated degradation under environment conditions. Furthermore, these compounds proved to be good self-sensitizers upon irradiation at 337 nm. Quantum yields (ΦΔ) for the production of O2(1Δg) were determined to be on the order of 0.25 in benzene.Keywords: dihydroxynaphthalenes, photooxidation, quenching, sensitization.not available

ISSN:0008-4042    

DOI:10.1139/v96-006

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

The electrochemical reduction at −30 °C of 2,6-dichlorobenzaldehyde (1a), benzaldehyde (1b), and terephthalaldehyde (2) in tetrahydrofuran with tetrabutylammonium perchlorate as supporting electrolyte, under an argon atmosphere and in the presence of fluorene or indene, gives carbinols resulting from the addition of fluorene or indene and requires only a catalytic amount of electricity. The chain reaction is initiated by proton abstraction from fluorene or indene by a base electrogenerated by reduction of the aldehyde and the propagation involves the addition of the carbanion to the aldehyde followed by regeneration of the nucleophile by proton transfer from the proton donor to the alkoxide anion (base-catalyzed addition). The voltammetric behavior of the aldehydes in the absence and in the presence of fluorene or indene is also presented.Keywords: electrochemical reduction, aromatic aldehydes, addition of fluorene (indene), base catalysis, chain reaction.

ISSN:0008-4042    

DOI:10.1139/v96-007

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Certain novel features of the Belousov–Zhabotinsky (B–Z) system employing different mixed substrates and Mn(II) as the catalyst are presented. Malic acid is the common substrate together with one of malonic, citric, cyanoacetic, maleic, acetic, oxalic, or tartaric acid as the second substrate. The correlation of the oscillatory behaviour with values of exchange current density establishes the oscillatory control by the redox couple, namely, Mn(III)/Mn(II) and (or) Br2/Br−. Each substrate when used alone gives rise to an oscillatory behaviour with characteristic features that can be compared with the system containing mixed substrate. The combination of substrates provides different modes of oscillatory behaviour such as entrainment, independent, partial inhibition, or complete inhibition. These observations are rationalized in terms of relevant steps involved in the mechanism of the reaction.Keywords: mixed substrate, exchange current density, entrainment, independent, inhibition.

ISSN:0008-4042    

DOI:10.1139/v96-008

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

SCF-MO computations were employed in a study of picric acid, the –1 picrate ion, and Li, Na, and K picrates. Atomic coordinates from crystallographic structural analyses were used as initial parameters in the computation of the minimum-energy conformations of each free molecule at the 6-31G level. The Mulliken charge numbers for each atom in the optimized model molecules were calculated at the 6-31G** level. The induction and resonance effects of the OX group (X = H, Li, Na, K) on the trinitrophenyl ring are discussed. It is demonstrated that molecular (electron) isodensity surfaces (MIDCOs) describe charge-density results nicely compatible with the point Mulliken charge numbers.Keywords: picrates, SCF-MO, charge distribution.

ISSN:0008-4042    

DOI:10.1139/v96-009

出版商:NRC Research Press    

年代:1996

数据来源: NRC

摘要:

Four substituted bicyclo[3.1.0]hexan-6-ones (cyclopropanones) were preparedinsitu, starting from the corresponding 2,6-dibromocyclohexanone and reductively removing the bromine atoms with the organometallic salt PPN+Cr(CO)4NO−The reaction is essentially instantaneous at −78 or−100 °C, and can be conveniently carried out in an NMR tube for easy characterization of the products by low-temperature1H and13C NMR spectroscopy. The1,5-di-tert-butyl and 1-tert-butyl analogs were thermally stable to ca. 0 °C, but the 1-tert-butyl-5-methyl and 1-tert-butyl-5-ethyl derivatives were extremely labile, rearranging at ca. −80 °C into a cross-conjugated enol, where the methyl (or ethyl) substituent was converted into an exomethylene group. These enols were also characterized asinsituspecies using1H and13C NMR spectroscopy, and by allowing the enol → α,β-unsaturated ketone rearrangement to take place at about 25 °C. The mechanism of the enol formation was investigated using a 1 -tert-butyl-5-CD3analog, and thekH/kDratio for enol formation was determined to be 6 ± 2. From this, the rate-determining step in the enol formation was postulated as a C-H → H-O transfer of a hydrogen atom in a cyclohexyl oxyallyl intermediate. The 1,5-di-tert-butylbicyclohexanone shows dynamic1H NMR line broadening, the origin of which is also proposed to involve a cyclohexyl oxyallyl intermediate.Keywords: cyclopropanone, oxyallyl, bicyclo[3.1.0]hexan-6-one, dienol, sigmatropic rearrangement.

ISSN:0008-4042    

DOI:10.1139/v96-010

出版商:NRC Research Press    

年代:1996

数据来源: NRC