摘要:

Factors involved in controlling and predicting the structural specificity and stereospecificity of synthetically useful enzymes, including pig liver esterase, subtilisin BPN′ and Carlsberg, α-chymotrypsin, and anL-lactate dehydrogenase, are discussed.

ISSN:0008-4042    

DOI:10.1139/v93-164

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The standard electrode potentials for the formation of the pure solid and molten compounds Li2ZrCl6, Li2HfCl6, Na2ZrCl6, Na2HfCl6, K2ZrCl6, K2HfCl6, Cs2ZrCl6, and Cs2HfCl6have been calculated from measured vapour pressures corresponding to their thermal decomposition at equilibrium and from available thermochemical data. Reversible potentials for the formation of Na2ZrCl6and of K2ZrCl6in solution according to the reactionwhere A is Na or K, have been calculated from available equilibrium vapour pressures as functions of the mole fractions of the alkali hexachlorocompounds. Standard potentials for the above reaction and "formal" potentials are also given. The latter are useful in predicting the electrochemical behaviour of dilute solutions of the hexachlorozirconates in alkali metal chlorides.

ISSN:0008-4042    

DOI:10.1139/v93-165

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Under homoenolization conditions (t-BuO−/t-BuOH/185 °C), the 3,3-dimethylbenzobicyclo[2.2.2]- and [3.2.1]octen-2-ones (7,8) and their [3.2.2] homolog (9) each undergo β-enolate rearrangement to a single ketone. These rearrangement processes, however, are in competition with Haller–Bauer cleavage, in contrast to the behavior of bicyclo-[2.2.2]octenone, which is not cleaved under these reaction conditions, although Haller–Bauer cleavage was found as a minor process for the [3.2.1] and [3.2.2] analogs. The structures of all products were determined primarily from their1H and13C magnetic resonance spectra.

ISSN:0008-4042    

DOI:10.1139/v93-166

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The reaction rate constants ofwith ammonium nitrate (~ 0.1 mol m−3) in 1-propanol-water and 2-propanol–water binary solvents correspond toreaction in the water-rich solvents, and toreaction in alcohol-rich solvents. The overall rate constant is smaller in solvents with 40–99 mol% water, with a minimum at 70 mol% water. The Arrhenius temperature coefficient is 26 kJ mol−1in each pure propanol solvent, increases to 29 kJ mol−1at 40 mol% water, then decreases to 17 kJ mol−1in pure water solvent. The high reaction rates in the single component solvents, alcohol or water, are limited mainly by solvent processes related to shear viscosity (diffusion) and dielectric relaxation (dipole reorientation). Rate constants reported for concentrated solutions (50–1000 mol m−3) of ammonium and nitrate salts in methanol (Duplâtre and Jonah. J. Phys. Chem.95, 897 (1991)) have been quantitatively reinterpreted in terms of the ion atmosphere model.

ISSN:0008-4042    

DOI:10.1139/v93-167

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Values of the rate constants,k2(106 m3mol−1s−1), of solvated electrons,with several related salts, in pure water and pure 1-butanol solvents at 298 K are, respectively, as follows: LiNO3, 9.2, 0.19; NH4NO3, 10, 8.3; NH4ClO4, 1.5 × 10−3, 12 in 20 mol% water; LiClO4, 1.0 × 10−4, < 1.0 × 10−4. The value ofin water solvent is 48 times larger than that in 1-butanol solvent, whereasin water is 10−4times smaller than the value in 1-butanol. This enormous reversal of solvent effects onreaction rates is the first observed for ionic reactants. The solvent participates chemically in thereaction, and the overall rate constantincreaseswithincreasingviscosity and dielectric relaxation time. This unusual behavior is attributed to a greatly increased probability of reaction of an encounter pair with increasing duration of the encounter. Effective reaction radii κRrforandwere estimated with the aid of measured electrical conductances of the salt solutions in all the solvents. Values of κRrare (2–7) × 10−10 m, except for NH4,s+in 100 and 99 mol% water, which are 2.6 and 2.7 × 10−14 m, respectively. The effective radii of the ions for mutual diffusion increase with increasing butanol content of the solvent, from ~50 pm in water to ~150 pm in 1-butanol, due to the increasing average size of the molecules that solvate the ions.

ISSN:0008-4042    

DOI:10.1139/v93-168

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The direct formation of 2,2-disubstituted 1,3-cyclopentanedione compounds by a Lewis acid catalysed reaction with 1,2-bis(trimethylsilyloxy)cyclobutene proceeds in good to excellent yields with unhindered ketals, but steric hindrance reduces the yields considerably. A carbonyl group α or β to the ketal, or a carbon–carbon double bond α to the ketal, stops the reaction completely. Orthoesters do not give geminally acylated products in synthetically useful yields.

ISSN:0008-4042    

DOI:10.1139/v93-169

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Five species of phthalocyanine-modified silica gels were prepared. The loadings of phthalocyanine on these silica gels were determined by acidic hydrolysis. The concept of using the aggregation phenomenon to advantage to aid chromatographic separation of unsymmetrical phthalocyanines was investigated by employing these modified silica gels as chromatographic media to purify unsymmetrical phthalocyanines. Four groups of phthalocyanine mixtures were prepared by mixed condensation. One of the desired unsymmetrical phthalocyanines was successfully purified by conventional column chromatography while others were separated by the new method described herein.

ISSN:0008-4042    

DOI:10.1139/v93-170

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The development of resistance to endocrine therapy as well as chemotherapy is presently a major problem in the treatment of breast cancer. To minimize this obstacle, new, more selective and potent, chemotherapeutic agents should be designed. One way to improve selectivity is to link a cytotoxic moiety to a molecule possessing an affinity to the estrogen receptor (ER). The latter would be used to direct the cytotoxic portion of the molecule towards the target cells. Our initial approach led us to the synthesis of new triphenylethylene–platinum(II) complexes1a–c. The commercially available desoxyanisoin (10) was efficiently transformed in seven steps into the platinum(II) complexes1a–cwith an overall yield exceeding 30%. The biological activity of compounds1a–cwas evaluated in vitro on ER+ and ER− human breast tumor cell lines: MCF-7 and MDA-MD-231.

ISSN:0008-4042    

DOI:10.1139/v93-171

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Infrared spectra of the cyclic peptide cyclosporin A and three analogues have been measured in a number of organic solvents (CCl4, CDCl3, acetonitrile, DMSO, and 50:50 acetonitrile:D2O). Seven of the eleven amide groups of cyclosporin A are methylated, the remaining four N-H protons forming strong intramolecular hydrogen bonds in the crystal and in CDCl3solution. These hydrogen bonds give rise to amide I (C=O stretching) bands at positions characteristic of β-turns, γ-turns, and β-sheet domains in proteins and model polypeptides. Increasing the polarity of the solvent eliminates some of these features; however, the spectra in DMSO and acetonitrile–D2O retain strong amide I absorptions characteristic of hydrogen-bonded carbonyl groups. The conformation of cyclosporin A in water cannot be observed directly due to low solubility; these findings suggest that the structure likely retains strong intramolecular hydrogen bonding. Spectra of the three analogues are consistent with this interpretation. Implications respecting the mechanism of pharmacological action of cyclosporin A are discussed.

ISSN:0008-4042    

DOI:10.1139/v93-172

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The photochemistry of several substituted α-(aryloxy)acetoveratrones that are models for chromophores in lignin has been studied using a combination of laser flash photolysis and product studies in solution and on silica, Na-X zeolite, and cellulose. The lifetimes of the triplet ketones vary substantially with the electron-donating ability of the substituents in the α-aryloxy ring, with values ranging from 30 ns for the 4-methoxy derivative to 5.5 µs for the 4-cyano ketone in acetonitrile. The triplet ketones are considerably longer lived on a silica surface than in solution and do not decay with first-order kinetics; however, they are all shorter lived than triplet acetoveratrone under the same conditions, indicating that β-phenyl quenching and (or) β-cleavage dominate their decay. The same triplets show quite different kinetic behavior in Na-X or cellulose and there is no indication in these systems for the importance of either β-phenyl quenching or β-cleavage in determining the rates of triplet decay. Product studies for α-guaiacoxyacetoveratrone indicate that β-cleavage from the singlet excited state is responsible for product formation; in contrast to solution results, the phenacyl–phenoxyl radical pairs react exclusively by geminate recombination to give rearranged ketones resulting fromorthoandpararing coupling of the two radicals. Product formation is very inefficient for this ketone in cellulose, indicating either a low singlet reactivity or efficient geminate recombination of the initial radical pair to regenerate starting material.

ISSN:0008-4042    

DOI:10.1139/v93-173

出版商:NRC Research Press    

年代:1993

数据来源: NRC