摘要:

This review outlines a decade of research that employs quantitative kinetic methods of autooxidation to phospholipids aggregated into membranes. The classical rate law for autoxidation:was found to apply to heterogeneous systems of bilayers and micelles, wherekpand 2kt. are the rate constants for chain propagation and termination, respectively, andRi, the rate of chain initiation, is controlled by thermal initiators. Theoxidizabilityof a typical lipid chain, linoleate 18:2, at 30 °C was similar (0.02–0.04 M−1/2s−1/2) in different media (solution, micelles, bilayers) and for different procedures using water-soluble or lipid-soluble initiators and inhibitors. A reduction in the absolute rate constant for termination, 2ktby a hundredfold in bilayers of dilinoleoylphosphatidyl choline (DLPC) compared to that intert-butyl alcohol solution, and a drop in thekpby a factor of five are interpreted in terms of diffusion of polarized peroxyl radicals from the hydrophobic bilayer phase to the aqueous surface, where peroxyls are strongly solvated by water. Such phenomena may also account for significant changes as observed by31P NMR spectra in bilayer lamellar structure accompanying extensive peroxidation. Analysis of the hydroperoxides formed by peroxidation of mixed bilayers of DLPC + DPPC (16:0) initiated by a water-soluble initiator, azobis(2-amidinopropane•HCl) (ABAP), showed a linear trend between the ratio ofcis,transtotrans,transgeometrical isomeric hydroperoxides and [DPLC] consistent with a peroxidation mechanism proposed in homogeneous solution.The antioxidant activities,kinh, of three classes of antioxidants: (a) polyalkyl-6-hydroxychromans (e.g., vitamin E), (b) polyalkyl-4-methoxyphenols, and (c) trialkylphenols, were measured in DLPC membranes. The results show an overall leveling and depression ofkinhvalues in DLPC membranes in the series (a) (by several orders of magnitude), (b), and (c) compared to known values in solution in chlorobenzene. In aqueous bilayers it is proposed thatkinhvalues are attenuated by hydrogen bonding by water at both theparaether oxygen and phenolic groups. Restricted diffusion (e.g., of α-tocopherol) may also reduce antioxidant activities in membranes. A synergistic effect between ascorbic acid and α-tocopherol was discovered under conditions of inhibited peroxidation of linoleate in SDS micelles. The natural peptide glutathione, GSH, however, acts as aco-antioxidantwith vitamin E by trapping peroxyls in the aqueous phase.Solid cholesterol was found to partition directly into PC lipsomes by shaking, above or below the phase transition temperature, and membrane-bound cholesterol, unlike the solid, undergoes facile peroxidation. A water-soluble form of α-tocopherol complexed with bovine serum albumin (α-toc:BSA) is an effective antioxidant for autoxidations of linoleate in SDS micelles. In contrast, α-toc:BSA required a long equilibration time (e.g., 12 h) with liposomes (DLPC) before the α-tocopherol was transferred to the liposomes to provide effective antioxidant action.

ISSN:0008-4042    

DOI:10.1139/v93-001

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

A quantitative ion-exchange/atomic absorption method is described for measuring the concentration of free (hydrated) calcium and magnesium in solution at micromolar levels. Sample solutions are pumped through a micro-column of strong acid-type cation-exchange resin until equilibrium has been achieved between resin and solution. After removal of interstitial solution by first air, then water, the sorbed metal ion is eluted from the resin with nitric acid directly into an atomic absorption spectrophotometer. In a 0.1 M 1:1 electrolyte, here KNO3, the amount of metal ion sorbed on the resin is directly proportional to the free metal ion concentration in solution over a concentration range of 1.25 to 5 × 10−5 mol/L (12–50 μmol/L). Selectivities for free calcium and magnesium in the presence of complexing ligands such as citrate and phosphate compare well with calculated values.

ISSN:0008-4042    

DOI:10.1139/v93-002

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

A method has been developed to determine the enthalpy of atom formation in a graphite furnace without invoking the order of reaction. The method is based on the equation In (AT) = −ΔH0/RT + C, whereAis the peak-height absorbance,Tis the absolute temperature, ΔH0is the enthalpy of atom formation which, for highly endothermic reactions, is considered to be equal to the activation energy,RandCare the universal gas constant and the constant of integration, respectively. The plot of In (AT) vs. 1/Tis similar to an Arrhenius-type plot and should yield the activation energy. Atomization mechanisms for eight elements, Cu, Co, Ga, Au, Ni, Fe, Ag, and Al, are proposed by correlating the experimental enthalpy with a very limited set of possible primary reactions. One significant result of this work is that the experimentally observed enthalpy of reaction correlates well with the activation energy, this means no excess energy other than the enthalpy of atomization is involved in the reaction.

ISSN:0008-4042    

DOI:10.1139/v93-003

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Complexes of 6-amino-2-thiouracil (H2ATU) with the metal ions Ni(II), Co(II), Zn(II), Cd(II), Cu(I), Ag(I), and Hg(II) have been isolated and characterized by means of elemental analysis, infrared, NMR, and visible spectroscopies, and magnetic measurements. The crystal structure of [Zn(HATU)2(H2O)] • 2H2O has been solved by X-ray diffraction showing that the metal atom is five-coordinated by a water molecule and two ligands that bind through the sulphur and N1 atoms, closing a four-member chelate ring. Diffuse reflectance spectra and magnetic measurements suggest that [Co(HATU)2(H2O)] • 2H2O has a similar structure to that of the zinc complex, and that the nickel complexes [Ni(HATU)2] and [Ni(HATU)2(H2O)2] have, respectively, a tetrahedral and aD4h-distorted octahedral environment.

ISSN:0008-4042    

DOI:10.1139/v93-004

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The esterification of stearic acid using titanates fixed on alumina or silica as catalysts has been studied. The anchoring of titanium, obtained by action of CH3Ti(OR)3with the acid sites of the support, has been studied using different contact times and different concentrations. The heterogeneized catalyst leads to a better efficacity than the homogeneous system. The activity of the catalyst is strongly increased by using crushed solid instead of balls. The catalyst has been recovered and recycled. The stability of the (M)–Ti(OR)3bond (with M = Al2O3or SiO2) has been shown by kinetic measures and exchange reactions of the alcoxy groups fixed on titanium with heavy alcohol.

ISSN:0008-4042    

DOI:10.1139/v93-005

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The acid-catalyzed hydrolysis of methyl α-(2,6-dimethoxyphenyl)vinyl ether in aqueous solution at 25 °C occurs with the hydronium ion catalytic coefficientand gives the solvent isotope effectthis indicates that reaction occurs by rate-determining proton transfer from the catalyst to the substrate to generate an alkoxycarbocation intermediate. An oxygen-18 tracer study shows further that, despite the steric hindrance provided by its twoorthosubstituents, this cation then reacts by addition of water to the cationic carbon atom to generate a hemiacetal, and not by nucleophilic attack of water on the methyl group remote from the carbocationic center:

ISSN:0008-4042    

DOI:10.1139/v93-006

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The compounds Me2Te[S2CNR2]2, Me2TeCl[S2CNR2], (p-MeOC6H4)2Te[S2CNR2]2, and (p-MeOC6H4)2TeCl[S2CNR2], where R = Me, Et, have been prepared and characterised primarily by vibrational and NMR spectroscopy. Variable temperature NMR spectroscopy and qualitative time studies indicate that in solution the dimethoxyphenyltellurium derivatives undergo the reductive-elimination noted for analogous phenyl derivatives but the reductive-elimination does not occur with the dimethyl tellurium derivatives. The crystal structures of Me2Te[S2CNMe2]2,1, and (p-MeOC6H4)2-Te[S2CNMe2]2,5, were completed. The cell parameters for1, which crystallizes as monoclinic in space groupP21/n, area = 13.550(2),b = 13.034(2),c = 17.854(2) Å, β = 106.61(1)°,V = 3021.6(7) Å3,Z = 8,R = 0.0368, andRw = 0.0402 and for5, which crystallizes as monoclinic in space groupC2/c, area = 35.36(1),b = 11.063(3),c = 12.778(3) Å, β = 96.16(3)°,V = 4963(3) Å3,Z = 8,R = 0.0627, andRw = 0.0646.

ISSN:0008-4042    

DOI:10.1139/v93-007

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The mechanism of reaction of boron trifluoride with amines, dialkyl ethers, and pyridine has been analyzed on the basis of the coordination model of reaction mechanisms. This model is tested mathematically by carrying out kinetic simulations of pathways P(X,C), accompanied by the calculation of structures of postulated intermediates by GAUSSIAN86 methods. Analyses of reaction mechanisms, including calculation of concentration vs. time curves, are reported for the following systems: NH3–BF3, MeNH2–BF3, Me2O–Et2O–BF3, Me2O–Et2O:BF3, pyridine–BF3, pyridine–Me3N–BF3, F3B:N–N:BF3, and (N–N)BF2+.

ISSN:0008-4042    

DOI:10.1139/v93-008

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

Enantioselective synthesis of simple epoxides can be achieved by Sharpless epoxidation of alkenylsilanols followed by protodesilylation of the chiral epoxysilanols. The approach has been applied to the synthesis of frontalin.

ISSN:0008-4042    

DOI:10.1139/v93-009

出版商:NRC Research Press    

年代:1993

数据来源: NRC

摘要:

The crystal structures of two quinoxalinedione derivatives, 6,7-dimethyl-1,4-dihydro-2,3-quinoxalinedione dihydrate (1) and 6,7-dimethoxy-1,4-dihydro-2,3-quinoxalinedione hydrate (2), and AM1 molecular orbital computational investigations of electronic properties and hydrogen bond strengths are reported. The space group for1iswitha = 7.0109(1) Å,b = 8.5901(3) Å,c = 10.7890(5) Å, α = 112.704(3)°, β = 96.053(3)°, and γ = 107.589(2)°. For2, the space group isPbcawitha = 12.2769(6) Å,b = 6.9291(3) Å, andc = 25.8644(7) Å. The geometries of these two compounds are similar; small differences are mostly due to the different electronic characters of the substituents and to intermolecular interactions in the crystals. There are two main factors determining the packing schemes of these compounds, π-stacking between neighboring molecules and hydrogen bonds, including bonds with the water molecules of crystallization. The AM1 semi-empirical calculations show differences between net atomic charges and dipole moments. For both structures, calculations confirm the asymmetry of hydrogen bonds that is observed in the crystal structures.

ISSN:0008-4042    

DOI:10.1139/v93-010

出版商:NRC Research Press    

年代:1993

数据来源: NRC