Artifactual oxidation of cholesterol during the analysis of cholesterol oxidation products: protective effect of antioxidants

To study the influence of the addition of various antioxidants and their combinations on the artifactual oxidation of cholesterol during analysis, 2 factorial experiments were performed in duplicate. In the first experiment, 2 amounts of the following antioxidants were assayed: ethylenediaminetetraacetic acid (EDTA) disodium salt (0 and 1 mg), pyrogallol (0 and 600 microg), and butylated hydroxytoluene (BHT; 0 and 600 microg); in the second, EDTA disodium salt (0 and 1 mg), ascorbyl palmitate (0 and 600 microg), and BHT (0 and 600 microg). Under low oxidative conditions of dim light, evaporation of solvents at low temperatures, and cold saponification in darkness under nitrogen atmosphere, the addition of antioxidants showed no further protective effect. Furthermore, the presence of ascorbyl palmitate significantly increased the formation of cholesterol-5beta,6beta-epoxide, and 7beta-hydroxycholesterol.     

Initiated oxidation and auto-oxidation of polyethylene in trichlorobenzene solution

Thermal auto-oxidation and initiated oxidation (by dicumyl peroxide) of low density polyethylene were studied in trichlorobenzene solution. The rate dependence on the pressure of oxygen, temperature and concentrations of initiator and polymer have been investigated. Oxidation of the initiator itself has also been studied. The experimental results obtained are well interpreted by highly simplified mechanisms. Extrapolation from the rate data obtained in relatively dilute solutions at 160°C to the rate of oxidation in the molten phase gave surprisingly good agreement with the previously measured experimental value.     

Modeling the surface of polystyrene and the adsorption of dye molecules on this surface

Models of the structures of complexes of the Nile red dye (NR) molecule on the surface of various types of polystyrene (PS) were constructed by molecular dynamics (MD). The surface of a polystyrene film and the surfaces of clumps of isolated polystyrene chains were examined as surfaces. The film and the clumps were obtained as a result of molecular-dynamics trajectories. The atomistic version of molecular dynamics was used. The surface of the film was comparatively uniform, and the clumps were irregular ellipsoids with varied local surface form. The variety of the surface forms makes it possible to obtain complexes of PS with local environments having different structures. A method of constructing the solvent-accessible surface (SAS) was proposed as method of evaluating the potential sensor characteristics of the material. In the PS clumps the NR molecule is almost completely submerged in the upper layer, while in the film the aromatic fragment of the dye is partly accessible to the analyte.     

The effect of flame coal oxidation on the solid and soluble products of its extraction

Solid and soluble products of THF and CH₂Cl₂ extraction of flame coal oxidised by four different oxidising agents (peroxyacetic acid (PAA), 5% HNO₃, O₂/Na₂CO₃, air/125 °C) were studied by elemental analysis and FTIR. The extraction yield with THF was much higher than that with CH₂Cl₂ for all samples. The greatest changes in elemental composition were in the extracts from coal oxidised by nitric acid. The sulphur content was lowest in extracts from coal oxidised with 5% HNO₃ and PAA. FTIR confirmed that coal treatment with nitric acid incorporates nitrogen into the coal structure.     

Initiated oxidation of dibutyl ether in the presence of inhibitors

It was found that unlike ionol, amines effectively inhibit the oxidation of dibutyl ether. The stoichiometric inhibition coefficient for amines is close to two. During the initiated oxidation of dibutyl ether in the presence of inhibitors, an exchange takes place of cyanoisopropyl peroxy radicals for the peroxy radicals of dibutyl ether. The mechanism of the reaction with p-phenylenediamine (p-PDA) probably consists in the formation of a fairly stable complex consisting of an amine molecule and two peroxy radicals. This reaction does not result in the formation of a hydroperoxide.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1268–1273, June, 1990.     

Initiated oxidation of polyenes formed in the thermal degradation of PVC

Polyenes formed in the thermal degradation of PVC are readily oxidized in the liquid phase in the presence of a radical initiator. In pure oxygen at a constant rate of initiation, different length polyenes are consumed by a first-order reaction: the rate of consumption is proportional to the polyene length and to the square root of initiator concentration. Applying the relationships given by the theory of chain reactions, we determined the ratio of the rate constant of chain propagation calculated for one double bond k₂ to the square root of the chain-termination rate constant k₄. The value obtained, which is relatively high in comparison to other unsaturated hydrocarbons, reflects very well the high reactivity of polyenes of the degraded PVC sample towards oxidation. Intramolecular chain propagation steps are also likely to play a role in the oxidation of polyenes.     

Oxidative decarboxylation of cyclohexane monocarboxylic acid III. Initiated oxidation

A mechanism is suggested for the oxidative decarboxylation of cyclohexane monocarboxylic acid initiated by decomposition of H₂O₂. The process is a non-branching chain reaction. Cyclohexanone and cyclohexanol form in termination reactions.     

Combined inhibitory effect of sulfur-containing phenol SO-4 with natural and synthetic antioxidants in the oxidation of methyl oleate

In a model system of initiated oxidation of methyl oleate, the antioxidant activities of 3-hydroxy-2-ethyl-6-methylpyridinesuccinate (mexidol) and bis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propyl] disulfide (SO-4) were studied and compared with those of α-tocopherol and 1-hydroxy-2,6-di-tert-butyl-4-methylbenzene (dibunol). A linear pattern of dependence of the inhibitory effect on the concentration of compounds was established. The ability of antioxidants to decompose hydroperoxides and inhibit their accumulation was revealed. The combined inhibitory effects of SO-4 with mexidol, α-tocopherol, and phospholipids were described for the first time. The rate constant for disproportionation of the SO-4 phenoxyl radicals, k ₉ = 0.90·10³ L mol⁻¹ s⁻¹, was determined by steady-state photolysis. The rate constant k ₁₀ ^eff for the reactions of SO-4 phenoxyl radicals with lipids characterized by different unsaturation degrees were determined for methyl oleate, linolic and arachidonic acids containing one, two, and four multiple bonds, and phospholiopids containing polyunsaturated fatty acids. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1330–1337, August, 2006.     

Evaluation of antioxidants using oxidation reaction rate constants

An evaluation method for the capacity of antioxidants to protect drugs against oxidation is presented. As a new viewpoint, to determine the priority of the competitive oxidations between the antioxidant and the protected drug, and to compare the drug-protection capacity of antioxidants, it is important to determine their oxidation rate constants using chemical kinetics instead of standard oxidation (or reduction) potentials. Sodium sulfite, sodium bisulfite and sodium pyrosulfite were used as models for the determination of oxidation reaction rate constants in aqueous solutions. In the experiments, sufficient air was continually infused into the solution to keep the concentration of dissolved oxygen constant. The residual concentrations of the antioxidants were determined by iodimetry, and the concentration of dissolved oxygen by oxygen electrode. The data were fitted by linear regressions to obtain the reaction rate constants. It was found that the degradation of sodium sulfite, sodium bisulfite or sodium pyrosulfite obeyed pseudo zero-order kinetics in the buffer solutions. Because of the ionization equilibrium, these three antioxidants have the same ion form in solutions at a definite pH value and therefore their apparent rate constants were essentially the same. The average apparent rate constants of the three antioxidants at 25°C are (1.34 ± 0.03) × 10⁻³ at pH 6.8, (1.20 ± 0.02) × 10⁻³ at pH 4.0 and (6.58 ± 0.02) × 10⁻³ mol·L⁻¹·h⁻¹ at pH 9.2, respectively. Translated from Acta Chimica Sinica, 2006, 64(6): 496–500 (in Chinese)     

Investigation of the thermal degradation process of polystyrene brominated on the ring

The results of investigation of the degradation process of polystyrene brominated on the ring via an ionic route have been presented. Using thermogravimetric (TG) and differential thermal analysis (DTA) methods, the course of degradation of polymer samples with different bromine content has been described. Introducing of bromine on the aromatic ring influenced the initial decomposition temperature (IDT) and the temperature corresponding to the maximum of decomposition rate (T _m). The samples have been pyrolyzed at 300°C and some pyrolysis products were identified by means of gas chromatography/mass spectrometry. Finally, the possible mechanism of degradation was presented.     

A novel process for synthesizing polystyrene and polyarylate block copolymers utilizing telechelic polystyrene

A novel process for synthesizing polystyrene (PS) and polyarylate (PAr) block copolymers utilizing telechelic polystyrene was proposed. This process was comprised of three steps. In the first step, carboxyl-terminated telechelic polystyrene (COOH PS COOH) was prepared by free radical polymerization with 4,4′-azobis(cyanovalelic acid) (ACVA). In the second step, COOH PS COOH was reacted with bisphenol-A by the use of triphenylphosphine, hexachloroethane, and triethylamine to convert carboxyl groups into phenol groups (OH PS OH). In the third step, to produce the PS PAr block copolymer, OH PS OH was added to a polyarylate synthesizing system where bisphenol-A and the mixture of tere/isophthaloyl dichloride (1 : 1 mole ratio) were polymerized by solution polycondensation. PS PAr block copolymers were successively obtained with relatively high PS copolymerization ratio. The ratio was over 70%, while there was a wide variety in molecular composition and molecular weight. Furthermore, by this process PS PAr block copolymers can be obtained from step 1 through step 3 consecutively without isolating the intermediates. This method has potential for industrial applications. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2839–2847, 1998     

Nanostructuring effect of plasma and solvent treatment on polystyrene

Plasma treatment of polymer surfaces is used to control the generation of topological surface structures: stripes, starlike morphologies, and pinnacles in the range from 100 nm up to several micrometers. These protrusions arise when the plasma-treated polymer surface is exposed to an organic solvent (liquid or vapor phase). The distribution density and the height of the observed structures on the surface are functions of the power density of the plasma reactor and the exposure time to the plasma, the duration of the development process, the type of the polymer, and its manufacturing. We suggest that the structures are generated by selective swelling of less cross-linked areas within the polymer surface and not by rearrangement or dissolution of polymer chain fragments created by plasma, or by amphiphilic moieties due to oxidation as a consequence of plasma treatment.     

Thermal degradation and oxidation of polystyrene studied by factor-jump thermogravimetry

Factor-jump thermogravimetry has been used to study the activation energy of polystyrene degrading in a vacuum, in N₂ flowing at 4 mm/s and in $\text{N}{}_2\text{O}{}_2$ mixtures. The results show the activation energy to be 44·9 ± 0·2 kcal/mole (188 ± 0·8 kJ/mole) for degradation above 350°C in vacuum or in flowing N₂. This agrees well with work reported in 1949 by Jellinek⁷ but with few results reported subsequently.The apparent activation energy for polystyrene losing weight above 280°C in an atmosphere of abundant O₂ is 21·5 ± 0·2 kcal/mole (90·2 ± 0·8 kJ/mole). In all cases where O₂ was deliberately introduced (partial pressures >4 mm Hg), the sample degraded to a black tar and the activation energy was ≤30 kcal/mole, depending on the amount of oxygen present and on the thermal history of the sample.     

Inhibiting the photosensitized oxidation of anthracene and tryptophan by means of natural antioxidants

It is shown that model reactions of photosensitized oxidation of anthracene and tryptophan can be used for evaluation and comparison of antioxidant activity of various classes of compounds. Inhibition of the oxidation of substrates in the presence of the familiar antioxidants tocopherol (vitamin E), ascorbic acid (vitamin C), and mixtures of these vitamins with methionine, and in the presence of reputed antioxidants dihydroquercetin and taurine, are considered. It is concluded that all of the above compounds except for taurine have antioxidant properties; i.e., they reduce the rate constants of the photosensitized oxidation of anthracene and tryptophan. It is found that the inhibition of oxidation is associated with the interaction between antioxidants and singlet oxygen. Analysis of the kinetic dependences of the photosensitized oxidation of substrates in the presence of antioxidants reveals that a mixture of vitamins inhibits the process most efficiently, and inhibition occurs at the initial stages due to more active interaction between singlet oxygen and vitamin C     

Destructive effect of polystyrene sulfonate on the structure of hemoproteins

The mechanism of the destruction of horse heart hemoglobin (Hb) and spermwhale muscle myoglobin (Mb), two hem-containing proteins, by polystyrene sulfonate, an anionic polyelectrolyte, was studied. Measurements of the optical absorption of the prostetic group of the hem in the visible spectrum and of the circular dichroism in the absorption bands of the peptide groups and aromatic amino acid residues demonstrated that the compact structure of both proteins experiences destruction in the presence of polystyrene sulfonate (PSS) at PSS concentrations ten times as low as that of the protein (in wt %) and that the content of α-helix structure in Hb and Mb decreases from 81% in the native state to 43% in their complexes with PSS. The distinctions in the mechanisms of the destruction of Hb and Mb by PSS were found to be as follows: (1) in contrast to Mb, Hb forms insoluble complexes with PSS at low PSS concentrations and (2) Mb-PSS solutions at Mb-to-PSS ratios >1 were found to contain free hems (that absorb at 397 nm), a feature not observed for Hb; the kinetics of the destruction of both the proteins by the polyelectrolyte was demonstrated to be a two-stage process. The first stage of the destruction of Hb (τ ≈ 24.5 s) was found to be four times as slow as that of Mb (τ ≈ 6 s); the second (slow) stage had a halftime of ～6 h for both the proteins under study. To determine the localization of regions at the protein molecule surface that are capable of binding polyelectrolyte molecules, the distribution of the electrostatic potential over the surface of the Hb and Mb molecules was numerically calculated with the help of the Poisson-Boltzmann equation at pH 6.2 and an ionic strength of 100 mmol/l. Based on experimental and theoretical studies of the mechanism of the interaction of the polyelectrolyte with the proteins, the structural-functional properties of proteins responsible for their destruction by the polyelectrolyte are determined.     

Confinement effect of polystyrene on the relaxation behavior of polyisobutylene

The dynamic mechanical loss tangent (tan δ) peak of polyisobutylene (PIB) reveals an asymmetrical broad structure with a maximum on the high‐temperature side and a shoulder on the low‐temperature side. By comparing with the literature results, it is suggested that the shoulder and the maximum originate from local segmental motion and Rouse modes, respectively. Blending polystyrene (PS) with PIB has two effects on the relaxation behavior of PIB. One effect is that the maximum and the shoulder are both suppressed, but the maximum is suppressed to a higher extent. After PS forms the continuous phase, the maximum becomes lower than the shoulder, and even almost disappears when the weight ratio of PIB/PS is under 20/80. The other effect is that, before PS forms the continuous phase, the temperature position of the maximum (T_s) and that of the shoulder (T_α) remains constant, but after PS forms the continuous phase, both of them are reduced with decreasing particle size of the PIB phase, in a way similar to nano‐confinement effect on the depression of glass transition temperature. The depression amplitude of T_s is larger than that of T_α. The aforementioned two effects can be interpreted in terms of the limited expansion of free volume of the PIB phase exerted by the PS phase, which affects the maximum to a higher extent than the shoulder because Rouse modes are more sensitive to the free volume than local segments. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010