Azo Initiator Research Articles

Redifferentiation of human hepatoma cells induced by green tea polyphenols

A novel approach for the treatment of cancer is the differentiation therapy in which cancer cells are induced to attain a mature phenotype when exposed to differentiation inducers. To examine the effects of polyphenols extracted from green tea, i.e. ( – )-epicatechin (EC), ( – )-epigallocatechin (EGC), ( – )-epicatechin gallate (ECG) and ( – )-epigallocatechin gallate (EGCG), on the proliferation and redifferentiation of human hepatoma cell line SMMC-7721, we measured the changes of cell growth, cell surface charge and cell morphography after treament with green tea polyphenols. It was found that the growth curve of treated cells was decreased remarkably, cell surface charge of treated cells was decreased and the microvilli on the surface of treated cells were reduced obviously. It confirmed that green tea polyphenols could reverse malignant phenotypic characteristics and induced redifferentiation of SMMC-7721 cells. The ability of green tea polyphenols to inhibit reactive oxygen species (ROS)-mediated oxidative damage of DNA was also assessedin vitro by measuring the conversion of supercoiled pBR322 plasmid DNA to the open circular and linear forms. It was found that green tea polyphenols could significantly inhibit the oxidative damage of DNA induced by a water-soluble azo initiator 2,2′-azobis(2-amidinopropane hydrochloride) (AAPH). However, they could promote the oxidative damage of DNA induced by H2O2 and Fe2+ at high concentrations. The relationship between the anti-cancer activity and antioxidation-prooxidation activity of green tea polyphenols is discussed.

Facile, controlled, room-temperature RAFT polymerization of N-isopropylacrylamide.

Poly(N-isopropyl acrylamide) is a thermoresponsive polymer that has been widely investigated for drug delivery. Herein, we report conditions facilitating the controlled, room-temperature RAFT polymerization of N-isopropylacrylamide (NIPAM). The key to success is the appropriate choice of both a suitable RAFT chain transfer agent (CTA) and initiating species. We show that the use of 2-dodecylsulfanylthiocarbonylsulfanyl-2-methyl propionic acid, a trithiocarbonate RAFT CTA, in conjunction with the room-temperature azo initiator 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), in DMF, at 25 degrees C, yields conditions leading to NIPAM homopolymerizations which bear all of the characteristics of a controlled/"living" polymerization. We also demonstrate facile size exclusion chromatographic analysis of PNIPAM samples in DMF at 60 degrees C, directly on aliquots withdrawn during the polymerizations, which avoids the problems previously reported in the literature.

Formation of Inorganic/Organic Nanocomposites by Nitroxide‐Mediated Polymerization in Bulk Using a Bimolecular System

AbstractSummary: A series of organic‐inorganic nanoparticles were synthesized by nitroxide‐mediated polymerization (NMP) of butyl acrylate initiated by a self‐assembled monolayer of an azo initiator. The azo initiator was immobilized on silica particles in the presence of a stable nitroxide radical, SG1 (an acyclic β‐phosphonylated nitroxide, N‐tert‐butyl‐N‐(1‐diethylphosphono‐2,2‐dimethyl)propyl nitroxide). After preliminary qualitative characterization by X‐ray spectroscopy (XPS) and Fourier‐transform infrared (FTIR) measurements, the nanoparticles were studied by thermogravimetric analysis (TGA) to determine the polymer grafting density and to permit a comparison with corresponding values of the initiator monolayer.It was demonstrated that the grafting from polymerization exhibits a controlled character with a low polydispersity ($\overline M _{\rm w} /\overline M _{\rm n}$ < 1.2) in a large range of molecular weights of the grafted chains (from 4 000 up to 145 000 g · mol−1) under the conditions when the stable radical SG1, acting as chain growth moderator tethered to the inorganic core, was used. magnified image

Azo initiator selection to control the curing order in dimethacrylate/epoxy interpenetrating polymer networks

AbstractDifferential Scanning calorimetry (DSC) and Fourier‐transform infrared (FT‐IR) spectroscopic studies have been undertaken of the cure of interpenetrating polymer networks (IPNs) formed with imidazole‐cured diglycidyl ether bisphenol‐A (DGEBA) and with either diethoxylated bisphenol‐A dimethacrylate (DEBPADM) or bisphenol‐A diglycidyl dimethacrylate (bisGMA), polymerized by a range of azo initiators (AIBN64, VAZ088, VR110 and AZO168). Due to the differing decomposition rates of the azo initiators, the neat dimethacrylate resin either cured faster than (with AIBN64 and VAZO88), or similar to (VR110), or slower than (AZO168), the neat epoxy resin. In the neat DGEBA/1‐methyl imidazole (1‐MeI), DEBPADM/AIBN64, DEBPADM/VAZO88 and DEBPADM/VR110 resins, close to full cure was achieved. For the neat, high‐temperature DEBPADM/AZO168 resin, full cure was not attained, possibly due to the compromise between using a high enough temperature for azo decomposition while avoiding depolymerization or decomposition of the methacrylate polymer. IPN cure studies showed that, by appropriate initiator selection, it was possible to interchange the order of cure of the components within the IPN so that either the dimethacrylate or epoxy cured first. In the isothermal cure of the 50:50 DEBPADM/AIBN64:DGEBA/1‐Mel IPN system, the cure rate of both species was less than in the parent resins, due to a dilution effect. For this system, the dimethacrylate cured first and to high conversion, due to plasticization by the unreacted epoxy, but the subsequent cure of the more slowly polymerizing epoxy component was restricted by the high crosslink density developed in the IPN. After post‐curing, however, high conversion of both reactive groups was observed and the fully cured IPN exhibited a single high‐temperature Tg, close to the Tg values of the parent resins. In the higher‐temperature, isothermal cure of the 50:50 DEBPADM/VR110:DGEBA/1‐Mel IPN system, the reactive groups cured at a similar rate and so the final conversions of both groups were restricted, while in the 50:50 DEBPADM/AZO168:DGEBA/1‐Mel system it was the epoxy which cured first. Both of these higher‐temperature azo‐initiated IPN systems exhibited single Tgs, indicating a single‐phase structure; however, the Tgs are significantly lower than expected, due to plasticization by residual methacrylate monomer and/or degradation products resulting from the high cure temperature. Copyright © 2004 Society of Chemical Industry

Reverse atom transfer radical polymerization of n‐butyl methacrylate in an aqueous dispersed system

AbstractReverse atom transfer radical polymerization (ATRP) of n‐butyl methacrylate (BMA) was conducted in an aqueous dispersed system. The influence of the surfactant, catalyst, reaction time and temperature on the colloidal stability and the control of polymerization was investigated. As a result, using an azo initiator (AIBN), a non‐ionic surfactant (Brij 35) and a hydrophobic ligand (dNbpy) to complex a copper halide, polymers with predetermined molecular weight and low polydispersity were obtained as stable latexes. Copyright © 2004 Society of Chemical Industry

A New Azo Initiator for the Synthesis of Polymers with Pyrene Terminal Groups

4,4′-azobis(4-cyanopyrenylmethyl pentanoate) (ACPMP) was synthesized by the reaction of pyrenylmethanol with 4,4′-azobis(4-cyanopentanoyl chloride) (ACPC). ACPMP was further used in the synthesis of polystyrene with pyrene terminal groups by using conventional free radical polymerization (FRP) and stable free radical polymerization (SFRP) techniques. Incorporatioil of terminal groups were confirmed by spectral measurements.

Strictinin as an efficient antioxidant in lipid peroxidation

The antioxidant effect of strictinin (SOH), which was extracted from green tea leaves, against the peroxidation of linoleic acid in sodium dodecyl sulfate (SDS) and cetyl trimethylammonium (CTAB) micelles, against the peroxidation of low-density lipoprotein (LDL) and against oxidative hemolysis of human red blood cells (RBCs), has been studied. The peroxidation of linoleic acid and LDL, and oxidative hemolysis of RBCs were initiated thermally by a water-soluble azo initiator 2,2′-azobis(2-amidinopropane hydrochloride) (AAPH), and the reaction kinetics in micelles and LDL were monitored by uptake of oxygen. The synergistic antioxidant effect of SOH with α-tocopherol (Vitamin E) was also studied by following the decay kinetics of α-tocopherol. Kinetic analysis of the antioxidation process demonstrates that SOH, used either alone or in combination with α-tocopherol, is an effective antioxidant against lipid peroxidation, but its effects significantly depend on the reaction medium.

Functionalization of Hydrogen-Terminated Silicon with Polybetaine Brushes via Surface-Initiated Reversible Addition−Fragmentation Chain-Transfer (RAFT) Polymerization

Controlled grafting of polybetaine brushes onto hydrogen-terminated Si(100) substrates (Si−H substrates) was carried out via surface-initiated reversible addition−fragmentation chain-transfer (RAFT) polymerization. The azo initiator was immobilized on the Si−H surface through a three-step process: (i) coupling of an ω-unsaturated alkyl ester to the Si−H surface under UV irradiation, (ii) reduction of the alkyl ester into a hydroxyl group by LiAlH4, and (iii) esterification of the hydroxyl group with the initiator 4,4‘-azo-bis(4-cyanopentanoic acid) after acid chlorination. In the presence of the chain-transfer agent, RAFT-mediated polymerization of the sulfobetaine monomer N,N‘-dimethyl(methylmethacryloyl ethyl) ammonium propane sulfonate (DMAPS) was initiated from the surface-immobilized azo initiator to produce DMAPS polymer (PDMAPS) brushes on the silicon substrate (Si-g-PDMAPS). X-ray photoelectron spectroscopy (XPS) analysis indicated that the PDMAPS brushes had been successfully grafted onto the si...

Polycationic salts. VI. Synthesis and in vitro studies of 2‐ionene oligomer derivatives of styrene as bile acid sequestering agents

AbstractSeveral vinylbenzyltri‐n‐alkylammonium salts were prepared via the Menshutkin reaction in high yields by the reaction of vinyl benzyl chloride (VBC) and tertiary amines. Additional salts based on 2‐ionene oligomers were synthesized via a sequential Menshutkin reaction. The 2‐ionene oligomers were further reacted with VBC to afford the vinyl benzyl chloride‐2‐ionene halide monomers in nearly quantitative yield. The monomers were polymerized in either aqueous or organic solvents with azo initiators in high yield with correspondingly high inherent viscosities. The polymers were measured spectrophotometrically for their bile acid sequestering activity with an in vitro enzymatic reaction. Several of the measured polymers exhibited 15–28% greater activity relative to cholestyramine. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 701–713, 2004

A novel method of following oxidation of low-density lipoprotein using a sensitive fluorescent probe, diphenyl-1-pyrenylphosphine.

Diphenyl-1-pyrenylphosphine (DPPP), which reacts with lipid hydroperoxide stoichiometrically to yield a fluorescent product DPPP oxide (DPPP=O) and the corresponding hydroxide, was used as a fluorescent probe for lipid peroxidation in low-density lipoprotein (LDL). DPPP was successfully incorporated into LDL using the dispersion reagent Pluronic F-127. Incorporation of DPPP into LDL was confirmed by gel filtration chromatography. Reaction of DPPP with hydroperoxide within an LDL particle was examined by monitoring the increase in fluorescence intensity of the LDL. It was found that lipid-soluble hydroperoxides such as methyl linoleate hydroperoxide preferably reacted with DPPP, whereas hydrogen peroxide did not. Fluorescence was increased at the early stages in the oxidation of DPPP-labeled LDL by an azo radical initiator or human neutrophils. LDL, which was labeled with DPPP or DPPP=O, was taken up by cells such as THP-1-derived macrophages and human umbilical vein endothelial cells. The fluorescence of DPPP=O could be observed in cells using fluorescence microscopy equipped with a cooled charge coupled device camera in a nondestructive manner. The present study shows that DPPP is a sensitive, selective, and quantitative probe for monitoring LDL oxidation and visualizing intracellular oxidation.

Reversible Tuning of Wetting Behavior of Polymer Surface with Responsive Polymer Brushes

We develop the route to fabricate mixed brushlike layers on polyamide substrates (PA-6, PA-6I, PA-66) by the “grafting from” approach. The PA substrates were functionalized by NH3 plasma. The azo initiator of radical polymerization was covalently bound to the functionalized PA surface. A two-step grafting procedure was applied to graft polystyrene in the first step and poly(2-vinylpyridine) in the second step. We found remarkable differences between grafting on Si wafers and on the PA substrates. “Grafting from” the PA surface results in a dramatic increase of the surface roughness of the film which can be explained by grafting in a swollen surface layer of PA. Because of this effect, we found a substantial amount of grafted polymers even on not functionalized PA substrates, which was explained by grafting via chain transfer reaction. The synthesized mixed polymer brushes form responsive coatings which switch their morphology due to the interplay between lateral and vertical phase segregation upon exposur...

Degradable Polymers Prepared from Alkyl Sorbates and Oxygen under Atmospheric Conditions and Precise Evaluation of Their Thermal Properties

We report here the alternating copolymers of diene monomers with oxygen as a new class of degradable polymer with great potential. The alternating copolymers of alkyl sorbates with oxygen are efficiently prepared without decomposition under atmospheric conditions in the presence of an azo initiator. The produced alternating copolymers exothermically decompose upon heating in a similar mechanism irrespective of the kind of the ester alkyl group of the sorbates; i.e., the methyl, ethyl, octadecyl, and trifluoroethyl esters. The initial and maximum decomposition temperatures (Tinit and Tmax) are determined by differential thermal analysis (DTA) and thermogravimetric analysis (TG) in a nitrogen stream: Tinit = 106–112 °C (DTA), Tmax = 147–152 °C (DTA) or 144–148 °C (TG). The TG curves obtained at a different heating rate are kinetically analyzed to evaluate an activation energy for decomposition by several methods. The decomposition rate constants, as well as overall activation energy, for the polymeric peroxide obtained from methyl sorbate are also precisely determined during the isothermal decomposition in a temperature range of 80–120 °C by two different methods; Ea = 86.7 and 84.7 kJ mol-1 on the basis of the weight loss of the polymer monitored by TG analysis, and the change in the molecular weight of the polymer determined by gel permeation chromatography, respectively. It has been revealed that the thermal cleavage of the peroxy linkage and the subsequent successive β-scission produce fumaraldehyde monoester and acetaldehyde during the thermal decomposition of the alternating copolymers as the polymeric peroxides.

Chemical studies of the antioxidant mechanism of tea catechins: radical reaction products of epicatechin with peroxyl radicals

Tea catechins, an important class of polyphenols, have been shown to have antioxidant activity and are thought to act as antioxidants in biological systems. However, the mechanisms of their antioxidant reactions remain unclear. The objective of this study was to characterize the reaction products of epicatechin with peroxyl radicals generated by thermolysis of the azo initiator azo-bisisobutyrylnitrile (AIBN). Structural elucidation of these products can provide insights into specific mechanisms of antioxidant reactions. Eight reaction products were isolated and identified using high-field 1D and 2D NMR spectral analysis. The observation of these compounds confirmed that the B-ring is the initial site for formation of reaction products in the peroxyl radical oxidant system.

Mechanisms by which cysteine can inhibit or promote the oxidation of low density lipoprotein by copper

Oxidised low density lipoprotein (LDL) may play a role in atherogenesis. We have investigated some of the mechanisms by which the thiol cysteine and the disulphide cystine can influence the oxidation of LDL by copper ions. Cysteine or cystine (100 μM) inhibited the oxidation of native LDL by copper in a simple phosphate buffer. One of the mechanisms by which cysteine (or more likely its oxidation products in the presence of copper) and cystine inhibited LDL oxidation was by decreasing the binding of copper to LDL (97% inhibition). Cysteine, but not cystine, rapidly reduced Cu 2+ to Cu +. This may help to explain the antioxidant effect of cysteine as it may limit the amount of Cu 2+ that is available to convert α-tocopherol in LDL into the prooxidant α-tocopherol radical. Cysteine (but not cystine) had a prooxidant effect, however, toward partially oxidised LDL in the presence of a low copper concentration, which may have been due to the rapid breakdown of lipid hydroperoxides in partially oxidised LDL by Cu + generated by cysteine. To prove that cysteine can cause the rapid breakdown of lipid hydroperoxides in LDL, we enriched LDL with lipid hydroperoxides using an azo initiator in the absence of copper. Cysteine, but not cystine, increased the rate of lipid hydroperoxide decomposition to thiobarbituric acid-reactive substances (TBARS) in the presence of copper.

Amino‐functionalized latex particles obtained by a multistep method: Development of a new immunoreagent

AbstractCationic latex particles with surface amino groups were prepared by a multistep batch emulsion polymerization. In the first one, two or three steps, monodisperse cationic latex particles to be used as the seed were synthesized. In the third and fourth steps, the amino‐functionalized monomer aminoethylmethacrylate hydrochloride was used to synthesize the final functionalized latex particles. Three different azo initiators 2,2′‐azobisisobutyramidine dihydrochloride, 2,2′‐azobisdimethylenisobutyramidine dihydrochloride, and 2,2′‐azobisisobutyronitrile were used as initiators. Hexadecyltrimethylammonium bromide was the emulsifier. To characterize the final latices, conversions were obtained gravimetrically, and particle size distributions and average particle diameters were determined by transmission electron microscopy and photon correlation spectroscopy. The amount of amino groups was determined by conductimetric titrations. Colloidal aspects were ascertained by measuring the electrophoretic mobilities. Activation of these particles with glutaraldehyde produced an efficient reagent for latex‐enhanced immunoassay. The covalent coupling efficiency (protein covalently bound with respect to the total amount of protein adsorbed) was compressed between 50 and 80%. The developed immunoreagent was applied to the measurement of serum ferritin concentration in a new turbidimetric procedure that was compared with a commercial nephelometric method; the results obtained with both methods demonstrated that the two procedures correlated well (r = 0.992). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2404–2411, 2003

Properties of poly(n-butyl methacrylate) prepared by reverse atom transfer radical polymerization in an aqueous dispersed system

Reverse atom transfer radical polymerization (ATRP) of n-butyl methacrylate (BMA) in waterborne media using Cu(II) complexes with azo initiators (i.e., reverse ATRP) was conducted. The influence of several factors, such as surfactant, catalyst, and reaction time, on the stability of the emulsion, the particle size, the morphology of the emulsion particles, and the control of the polymerization was investigated. The results showed great differences between ATRP and conventional emulsion polymerization, especially the nucleation mechanism and the kinetics. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1542–1547, 2003

Production of micron-sized polystyrene particles containing PEG near the particles surface

Polystyrene (PS) particles in the size range of 1–7 µm, containing poly(ethylene glycol) or PEG on the particles surface, were prepared by multi-step seeded polymerizations. Micron-sized PS particles were first prepared by dispersion polymerization using 2,2′-azobisisobutyronitrile as initiator and polyvinyl pyrrolidone as stabilizer. Conventional swelling method was then used to increase the size of the PS particles with a large amount of styrene in presence of oil soluble initiator, benzoyl peroxide. In the final step, the PS particles have been used to carry out seeded polymerization with small amount of styrene in presence of poly(ethylene glycol)-azo or PEGA initiator with average molecular weights of the PEG chains of 200 and 3000 g mol–1 , respectively. The average size, size distribution, and surface morphology indicate that seeded polymerization in the final step with small amount of styrene in presence of PEGA is the best way to produce monodisperse polystyrene particles containing PEG near the particles surface.

Perfluorooctyl bromide (perflubron) attenuates oxidative injury to biological and nonbiological systems.

To examine whether perfluorooctyl bromide (perflubron) is capable of protecting biological and nonbiological systems against oxidative damage through a mechanism independent of its known anti-inflammatory property. A controlled, in vitro laboratory study. Research laboratory of a health sciences university. Rat pulmonary artery endothelial cell cultures (biological system) and linoleic acid in sodium dodecyl sulfate micelles (nonbiological system). Rat pulmonary artery endothelial cells labeled with dichlorofluorescein diacetate and incubated with perflubron or culture media (control) were exposed to H2O2. H2O2-induced fluorescence of dichlorofluorescein diacetate was measured as an index of intracellular oxidative stress. In another experiment, linoleic acid in sodium dodecyl sulfate micelles was exposed to various concentrations of the azo initiator 2,2'-diazo-bis-(2-amidinopropane) dihydrochloride (2, 4, 20, and 50 mM) in the presence or absence of perflubron. Malondialdehyde measurements were obtained as a marker of oxidative damage to linoleic acid. Cell monolayers incubated with perflubron exhibited 66.6% attenuation in intracellular fluorescence compared with controls (p < .05). Linoleic acid in sodium dodecyl sulfate micelles incubated with perflubron and exposed to 2, 4, 20, or 50 mM of 2,2'-diazo-bis-(2-amidinopropane) dihydrochloride showed less evidence of lipid peroxidation as indicated by lower malondialdehyde measurements at 240 mins (10.6%, 16%, 41%, and 14.2%, respectively) compared with controls. Perflubron attenuates oxidative damage to both biological and nonbiological systems. This newly recognized property of perflubron is independent of its anti-inflammatory properties.

Radical Grafting Polymerization of Vinylformamide with Functionalized Silica Particles

AbstractTwo different radical polymerization techniques have been applied to covalently graft vinylformamide (VFA) onto silica particles. Grafting by the polymerization of VFA using an immobilized azo initiator on silica has been found less effective, because monomer conversion is limited in non‐aqueous solvents and grafting yields are low in water. Radical copolymerization of VFA with vinyltriethoxysilane (VTS)‐functionalized silica particles is suitable to produce poly(vinylformamide) (PVFA) silica hybrid particles in respectable yield. The VFA/VTS‐silica ratio determines the degree of grafting. The PVFA‐VTS‐co‐grafted silica particles can be acidically hydrolyzed into poly(vinylamine)‐grafted silica particles. Molecular structures of the surface groups and grafted polymer chains have been confirmed by means of solid state 13C{1H} cross‐polarization magic‐angle spinning (CP MAS) NMR spectroscopy. Zeta potential measurements show the altering of the former silica particles surface charges arising from the introduction of basic groups on the surface.image

Inhibitory effects of Oren-Gedoku-To (Huanglian-Jie-Du-Tang) on free radical-induced lysis of human red blood cells.

Oren-gedoku-to (Huanglian-Jie-Du-Tang, OGT) has been used for the treatment of cerebrovascular disease, hypertension, gastritis and liver disease in Japan. The present study was to test our hypothesis that ingestion of Oren-gedoku-to extract (TJ-15) would protect red blood cell (RBC) membrane from free radical-induced oxidation if antioxidants in OGT could be absorbed and circulated in blood. When incubated with RBC suspension, OGT and its four constituting herbs provided strong protection for RBC membrane to hemolysis induced by 2,2-azo-bis (2-amidinopropane) dihydrochloride (AAPH), an azo free radical initiator. The inhibitory effect was in a dose-dependent manner at concentrations of 5 microgram/ml to 500 microgram/ml. Furthermore, the ingestion of 7.5 g of OGT (daily dose) was associated with a significant decrease in susceptibility of RBC to hemolysis in humans. The direct protection of RBC membrane from free-radical attack as observed in the present study could provide an important pathophysiological basis for making use of the favorable hemorheological effect of OGT.