Scission Products Research Articles

Biphasic superoxide generation in potato tubers. A self-amplifying response to stress.

Potato (Solanum tuberosum) cultivars differ quantitatively in their responses to mechanical stress including the ability to synthesize melanin pigments in tuber tissues. Investigations into the cellular events induced by mechanical stress on tuber tissues have shown that an early cellular response is a significant and rapid synthesis of superoxide radicals. This burst of radical production distinctively displays a reproducible biphasic pattern over time with peaks of generation at 2 and 5 h. A concomitant consequence of the generation of these free radicals is elevated levels of oxidatively modified tuber proteins. Both radical generation and protein modification vary between cultivars but both are directly proportional to the amount of melanin pigments produced. Cell-free extracts of mechanically stressed tissues, pectic fragments, and scission products generated from cell walls are able to induce superoxide generation in non-stressed tissues, indicating the participation of a biologically active factor that induces a further a phase of radical synthesis.

Computer simulation of thermal degradation of poly(butylene terephthalate) and analytical problems of terephthalic acid in scission products

Computer simulation by “Molic-mouse method” was applied to the thermal degradation of poly(butylene terephthalate) (PBT). Two kinds of simulation models were attempted. One was a sophisticated model by which experimental data could be explained with good precision. However, it did not solve the problem of why terephthalic acid (TPA) was not observed in the scission products. The other was a simple model whose simulation showed that TPA should form 25% of the scission products although the simulation result did not coincide with the experimental data. The analytical method and conditions were studied so as to find the conditions under which TPA could be observed. 25% of TPA was detected under the improved conditions. The distribution of PBT scission products as well as the role of computer simulation on polymer degradation phenomena are discussed based on the results.

Deep ion beam lithography in PMMA: Irradiation effects

Using H+- and He+-ions in the energy range between 100keV and 10MeV, the effects of irradiation on polymethylmethacrylate (PMMA) were extensively studied. The surface shrinking phenomenon due to outgassing and the diffusion of the volatile scission products caused by the irradiation process have been investigated as a function of ion energy, ion fluence, dose rate and thermal treatment after the irradiation. Diffusion constants, mean diffusion path lengths and activation energies have been deduced from the shrinking dependency on the temperature. The influence of the ion fluence, the dose rate distinguished in current density and scan density and the particle energy is discussed. With respect to applications in deep ion beam lithography (DIBL) the results lead to the conclusion that one should prefer a rather dispersed instead of a sharply collimated beam in order to prevent an abrupt rupture during the irradiation. On the other hand, long irradiation times should be avoided to prevent inaccuracies with respect to the depth of the microstructures due to shrinking of the surface during irradiation.

Extraction of Material from Oil Paints by Solvents

ABSTRACTDrying oils used to formulate oil paints consist primarily of polyunsaturated triglycerides, along with smaller amounts of mono- and diglycerides, free fatty acids, and other compounds such as sterols. The drying of oils occurs through an oxidative crosslinking process that also produces smaller scission products such as short chain fatty acids and diacids. Compounds that are not attached to the crosslinked oil matrix include glycerin, the unreactive free saturated fatty acids and their glycerides, soaps of fatty acids produced by reaction with metal ions from the pigment, and scission products. As the oil ages, additional soluble material is produced by hydrolysis of glyceride esters. These relatively low molecular weight compounds can be extracted by exposure to solvents, as occurs during the cleaning of paintings. Excessive extraction of material canembrittle the paint film and affect the appearance of the paint. This paper examines the types and amounts of compounds extracted from oil paint films as a function of solvent, exposure time, pigment, and age of the paint. The results indicate that solvents vary more in the speed that they extract material than in their selectivity. The composition of the extracts is consistent with predictions based on the mechanisms of the drying and aging processes. These results confirm the importance of using the least polar solvent possible to clean paintings and to formulate varnishes.

Distribution Kinetics of Polymer Thermogravimetric Analysis: A Model for Chain-End and Random Scission

Thermogravimetric analysis (TGA), through measurements of mass volatilized as a function of time, is an essential method for understanding pyrolysis of macromolecular materials. In this paper, we show that two differential equations for mass and moles of test sample allow interpretation of isothermal and nonisothermal TGA data for polymers. The equations, related to radical mechanisms and derived by means of distribution kinetics, are based on the actual fundamental chemical reactions that occur in the pyrolyzing sample: random-chain scission, recombination, and chain-end scission. The model describes the isothermal and constant heating rate TGA curves for polymer mass as a function of time or temperature. For isothermal TGA, the equations have an analytical solution identical to an empirical kinetics model that successfully correlates TGA data. Nonisothermal TGA requires a numerical integration of the two differential equations. Procedures for determining activation energies from TGA data are suggested by the new theory. Low-molecular-weight (MW) products of chain-end scission are assumed to volatilize instantaneously. TGA data for polyethylene, polystyrene, and polyether−ether−ketone are examined by nonlinear fitting precedures that yield activation energies for the three chemical reactions.

Radiation protection effects by addition of aromatic compounds to n-dodecane

Protection effects by aromatic compounds in irradiated liquid n-dodecane were studied as a model experiment for the radiolysis of aliphatic polymers. Final products of γ-radiolysis were analyzed by gas chromatography, while reactivity of additives towards reactive intermediate species was analyzed by pulse radiolysis. The yield of scission products was well reduced by increase of the degree of aromaticity and concentration of additives; moreover, the higher the degree of aromaticity of additive, the higher was the reactivity of additive towards excited molecules, radical cations, and electrons. In conclusion, reactivity between additive and reactive intermediate species is an important factor of radiation protection.

Solid state 19F NMR determination of new structure formation in FEP following radiolysis at 300 and 363 K

The radiation chemistry of FEP copolymer with a mole fraction TFE of 0.90 has been studied using 60Co γ-radiation at temperatures of 300 and 363 K. New structure formation in the copolymers was analysed by solid state 19F NMR. New chain scission products were the principal new structures found. The G-value for the formation of new –CF 3 groups was 2.2 and 2.1 for the radiolysis of FEP at 300 and 363 K, respectively, and the G-value for the loss of original –CF 3 groups was G(–CF 3)=1.0 and 0.9 at these two temperatures, respectively. There was a nett loss of –CF– groups on irradiation, with G(–CF) of 1.3 and 0.9 at 300 and 363 K, respectively.

Computer simulation of rearrangement reactions and degradation processes in poly(phenylene ether)

Quantitative analysis of thermal degradation of poly(phenylene ether) (PPE) has been attempted by a computer simulation model named “Molic mouse model”. The model was designed to simulate individual steps, each corresponding to an organic reaction in the thermal degradation. The rearrangement reaction of the main chain, the cleavage reactions of ether and methylene bridge between benzene rings, and the elimination of a methyl group from a side chain were selected. Each reaction was assumed to occur if a generated random number was greater than its probability, which had been determined by fitting the distribution of monomeric scission products in prior simulation. The result was verified by predicting the distribution of dimeric scission products. The individual reaction rates during PPE thermal degradation could be estimated by the Molic mouse model for the first time.

The radiation chemistry of poly(dimethyl-siloxane)

The radiation chemistry of poly(dimethyl siloxane) has been investigated with respect to identification of the nature of the small molecule chain scission products. Low molecular weight linear and cyclic products have been identified through the use of 29Si solution NMR, GPC and MALDI-TOF mass spectrometry. It has been suggested that the low molecular weight cyclic products are formed by back-biting depolymerization reactions.

Semi-quantitative analysis of thermal degradation in polyphenylene-ether

The thermal degradation of polyphenylene-ether based on 2,6-dimethylphenol has been semi-quantitatively studied by experiments and simple computer simulation. The polymer was thermally aged under several different conditions prior to thermal degradation, because rearrangement during ageing was found to be one of the most important processes. Although thermal ageing increased the yield of dimeric scission products, whose origins were supposed to be methylene bridges, there was not a clear increase in monomeric products. The thermal degradation was too complicated to analyse without computer simulation. A simple degradation model was designed and the cleavage rates of the ether, the methylene bridge and the side chain were simulated in order to be able to explain the distribution of the scission products. The results were not totally conclusive, but the degradation rates gave information on the cleavage. Several figures were interesting, for example: (1) the rearranged chain was calculated to be about 60 or 80% of all chains, (2) the cleavage rate of the ether bridge was about 9 times lower than that of the methylene bridge and (3) the cleavage rate of the side chain was almost the same as that of the ether bridge.

Cleavage of surfactant-incorporating fibrin by different fibrinolytic agents. Kinetics of lysis and rescue of surface activity.

Incorporation of surfactant into polymerizing fibrin causes loss of surface activity and marked retardation of clot lysis by plasmin (Günther and colleagues, Am. J. Physiol. 1994;267:L618-L624). We compared the efficacy of tissue-type plasminogen activator (t-PA), urokinase-type plasminogen activator (u-PA), activated anisoylated streptokinase-plasminogen activator complex (APSAC), and plasmin to dissolve surfactant-incorporating fibrin. Alveofact was employed as a natural surfactant source, and plasminogen was coincorporated into the fibrin matrix at a physiologic ratio to fibrin. Fibrinolysis was quantified by the release of tracer from (125)I-labeled fibrin, and the pattern of split products was characterized by sodium dodecyl sulfate polyacrylamide gel electrophoresis. In addition, we investigated the fibrinolysis-related restoration of surface activity by measurement in the pulsating bubble surfactometer. Concentrations of all fibrinolytic agents were chosen to effect approximately 40% lysis of clot material in the absence of surfactant (control). When incorporated into the fibrin matrix, but not when admixed after clot formation, surfactant inhibited the cleavage of fibrin by all fibrinolytic agents in a dose-dependent manner. Interestingly, t-PA and u-PA were significantly less inhibited than was plasmin or APSAC. The pattern of arising fibrin scission products was identical for all fibrinolytic approaches and was independent of surfactant incorporation. Adsorption and minimum surface tension-lowering properties of Alveofact were almost completely lost upon incorporation into fibrin, but surface activity was fully restored upon sustained clot lysis with all fibrinolytic agents. We conclude that the fibrinolytic capacity of all agents investigated is markedly inhibited by surfactant incorporation in fibrin, but this inhibition is significantly less pronounced in the agents employing preincorporated plasminogen (t-PA and u-PA), as compared with plasmin and APSAC. The plasminogen activators may thus proffer to "rescue" pulmonary surfactant function by induction of fibrinolysis in the alveolar compartment.

Radiation effects on liquid paraffins as polymer model compounds

The effects of gamma rays on n-C16H34 and C30H62 have been studied under vacuum and oxygenated conditions. Aromatic compounds were dissolved in various liquids to investigate the protection mechanism. The radiolysis and protection mechanisms were explored from product analysis. Gas products were analysed by gas chromatography (GC), and combinations of GC, gel permeation chromatography (GPC) and mass spectrometry (MS) were used to analyse the liquid products. Hydrogen gas was the main component of the gas product regardless of the irradiation conditions. Oligomerization was dominant under vacuum, whereas chain-scission was dominant under oxidation condition. Double bond enrichment of the oligomeric products is the main source of H2 formation as shown by vacuum radiolysis. The scission products under vacuum were saturated alkanes, and the presence of O2 during oxidation did not affect the scission pattern. Energy (excitation and charge) transfer to the aromatic molecules reduces the solvent excitation that preceeds the formation of radiolysis products. In vacuum radiolysis, reactions of additive-solvent molecules to form adduct compounds were observed. Mass analysis showed that the adduct compounds had fewer double bonds due to hydrogenation, and the protection efficiency depended on the degree of aromaticity. The low O2 uptake [G(O2) ≈ 6.0] indicates that liquid oxidation does not exhibit chain kinetics. More than 90% of the consumed O2 was converted into the oxidation products in the liquid phase, mainly carboxylic acids. Hydroaromatic compounds are energy- and charge-scavengers, but they are selectively oxidized leading to large O2 uptake and corresponding ketone formation. The G values of products in the vacuum radiolysis of liquid paraffins were hardly comparable to those in the polymeric systems. However, similar tendencies can be seen, and the comparable G(O2) in the radiation-induced oxidation indicates that the chemical reactions taken place in the liquid paraffins reflect those in the amorphous part of polymers. The results of mechanical testing on radiation-oxidized polypropylene (PP) matrix (stabilized and unstabilized) confirmed that alkane radiolysis and the protection mechanism also occurred in the practical application of polymers. © 1999 Society of Chemical Industry

NMR Study of the Radiation-Induced Cross-Linking of Poly(tetrafluoroethylene-co-perfluoromethyl vinyl ether)

The gamma-radiolysis of poly(tetrafluoroethylene-co-perfuoromethyl vinyl ether) (TFE/PMVE) was investigated using solid state F-19 and C-13 NMR spectroscopy. Chain scission products identified in the polymer were saturated chain ends -CF2CF3 (G = 1.0), methyl ether end groups -CF2OCF3 (G = 0.9), acid end groups -CF2COOH (G = 0.5), and a small amount of terminal unsaturation -CF=CF2 (G = 0.2). A mechanism for the formation of these scission products was proposed and the G value for main chain scission, G(S), was determined to be 1.4. Cross-linking of TFE/PMVE was found to proceed via a Y-linking mechanism. The G value for cross-linking, G(X), was determined to be 0.9. A maximum of 0.2 mol % cross-links were formed under the experimental conditions.

Oxidation products of cholesteryl linoleate are resistant to hydrolysis in macrophages, form complexes with proteins, and are present in human atherosclerotic lesions

Accumulation of the insoluble lipid-protein complex, ceroid, is a characteristic of atherosclerotic plaques. To determine whether deficient processing of cholesteryl esters in oxidized (ox) low density lipoprotein (LDL) contributes to ceroid formation, we studied the hydrolysis of internalized [3H] cholesteryl linoleate (CL) in oxLDL by mouse peritoneal macrophages (MPM). The hydrolysis by MPM of [3H]CL incorporated into oxLDL or LDL did not differ, suggesting that products of lipid and/or apoB oxidation had no impact on the lysosomal hydrolysis of [3H]CL. To evaluate the hydrolysis of oxCL by MPM, we subjected extensively ox[3H]CL to fractionation by TLC. The predominant fraction (D) consisted of sterols and oxysterols esterified to scission products of oxidized fatty acids containing terminal carbonyl groups, i.e., lipid core aldehydes. The extent of hydrolysis of [3H]-fraction D by MPM cultures, as well as by MPM extracts at pH 4.0, was significantly reduced when compared to the hydrolysis of intact [3H]CL. Fraction D also formed complexes with serum proteins, and the purified core aldehyde, cholesteryl 9-oxononanoate reacted with epsilon-amino group of lysines. Finally, several cholesteryl ester aldehydes were detected in lipid extracts of human atheroma. These results suggest that decomposition products of extensively oxidized cholesteryl linoleate that are also present in atherosclerotic lesions, are not adequately degraded by mouse peritoneal macrophage lysosomes and could interact with proteins to form ceroid.

Flame retardancy and rearrangement reaction of polyphenylene-ether/polystyrene alloy

The flame retardancy and the rearrangement reaction of polyphenylene-ether [poly-(oxy-2,6-dimethyl-1,4-phenylene), PPE] and polyphenylene-ether/polystyrene (PS) alloys have been studied. The flame retardancy of PPE blended with phosphates was proportional to PPE content as well as to the phosphates. The surface temperatures of PPE during a combustion was higher than that of PS, whereas PPE is more flammable than PS. The element analysis of the polymer surface showed that carbon was richer compared with the content of the newly synthesized PPE. Four monomeric and eight dimeric scission products were recovered by thermogravimetric-mass (TGA-MASS) analysis at high temperature in inert atmosphere. These structures of the scission products suggested that the rearrangement reaction occurred in combustion at high temperature. Namely, the formation of carbonaceous materials on the surface followed the rearrangement reaction and dehydration which was accelerated by the addition of aromatic phosphates. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1175–1183, 1997

Isolation and characterization of the products of anoxic irradiation of d(CpGpTpA)

An improved method for separating the products of DNA oligomers irradiated in aqueous solution has been devised. Altogether 39 products were isolated from the tetramer d(CpGpTpA) X-irradiated in dilute anoxic solution. Of these, 16, including most of the major products, were identified through the use of proton nmr spectroscopy. The identified products fall into four categories: (1) base products, (2) strand scission products, (3) base modifications and (4) tandem lesion. A tandem lesion in which the methyl carbon atom of thymine is covalently linked to the guanine C8 carbon atom was produced in larger yield than any of the simple base modifications.

Continuous kinetics of cracking reactions: Thermolysis and pyrolysis

The objective of this work is to illustrate several features of continuous-mixture kinetics for thermolytic and pyrolytic reactions. Cracking (bond scission) reactions occur during thermal decomposition (thermolytic) processes such as pyrolysis. We consider pyrolysis to be thermal decomposition of a solid material in contact with a gas. Such thermolysis usually results in a complex mixture of many products, which can often be described as a continuous function (frequency distribution) of a property, such as molecular weight. A population balance equation governs how the molecular-weight distribution (MWD) evolves in time and space. Continuous kinetics theories require the formulation of (1) a stoichiometric coefficient expression that shows how the products of scission are distributed, and (2) an expression for the rate coefficient indicating how the size of the cracking molecule affects the rate. This approach is herein applied to a pyrolytic reaction that yields tar with a measurable MWD and where char and gaseous products are lost from the reactor.

A Simulation Study on Random Scission of Branched Chains

This paper deals with the mathematical modelling of the random scission of branched chains. The objective is to predict the reaction products as well as the molecular weigth distribution for processes such as the chemical modification of polyolefins by free radical mechanism. In comparison with linear chains, where the number of scission possibilities is clear and scission products are linear, for branched chains the problem is much more complex. Two modelling approaches for branched chains were used. The first one, a kinetic, deterministic approach assumes a simple, regular morphology for all chains in order to compute its effect on the process. The second, more realistic approach uses Monte Carlo methods, and is able to keep track of the structure of each chain during the scission process. Stars and combs were used as idealized structures in the simulations with both approaches. The two approaches are applied, compared and discussed. The simulated results show that the chain morphology plays an important role in the scission of branched chains.

Specific cleavage of a DNA triple helix by FeII.bleomycin.

The specific cleavage of a DNA triple helix by FeII.bleomycin (BLM) is demonstrated. Triplex-specific cleavage was observed on both strands of the 32-base pair (bp) duplex at the duplex-triplex junctions. Strand scission products and alkali labile lesions were both formed. The strongest BLM cleavage site was located at the 5'-duplex-triplex junction, which is also the preferred triplex binding site of intercalating agents [Collier, D. A., Mergny, J.-L., Thuong, N. T., & Hélène, C. (1991) Nucleic Acids Res. 19, 4219-4224]. The preference of BLM for the 5'-junction does not appear to derive from selective intercalative binding at this site. This is supported by the observation that phleomycin, which contains a thiazolinylthiazole moiety rather than a planar bithiazole ring system, exhibited the same selectivity of triplex cleavage as BLM. Cleavage of the triple helix by FeII.BLM was unaffected by concentrations of Mg2+ up to 5 mM, suggesting possible therapeutic applications of this novel DNA target. Molecular-modeling calculations of the triplex region suggested that dramatic variations in minor groove width and depth occur at the duplex-triplex junctions, particularly at the 5'-junction. Moreover, the minor groove at these sites was calculated to be somewhat shallower and wider than the minor groove of B-DNA. These results suggest that the preference of BLM for the duplex-triplex junctions derives from selective recognition of minor groove shape at these sites and thus reflects conformation-selective, rather than sequence-selective, DNA recognition by FeII.BLM.

Rearrangement of the main-chain and subsequent thermal degradation of polyphenylene-ether

Studies of the thermal rearrangement and degradation of polyphenylene-ether (PPE) have been carried out by TGA (thermo-gravimetric analysis), GC-MS (gas chromatography-mass spectroscopy) and 1H-NMR. 2,6-Xylenol was a major scission product in the temperature range 420–700 °C, and 3,5-xylenol, as a scission product, decreased with increasing temperature. Four monomeric and eight dimeric scission products were observed at relatively high temperatures. The distribution of the scission products, which changed with increasing temperature, led to the suggestion that rearrangement of the PPE main-chain occurred to form diphenyl methylene groups, followed by thermal degradation.