Reactive Entities Research Articles

Kinetics and mechanism of the oxidation of oxalic acid by tetrachloroaurate(III) ion

The oxidation of oxalic acid by tetrachloroaurate(III) ion in 0.005 ⩽ [HClO 4] ⩽ 0.5 mol dm −3 is first order in [ AuCl 4 - ] and a fractional order in [oxalic acid], the reactive entities being AuCl 3(OH) − and HC 2 O 4 - ions. The pseudo first-order rate, k obs, with respect to [Au(III)], is retarded by increasing [H +] and [Cl −]. The retardation by H + ion is caused by the dissociation equilibrium ( COOH ) 2 ⇌ HC 2 O 4 - + H + . A mechanism in which a substitution complex, AuCl 3 C 2 O 4 2 - is formed from AuCl 3(OH) − and HC 2 O 4 - ions prior to its rate limiting disproportionation into products is suggested. The rate limiting constant, k, has been evaluated and its activation parameters are reported. The equilibrium constant K 1 for the formation of the substitution complex AuCl 3 C 2 O 4 2 - and its thermodynamic parameters are also reported.

Mechanism of Mitochondrial Uncouplers, Inhibitors, and Toxins: Focus on Electron Transfer, Free Radicals, and Structure -Activity Relationships

The biology of the mitochondrial electron transport chain is summarized. Our approach to the mechanism of uncouplers, inhibitors, and toxins is based on electron transfer (ET) and reactive oxygen species (ROS). Extensive supporting evidence, which is broadly applicable, is cited. ROS can be generated either endogenously or exogenously. Generally, the reactive entities arise via redox cycling by ET functionalities, such as, quinones (or precursors), metal compounds, imines (or iminiums), and aromatic nitro compounds (or reduced metabolites). In most cases, the ET functions are formed metabolically. The toxic substances belong to many categories, e.g., medicinals, industrial chemicals, abused drugs, and pesticides. Structure-activity relationships are presented from the ET-ROS perspective, and also quantitatively. Evidence for the theoretical framework is provided by the protective effect of antioxidants. Among other topics addressed are proton flux, membrane pores, and apoptosis. There is support for the thesis that mitochondrial insult may contribute to illnesses and aging.

Flocculation of hematite particles by a comparatively large rigid polysaccharide: schizophyllan

We studied the flocculation kinetics and structure of hematite aggregates induced by a large rigid extracellular polysaccharide, schizophyllan. Transmission electron microscopy (TEM), atomic force microscopy (AFM), photon correlation spectroscopy (PCS), and static light scattering (SLS) were used to characterize hematite particles, schizophyllan chains, and their flocs, to follow the time evolution of floc sizes, and to determine floc fractal dimensions. A maximum flocculation rate was found at a certain schizophyllan/hematite ratio. The maximum rate was considerably smaller than the rate of diffusion-limited aggregation (DLA) of hematite particles induced by simple electrolytes. To interpret the experimental results and to reveal various factors affecting the optimal dosage, Monte Carlo simulations were performed on the flocculation of small colloidal particles by relatively long, monodisperse linear polymers. The existence of the maximum flocculation rate was confirmed by computer simulation. However, a higher optimal dosage of schizophyllan was obtained in the experiments. The difference in the optimal dosage can be attributed mostly to the higher adsorption affinity of the hematite on schizophyllan aggregates present in the initial solution and the presence of a large fraction of free polymer chains which do not participate in the flocculation process. Both experiments and computer simulations demonstrated the fractal nature of the schizophyllan–hematite flocs. The fractal dimensions of the flocs at the optimal dosage were determined. A higher fractal dimension was obtained from experiments than from computer simulations, suggesting a reconstruction of the floc structure. Finally, a two-stage flocculation mechanism for hematite particles in the presence of a relatively long schizophyllan polymer was proposed. In the first flocculation stage, the hematite particles are preferentially adsorbed onto the schizophyllan aggregates in solution. The second stage consists of the association of these reactive entities with each other and also with naked chains to form fractal flocs by a bridging mechanism, where the hematite particles play the role of ligands.

Synthesis and Thermal Behavior of Novel Si−B−C−N Ceramic Precursors

Several boron-modified polysilazanes of general type {B[C2H4Si(R)NH]3}n (C2H4 = CHCH3 or CH2CH2) were synthesized and their thermal behavior studied. In contrast to the known derivatives with R = alkyl or aryl, we describe ceramic precursors in which the bulky moieties R are substituted with lower weight groups and/or reactive entities. Reactive units enable further cross-linking of the polymeric framework and therefore minimize depolymerization during ceramization. The polymer-to-ceramic conversion of all synthesized polymers was monitored by thermogravimetric analysis. Both low molecular weight substituents and/or cross-linking units increase the ceramic yield from 50% (R = CH3) to 83−88%. High-temperature thermogravimetric analysis in an inert gas atmosphere indicates the ceramics obtained are stable up to ∼2000 °C. XRD studies of the fully amorphous materials point out that, with increasing temperature, formation of α-SiC or α-SiC/β-Si3N4 crystalline phases occurs at 1550−1750 °C, depending on the material's composition. The resistance of these novel materials toward oxidative attack was investigated by TGA in air up to 1700 °C and SEM/EDX, indicating that the materials efficiently self-protect toward oxidation.

A semiotic approach to the internal functioning of publics: implications for strategic communication and public relations

Employing public relations as a paradigmatic instance of strategic communication, this article challenges the prevailing view of publics as reactive entities. The article briefly reviews the two primary schools of semiotics, Saussurean and Peircean, and argues for the utility of the latter for building a language-centered understanding of publics. The Peircean ideas of unlimited semiosis, semiotic rhetoric, and community are employed to argue that publics ought to be understood primarily as self-actuated and interactive social entities with values and internal dynamics at least as complex and important to communication campaigns as are message content or client/practitioner intentions. The article concludes that we can best understand a public as an ongoing process of agreement upon an interpretation, and that during this process a public may well develop an interpretation that is more sophisticated, insightful, and socially linked than the understanding with which the practitioner/client started. As a result, we argue, publics should be accorded the preeminent position when studying or practicing any form of strategic communication, including public relations. Carl Botan is Associate Professor at Purdue University. Francisco Soto (MA, Purdue, 1995) is an Account Director for Aleph Communicacion, a public relations firm in Madrid, Spain and lectures at several higher education institutions.

Organosilicon Polymers?Synthesis, Architecture, Reactivity and Applications

Tailoring of polysilanes with given architectures and reactivities is a great challenge in the field of SiC pre-ceramic polymers. This paper reviews recent polysilane and related copolymer synthesis reactions. It is shown that the Wurtz-type polymerization of dichloro-, trichloro- or tetrachloro-silanes, so far the most extensively studied, enables access to a large variety of architectures ranging from one- to three-dimensional (3D) topologies, and based on secondary >SiR2, tertiary RSi(Si)3 or quaternary Si(Si)4 silicon units in the polymer backbone. These polysilanes usually present an intrinsic low reactivity, detrimental for fiber processing. Examples are given to illustrate how this reactivity can be increased by secondary substitution reactions, which create reactive entities that can favor further crosslinking reactions. Secondly a novel route involving heterogeneously catalyzed disproportionation of chloromethyldisilanes, developed in our laboratory, is reviewed which offers a direct access to polysilyne-type 3D architecture constituted by arrangements of fused rings. The Lewis-base catalyzed disproportionation mechanism is discussed and seems to involve donor-stabilized silylenes as key intermediates in the polymer formation process. The experimental results are supported by ab-initio quantum chemical calculations. Silylenes attack the Si sites of higher functionality causing a high regioselectivity for the exclusive formation of branched oligosilanes. The oligomers undergo thermally induced branching and crosslinking reactions leading to poly(chloromethylsilane)s. Obviously, there are analogies to the oligomer and polymer formation of the transition-metal complex catalyzed dehydropolymerzation of methyldisilanes. Poly(chloromethylsilane)s exhibit a high reactivity due to the presence of Si–Cl bonds. Disproportionation of chloromethyldisilanes in presence of olefins such as styrene provides promising polymer precursors for SiC fibers. Their rheological properties have been investigated for various styrene contents. The polymer fibers spun from melt are cured under ammonia, and then pyrolyzed to silicon carbide fibers, showing temperature resistance up to 1500 °C. © 1996 by John Wiley & Sons, Ltd.

Influence of the pH on the photodynamic effect in lysozyme A comparative kinetic study with the sensitized photooxidation of isolated amino acids

The kinetics of the eosin-sensitized photooxidation ([O2((1)Δg)]-mediated) of the protein lysozyme (Lyso) was investigated under two different pH conditions (pH 7 and pH 11). Rates of oxygen consumption and the fade in the protein fluorescence spectrum upon sensitized irradiation were monitored. Parallel studies on both denatured Lyso (absence of the four-S-S- bridges in the protein) and different mixtures of the photooxidizable amino acids of Lyso were also carried out. The mixtures maintained the same molar ratio as in the native protein, and were selected just in order to throw into relief the preferential amino acids that were being photooxidized at both pH values.Under work conditions Lyso was only photooxidizable at pH 7, whereas the opposite accounted for the denatured protein: only measurable oxygen consumption was detected at pH 11. Nevertheless, Lyso at pH 11, evidenced an important physical quenching of O2((1)Δg) due to the Tyr and Trp residues.The results for the native protein were interpreted on the basis of a previously described dark complex Eosin-Lyso, which selectively favours the photooxidation of the bounded protein. The Trp residues were the main reactive entities in the native protein. The photodinamic effect in denatured Lyso was characterized by the prevalence of Tyr residues as photooxidizable targets.In the discussion of the results, a comparisson with the photooxidation kinetics of the mixtures of free amino acids was made.

New Aspects of the Aromatic Photosubstitution with Iodopyridines.

Reactive entities in the photoreaction of 2-, 3- and 4-iodopyridines with substituted benzenes were investigated; 3- and 4-pyridylation could be explained in terms of radical reaction, while the 2-pyridyl cation was an important intermediate in the 2-pyridylation. The importance of the unshared electron pair of the nitrogen adjacent to the radical carbon for the conversion of the 2-pyridyl radical initially produced into the cation is discussed on the basis of molecular orbital (MO) calculations.

Distribution of oxygen forms in Alberta low rank coals

The partitioning of oxygen contents between oxygen-bearing functional groups in a suite of Western Canadian low rank coals and in some representative Carboniferous and Cretaceous bituminous coals has been studied. Distribution of [O] between [O COOH], [O OH], [O CO] and a nominally unreactive [O resid] not encompassed by the three reactive entities suggests considerable chemical diversity among analytically similar low rank coals and may go some way towards explaining the frequently quite disparate behaviour of such coals in chemical processing. Additional causes of such disparate behaviour may lie in the dissimilarly changing relative concentrations of [O COOH], [O OH], [O CO] and [O resid] during mild pyrolysis to 300 °C. These changes reflect significant, but seemingly rank-independent structural alteration of the coal substance at temperatures far below the onset of active decomposition (usually placed at or near 400 °C). The possible beneficial impact of partial thermal deoxygenation (by removal of COOH) on hydrogen traffic in subsequent coal processing is briefly noted.

Utilisation du rayonnement ultraviolet dans le traitement des eaux: mesure du flux photonique par actinometrie chimique au peroxyde d'hydrogene

Disinfection of water by u.v. irradiation has known significant technological developments and industrial applications during these last years (Matsuura and Smith, 1970; Jepson, 1973; Severin et al., 1984). More recently, advanced oxidation processes involving the generation of hydroxyl radicals or other reactive entities by the combination of u.v. radiation and a chemical oxidant (ozone or hydrogen peroxide) have been shown to be effective for the degradation of organic micropollutants resistant to classical oxidants (Glaze et al., 1984, 1987; Bourbigot et al., 1985; Sundstrom et al., 1986; Guittonneau et al., 1988). The effectiveness of these oxidation processes depends on some physico-chemical parameters (initial oxidant dosage, pH, concentration of bicarbonate ions ...) and on the conditions of irradiation (exposure time or irradiation dose). Meanwhile, the comparison of the results of photochemical—oxidative degradation rates is difficult because of the great diversity of photochemical reactors used in the studies and of the lack of data about the characteristics of u.v. light sources (photonic fluxes). Therefore, this study describes an experimental procedure for the determination of photonic fluxes at 253.7 nm emitted by low-pressure mercury vapour lamps and for the evaluation of the reflecting power of the internal wall of photochemical reactors.

In Vitro Covalent Binding of New Brain Tracer, Para125I-Amphetamine, to Rat Liver and Lung Microsomes

p-125I-amphetamine (I-Amp) is retained significantly in liver and lung during brain tomoscintigraphy. To attempt to explain this clinical observation, we have investigated the interaction of I-Amp with rat liver and lung microsomal proteins. Studies using spectral shift technique indicate that low concentration of I-Amp gives a type I complex and high concentration appears very stable type II complex with cytochrome P-450 Fe III. In the presence of NADPH, I-Amp gives rise to a 455 nm absorbing complex with similar properties to the Fe-RNO complexes. This complex formation was greatly enhanced with phenobarbital treated liver microsomes. The in vitro binding study shows that I-Amp and/or its metabolites was covalently bound to macromolecules in the presence of the molecular oxygen and NADPH-generating system. Incubation in the presence of glutathione, cystein and radical scavengers decreases binding. Mixed function oxydase (MFO) inhibitors diminish the amount of covalent binding and alter the extent of metabolite formation. The total covalent binding level increased with liver microsomes from PB pretreated rats as it was observed with the 455nm complex formation. The radioactivity distribution on microsomal proteins was examinated with SDS polyacrylamide gel electrophoresis and autoradiography. This experiment proves that the radiolabelled compounds are bound on the cytochrome P-450. The radioactivity bound increased when the PB induced rat liver microsomes were used. All these results indicate that I-Amp was activated by an oxydative process dependent on the MFO system which suggests a N-oxydation of I-Amp and the formation of reactive entities which covalently bind to proteins.

Isotopic characterisation of kerogen-like material in the Murchison carbonaceous chondrite

Isotopic data for C, H and N in acid-resistant residues from carbonaceous chondrites show substantial variability during stepwise pyrolysis and/or combustion. After subtraction of contributions due apparently to inorganic C grains, of probably circumstellar origin, considerable isotopic variability remains, attributable to the kerogen-like organic fraction. That variability may be interpreted in terms of three or four distinct components, based on C, H and N isotopes. The relative proportions of those components vary significantly from sample to sample. The different isotopic components are tentatively identified in terms of specific chemical/structural moieties within the kerogen-like material. This combination of chemical, structural and isotopic information suggests a complex history for meteoritic organic matter. At least three components within the organic population as a whole still carry a signature of apparently interstellar Denrichment. Part, at least, of the interstellar carrier consisted of reactive entities, not solely polymers.

Trapped radicals in titania gels

Free radicals are in general very reactive entities capable only of transient existence. Nevertheless, several methods have been devised for their detection and identification, including stabilization by matrix isolation procedures1 and detection by spectroscopic methods. Radicals can be readily produced by oxidation–reduction processes where electrons are transferred from the solute to the matrix or conversely. Here we report the first production of trapped O−2 free radicals generated in a titania-gel matrix by the initial reaction of Ti(III) ions with H2O2 and the subsequent precipitation with NH4OH. We also have indications that the detected paramagnetic species may be complexed to the Ti(IV) ions. A striking feature in our results is the very high 2-year stability of these trapped O−2 radicals. We are able to confirm unequivocally the mechanism proposed by Haber and Weiss to explain the Fenton reagent reactions2.

Factors affecting radiolytic effects in food

On basis of radiation chemical considerations supported by model system studies, several general predictions can be made regarding the influence of specific parameters on the chemical effects in irradiated foods. Among the parameters considered are: food composition, physical state of components, irradiation temperature, dose-rate, and total dose. Composition determines the type and extent of reactions initiated by both direct and indirect radiolytic interaction. The multiphasic character of foods influences cross reactions between components. State of hydration introduces both physical and chemical aspects into these considerations. Irradiating systems in which the water is frozen changes substantially the chemical effects because the diffusion of reactive entities is impeded. Temperature, especially is frozen systems, has a specific effect on the type and extent of reactions that could occur. Dose and dose-rate, insofar as they affect the pathways for reaction of intermediate species with the original or new components, also determine the type and amount of final radiolysis products. Illustrations of many of these features of the chemistry have been drawn from experiments on model systems of proteins, frozen aqueous solutions, and compounds derived from lipids, as well as from irradiated meats and poultry. These considerations and related experimental results explain the validity of using chemical data to obtain clearances and to extend clearances from approved foods to other foods or to other irradiation conditions.

The Biology and Pathology of Oxygen Radicals

Superoxide radicals (O2-) are commonplace products of the biological reduction of oxygen. Their intrinsic reactivity and ability to generate other more reactive entities constitute a threat to cellular integrity. Superoxide dismutases, enzymes that catalytically scavenge these radicals, have evolved to meet this threat. These metalloenzymes are essential for respiring organisms to survive. Several compounds, such as the antibiotic streptonigrin and the herbicide paraquat, augment the production rate of O2- inside cells. This accounts for the oxygen-enhancement of their lethality. Some bacteria respond to this artificially increased rate of O2- production by synthesizing additional superoxide dismutase. Ionizing radiation generates O2- in its passage through oxygenated aqueous media, and superoxide dismutase added to the suspending medium, decreases the oxygen-enhancement of the lethality of such irradiation of the bacterium Escherichia coli. Production of O2- by activated neutrophils is clinically significant, since it is an important component of the bactericidal actions of these cells and the inflammatory process. Superoxide dismutases exert an anti-inflammatory action that may be useful in managing inflammations.

Charge scavenging in non-polar liquid studied by electrical conductivity measurement

Abstract Radiation-induced electrical conductivity in n-hexane has been measured as a function of the concentration of charge scavenger of low diffusion constant, such as 1,2-benzanthracene, 1,2-benzphenanthrene, pyrene and naphthacene. With increasing scavenger concentration, a gradual decrease of relative electrical conductivity, down to an individual value corresponding to each scavenger, has been observed. This is interpreted as implying that both negative and positive charge carriers, or free ions, are reactive entities throughout their life time. The charge scavenging efficiencies have been found to lie in the following order; 1,2-benzanthracene = 1,2-benzphenanthrene < pyrene < naphthacene.

The Immunologic Properties and Identity of Mucopolysaccharides Associated with C-Reactive Protein in Acute Phase Human Serum

Summary The immunologic and serologic characterization of the mucopolysaccharides associated with CRP as complexes in acute phase serum have been determined by utilization of various hemagglutination techniques. These studies show that the reactive entities have many characteristic features specifically associated with H, Lea and Leb blood group substances. Possible functions of CRP as related to their interaction with these blood group substances are discussed.