Stable Reaction Products Research Articles

Relative reactivity of arsenic and gallium dimers and backbonds during the adsorption of molecular oxygen on GaAs(100)(6×6)

The chemisorption sites of molecular oxygen on the mixed GaAs(100)(6×6) surface were imaged at room temperature using scanning tunneling microscopy (STM). This surface is terminated by both gallium dimers and arsenic dimers, neither of which react with oxygen. Instead, the As–Ga backbonds are shown to react with O2 with 100% chemical selectivity. The reason for this selectivity is found in the interaction of the highly electronegative oxygen atoms with the higher electron density at the arsenic atoms. One oxygen atom displaces the attacked arsenic atom while the other oxygen atom bonds to two nearby gallium atoms, resulting in the thermodynamically most stable reaction products: metallic arsenic clusters and gallium oxide.

Surface Kinetics of Polyphenylene Oxide Etching in a CF 4 / O 2 / Ar Downstream Microwave Plasma

A downstream microwave plasma etcher with in situ diagnostics has been constructed to elucidate the chemical mechanisms in plasma etching of polyphenylene oxide (PPO). reactant gases are used. Stable reaction products are investigated with mass spectrometry while reactive‐free radical information is obtained using optical emission spectroscopy. Additionally, the weight loss of PPO is measured to determine average etch rate. A linear correlation between weight loss measurements of PPO laminates and integration of CO and formation measured by mass spectrometry suggests that real‐time monitoring of polymer etching can be achieved. Etching dynamics are studied with gas compositions of varied from 6.6 to 30%. The etch process exhibits a dynamic reduction in rate with . spectra of X‐ray photoelectron spectroscopy show an increase in fluorination of the etched surface with . A kinetic model based on the dynamic change of the number of carbon sites that are fluorinated is proposed. © 2000 The Electrochemical Society. All rights reserved.

High-performance thin-layer chromatographic determination of six major ginsenosides in Panax ginseng

A densitometric determination of six major ginsenosides in Panax ginseng, separated by high-performance thin-layer chromatography (HPTLC), was optimized. Simple extraction and clean-up methods of the target constituents and the development of standardized conditions of chromatoplates with a quaternary-solvents system allowed an efficient saponins recovery from the plant material and their selective separation. After exposure of the chromatograms to thionyl chloride vapors and further heating, stable reaction products of ginsenosides, which showed absorption maxima at λ=275 nm as well as a fluorescence ( λ excitation=366 nm, λ emission=400 nm), allowed the application of a sensitive and reproducible method for their simultaneous determination. The method was validated by spiking the ginseng extracts with pure standards.

Oxidation of methanol by hydroxyl radicals in aqueous solution under simulated cloud droplet conditions

The results of a detailed mechanistic study of aqueous-phase OH-oxidation of methanol are presented. Analysis of reaction products by specific chromatographic methods revealed that hydrated formaldehyde is not the only stable primary reaction product. Formic acid and/or formate ion are also stable primary molecular reaction products of methanol OH-oxidation. The branching ratios for their formation are highly pH dependent. At pH=7, hydrated formaldehyde is the dominant molecular reaction product (ratio 4.5 : 1 for hydrated formaldehyde : formate ion), whereas at pH=2, formic acid is the dominant product (ratio 3.7 : 1 for formic acid : hydrated formaldehyde). At all pH studied, the sum of the primary stable products represents 49 (±11)% of methanol removal, in agreement with the amount of OOCH 2OH radicals formed relative to methanol removal 48(±2)%. The formation of primary formic acid at pH=2 is attributed to OOCH 2OH self-reaction, and the strong pH effect is attributed to the base-catalyzed decomposition of OOCH 2OH leading to the formation of hydrated formaldehyde. Evaporation and/or an addition reaction between CH 2OH and HO 2 radicals leading to the formation of hydroxymethyl hydroperoxide is proposed to explain the missing yields. The implications of this mechanism to atmospheric chemistry are discussed.

Evolution of CO2 during Birnessite‐Induced Oxidation of 14C‐Labeled Catechol

Phenolic compounds undergo several transformation processes in soil and water (i.e., partial degradation, mineralization, and polymerization), many of which have been attributed primarily to biological activity. Results from previous work indicate that naturally occurring Mn oxides are also capable of oxidizing phenolic compounds. In the present study, 14C‐labeled catechol was reacted with birnessite (manganese oxide) in aqueous suspension at pH 4. The mass of catechol‐derived C in solid, solution, and gas phases was quantified as a function of time. Between 5 and 16% of the total catechol C was liberated as CO2 from oxidation and abiotic ring cleavage under various conditions. Most of the 14C (55–83%) was incorporated into the solid phase in the form of stable organic reaction products whereas solution phase 14C concentrations increased from 16 to 39% with a doubling of total catechol added. Polymerization and CO2 evolution appear to be competitive pathways in the transformation of catechol since their relative importance was strongly dependent on initial birnessite–catechol reaction conditions. Solid phase Fourier transform infrared (FTIR) spectra are consistent with the presence of phenolic, quinone, and aromatic ring cleavage products. Carbon dioxide release appears to be limited by availability of reactive birnessite surface sites and it is diminished in the presence of polymerized reaction products.

Pathways of Nitrosobenzene Reduction by Thiols in Alcoholic Media.

The biologically important reaction of nitrosobenzenes with thiols has been investigated in 2-propanol solution at room temperature, experimental conditions which allow for the detection and characterization of key intermediates. Final stable products of such complex reactions include azoxybenzenes and anilines, formed in relative proportions and at a rate which depend on the reagents initial molar ratio. A detailed description of the reaction of 4-chloronitrosobenzene with benzenethiol in 2-propanol was achieved by means of (1)H NMR in situ analysis. The reaction is initiated by rapid and quantitative coupling of the two reagents into a covalent adduct, an N-hydroxysulfenamide (N(OH)S), which decays to N-(4-chlorophenyl)hydroxylamine. This second intermediate is then converted via competing paths to 4,4'-dichloroazoxybenzene and to N-(4-chlorophenyl)benzenesulfenamide (4-ClC(6)H(4)NHSPh), which in turn decays to 4-chloroaniline. Interestingly, sulfinamides (ArNHS(O)R), major products of the reaction in aqueous media, do not form in 2-propanol.

Ajoene is an inhibitor and subversive substrate of human glutathione reductase and Trypanosoma cruzi trypanothione reductase: crystallographic, kinetic, and spectroscopic studies.

Ajoene ((E,Z)-4,5,9-trithiadodeca-1,6,11-triene 9-oxide), a garlic-derived natural compound, is a covalent inhibitor as well as a substrate of human glutathione reductase (GR) and Trypanosoma cruzi trypanothione reductase (TR). The 2.1-A resolution crystal structure of GR inhibited by (E)-ajoene revealed a mixed disulfide between the active site Cys58 and the CH2=CH-CH2-SO-CH2-CH=CH-S moiety of ajoene. The modified enzyme has a markedly increased oxidase activity when compared to free GR. GR reduces (Z)-ajoene with a kcat/Km of 6.8 x 10(3) M-1 s-1 yielding 4,5,9-trithiadodeca-1, 6,11-triene (deoxyajoene) and 4,8,9,13-tetrathiahexadeca-1,6,10, 15-tetraene as stable reaction products. The reaction leads also to the formation of single-electron reduced products and concomitantly superoxide anion radicals as shown by coupling the reaction to the reduction of cytochrome c. The interactions between the flavoenzymes and ajoene are expected to increase the oxidative stress of the respective cell. The antiparasitic and cytostatic actions of ajoene may at least in part be due to the multiple effects on key enzymes of the antioxidant thiol metabolism.

Ring addition of the alpha-amino group of glutathione increases the reactivity of benzoquinone thioethers.

2-(Glutathion-S-yl)-1,4-benzoquinone was found to be remarkably unstable in phosphate buffer (pH 7.4) even in the absence of oxygen. Intramolecular addition of the alpha-amino group of the glutamate residue to the quinone ring yielded ultimately 2,3-(glutathion-N, S-yl)-1,4-benzoquinone and 2,6-(glutathion-N,S-yl)-1,4-benzoquinone in a 3:1 ratio along with 2-(glutathion-S-yl)-1,4-hydroquinone. Kinetic studies indicated that the cyclization reactions proceeded at a rate k1 of 0.093 min-1, while intermolecular reactions followed a second-order kinetics with a k2 of 94 M-1 min-1 (pH 7.4, 37 degreesC), resulting in multiple polymerization products. Both intramolecular amino adducts of 2-(glutathion-S-yl)-1,4-benzoquinone are prone to hydrolysis, leading to the insertion of an additional OH group in the ring. These S-substituted trihydroxybenzene derivatives are particularly susceptible to autoxidation. The model compound 6-(N-acetylcystein-S-yl)-2-hydroxy-1,4-hydroquinone was shown to form readily two atropoisomeric biphenyls upon autoxidation: 2,4'-bis(N-acetylcystein-S-yl)-2',3,3',4,6, 6'-hexahydroxybiphenyl, indicating C-C coupling, presumably via semiquinone radical intermediates. Thus, the sequence of glutathione S-addition, followed by oxidation, N-addition, oxidation, and hydrolysis, constitutes a novel and very effective activation pathway of quinones for eliciting oxidative stress. These data underline the fact that glutathione conjugates of autoxidizable aromatics are no obligatory stable end products of a detoxication reaction. The possible toxicological impacts of intra- and intermolecular addition reactions of quinoid thiol conjugates are discussed.

Crystallization of xylanase from Erwinia chrysanthemi: influence of heat and polymeric substrate.

Xylanase from the bacterial plant pathogen Erwinia chrysanthemi (E.C. 3.2.1.8), expressed in E. coli, has been crystallized for X-ray diffraction analysis both in the presence and the absence of its polymeric substrate 4-O-methyl glucuronoxylan. In all cases it was found that the quality, time of appearance, and reproducibility of both the native and complex crystals were significantly enhanced by heating of the protein to 323 K prior to dispensing the crystallization trials. Crystals of the native protein are ideal for X-ray diffraction analysis, producing Bragg reflections beyond 1.5 A resolution with virtually no degradation with time. The native crystals are in space group P2(1), with a = 39.33, b = 49.46, c = 90.85 A and beta = 101.58 degrees. Other polymorphs have also been obtained and their cell parameters determined. Crystallization of the enzyme in the presence of polymeric substrate yields two distinctly different crystals at different concentrations of the xylan. These are thought to be complexes of the protein with stable products of the enzymatic reaction. A similar result had been obtained previously with pancreatic alpha-amylase and its substrate glycogen.

Laser Flash Photolysis Study of Phenylcarbene and Pentafluorophenylcarbene

Laser flash photolysis (LFP) (XeCl, 308 nm, 17 ns) of phenyldiazomethane and pentafluorophenyldiazomethane releases phenylcarbene (PC) and pentafluorophenylcarbene (PFPC), respectively. In acetonitrile solvent the carbenes react rapidly to form nitrile ylides which have convenient absorption maxima for optical detection (λmax = 350 nm). Phenylcarbene and pentafluorophenylcarbene each react with acetonitrile with an absolute rate constant of 2.4 × 106 M-1 s-1 in CF2ClCFCl2 (Freon-113) at ambient temperature. The lifetimes of spin-equilibrated PC and PFPC are 190 and 500 ns in Freon-113, respectively, and the lifetime of each carbene is deduced to be 22 ns in neat acetonitrile. LFP of phenyl and pentafluorophenyldiazomethane in the presence of pyridine leads to the expected pyridine ylide. The observed absolute rate constants of reaction of spin equilibrated PC and PFPC with pyridine are 1.9 × 107 and 5.1 × 107 M-1 s-1, respectively, in Freon-113 at ambient temperature. From this data, one can deduce singlet−triplet splittings (ΔGST, 298 K) of 2.3 and 3.1 kcal/mol for PC and PFPC, respectively, which is the difference in energy between the triplet minimum and the point of intersection of the singlet and triplet carbene plus pyridine surfaces. LFP of phenyldiazomethane and pentafluorophenyldiazomethane in acetonitrile containing carbene scavengers (e.g., alcohols, alkenes, and silanes) gives reduced yields of nitrile ylides. Analysis of the yield of ylide as a function of quencher by the Stern−Volmer method gives kQτ values of carbene quenching. Low-temperature (77 K) photolysis of pentafluorophenyldiazomethane generates the persistent EPR spectrum of triplet pentafluorophenylcarbene which is the ground state of this carbene. However, photolysis of pentafluorophenyldiazomethane at ambient temperature generates stable reaction products derived from capture of singlet pentafluorophenylcarbene. Thus the properties of pentafluorophenylcarbene are remarkably similar to those of phenylcarbene. Each carbene has a triplet ground state but reacts in solution at ambient temperature through a low-lying excited singlet state. The lack of a fluorine substituent effect on the behavior of phenylcarbene is compared to that in singlet phenylnitrene.

Products of the Gas-Phase Reactions of NO3 Radicals with Furan and Tetramethylfuran

Furan and substituted furans were found to be atmospheric reaction products of the OH radical-initiated degradation of conjugated dienes, such as 1,3-butadiene, cis-1,3-pentadiene, and isoprene. They are also products of combustion processes. The purpose of this work is to identify the primary formed stable reaction products of the reaction of NO3 radicals toward furan and tetramethylfuran. Experiments were performed under flow conditions in the pressure range 5.5 < P (mbar) < 100 at 298 K. GC−MS/FID, direct MS, and long-path FT-IR served as detection techniques. cis-Butenedial and 3H-furan-2-one were found as main products of the reaction of NO3 radicals with furan in N2 with averaged formation yields of 0.77 and 0.19, respectively, independent of total pressure in the tube. In experiments with a maximum O2 concentration of 2.2 × 1018 molecule cm-3, an increase of the 3H-furan-2-one yield was observed up to 0.38. There were no indications for the formation of nitrates or peroxynitrates. In the reaction o...

Mechanisms for the heterogeneous hydrolysis of hydrogen chloride, chlorine nitrate and dinitrogen pentoxide on water‐rich atmospheric particle surfaces

The heterogeneous interaction of the stratospheric reservoir species HCl, ClONO2, and N2O5 with water‐rich polar stratospheric particle mimics is characterized by the formation of solvated ionic products. Simple semiempirical calculations have been used to explain the nature of the species observed in infrared spectroscopic measurements and to elucidate the mechanism by which they are formed. The initial stages of the interaction appear to involve an SN2‐type nucleophilic attack by the oxygen atom of the surface water molecule upon the most accessible electrophilic site of the adsorbing reactant. Mechanistic schemes involving protonated acid intermediates and their subsequent decomposition or hydrolysis can be used to accurately predict and explain the stable reaction products observed spectroscopically under stratospheric conditions.

Laser-Evaporated Aluminum Atom Reactions with Halogen Molecules. Infrared Spectra of AlXn (X = F, Cl, Br, I; n = 1−3) in Solid Argon

Reactions of laser-evaporated aluminum atoms with molecular fluorine, chlorine, bromine, and iodine in excess argon during condensation onto a CsI window at 11 ± 1 K produced the AlXn (X = F, Cl, Br, I and n = 1−3) and Al2X6 molecules, which were characterized by infrared absorption spectroscopy. Low laser power favored the primary reaction product AlX and AlX2 transient species over the more stable secondary reaction product AlX3 molecules. Chlorine and bromine isotopic structures substantiated the molecular identifications and vibrational assignments. DFT calculations were done for the AlCln and AlBrn series to support the vibrational assignments. Possible Al2Cl4 structures were also explored by DFT calculations, and evidence is presented for a distorted Cl2AlCl2Al structure. The present study provides further evidence for transient aluminum dihalide radicals for the four halogens.

In situ analysis of the tribochemical films formed by SiC sliding against Mo in partial pressures of SO2, O2, and H2S gases

X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) were used to identify gas reaction layers and tribochemical films formed during reciprocating sliding tests in an ultrahigh vacuum (UHV) tribometer. Tests were performed on UHV cleaned SiC pins and Mo flats during or after exposure to SO2, O2, or H2S gas at pressures around 40 Pa. XPS identified the gas reaction layers on Mo to be chemisorbed MoS2 and/or MoO2 phases less than 1 nm thick. AES of Mo wear tracks showed tribochemical films similar in composition to, but thicker than, the reaction layers. AES of SiC wear scars in all three gases indicated tribochemical films containing Si oxide and/or Si sulfide and possibly graphite. In addition, transfer films of Mo oxysulfide and Mo oxide were found in SO2 and O2 tests, respectively, but no transfer films were detected in H2S tests. Thermochemical calculations of stable reaction products of the gas–solid reactions were in good agreement with the phases inferred from XPS and AES. An explanation for the agreement between thermochemical predictions and tribochemical results is given.

Evaluation of the stable reaction products of histidine with formaldehyde or with other carbonyl compounds in dairy products

Combined formaldehyde (FA) can be evaluated in cheese and other dairy products by determining spinacine (6-carboxy-1,2,3,4-tetrahydroimidazopyridine), an imino acid arising from the reaction of FA with the alpha-amino group of histidine. Other carbonyl compounds usually produced by fermentations can react themselves with histidine, forming molecules which interfere with the determination of spinacine. A sensitive (minimum detectable amount, 5 fmol spinacine) and interference-free HPLC method, with precolumn derivatization with o-phtalaldehyde (OPA) and 9-fluorenylmethylchloroformate (FMOC), to evaluate these histidine reaction-compounds in dairy products is described. The clean-up of the FMOC-derivatized sample has been performed for the first time with solid-phase extraction (SPE) on an amino cartridge. The method was applied to samples of casein and differently ripened cheese, the origins of which were either known or commercial, in order to quantify the natural level of spinacine (0.7-2.7 ppm) probably deriving from biogenic FA, and to detect whether FA was used in processing.

Engineered flux-pinning centers in Bi2Sr2CaCu2Ox and TIBa2Ca2Cu3Ox superconductors

Three methods were used to introduce flux-pinning centers into Bi2Sr2CaCu2Ox (Bi-2212) and TlBa2Ca2Cu3Ox (Tl-1223) samples. It was found that carbon induced local decomposition, that nanosized Al2O3 additions created stable reaction products, and that second phases could be isolated in Tl-1223 during synthesis. Each of these defects enhanced flux pinning and was of most benefit at temperatures ≤ 35K.

Tumor Necrosis Factor-α and Interferon-γ Modulation of Nitric Oxide and Allograft Survival

Tumor necrosis factor-α (TNF) and interferon-γ (IFN) modulate immune responses, as TNF is postulated as a first messenger for priming immune cells and IFN as the second messenger for activation. Nitric oxide (NO) is also an important mediator of immune function, as NO produced by immune cells in response to cytokines such as TNF and IFN may be a cytotoxic end-product effector of immune activation. To examine TNF and IFN modulation of NO production and effects upon allograft survival, we studied the in vitro effect of TNF, IFN, and lipopolysaccharide (LPS, as a nonspecific immune stimulator and TNF promoter) singly or together on NO production in unstimulated rat splenocytes or in response to allogeneic stimulation in MHC mismatch (Brown-Norway, BN, vs Lewis, LEW) mixed lymphocyte reactions. Nitrite as a stable reaction product of NO was determined by a colorimetric method based on the Griess reaction. Interestingly, unstimulated cells had little NO production in response to TNF or IFN; however, following nonspecific (LPS) or allogeneic stimulation there was a significant upregulation of NO production that was synergistically enhanced by the presence of both TNF and IFN. Significantly, anti-TNF antibody inhibited NO production up to 60% in all groups. In vivo studies used a cardiac heterotopic allotransplant model, again BN to LEW. Control recipients received no immunotherapy. Experimental recipients received IFN alone, anti-IFN antibody, anti-TNF antibody, or anti-TNF and anti-IFN antibody. Results from these allograft experiments showed no influence on survival by IFN alone or anti-FN antibody alone. However, anti-TNF antibody extended allograft survival, which was synergistically and significantly enhanced with the addition of anti-IFN antibody. These results demonstrate that cytokines synergistically modulate immune response via NO synthesis, as NO is markedly increased by IFN in the presence of allogeneic stimulation or TNF/LPS. Furthermore, as anti-IFN was only beneficial to allograft survival in the presence of anti-TNF, the priming and activating roles of these cytokines in modulating the alloimmune response via NO production suggest that transplant rejection may be mediated through a distinct cascade. Modulation of cytokine-NO cascades may lead to new therapies.

FT-EPR Study of the Photolysis of 4-Chlorophenol

A number of publications have been concerned with the photolysis of 4-chlorophenol (4CP). The interest in the photochemistry of 4CP, and halogenated phenols in general, stems from efforts to develop a method for the photodegradation of these important pollutants. The study provides evidence that photoexcitation of 4CP in aqueous solution produces the 4-oxocyclohexa-2,5-dienylidene (carbene) as the first detectable transient reaction product. Reactions of the (triplet state) carbene account for the stable reaction products found under different conditions. It is of special note that quenching by alcohols of the transient absorption assigned to the carbene is accompanied by a growing in of the absorption spectrum of the phenoxyl radical. This is attributed to the hydrogen abstraction reaction. FT-EPR by identifying the transient free radicals formed in the photolysis of 4CP and the spin state of their precursor, provides strong support for the novel reaction scheme. 10 refs., 1 fig.

High-resolution infrared spectroscopy of etching plasmas

Infrared absorption spectroscopy has been used to measure the absolute densities of neutral particles in various fluorocarbon RF plasmas. The densities of CF2 radicals have been measured using a tunable diode laser. Moreover, broadband absorption spectra obtained using a Fourier transform spectrometer have been used to determine the densities of stable reaction products and to assess the degree of dissociation in the plasma. The results indicate that the rotational temperatures of all species involved are slightly above room temperature. The density of CF2 at high gas pressures increases close to the electrodes, indicating production near or on the electrodes and loss in the gas phase. The dissociation degree in plasmas of gases with a high C/F ratio can be as high as 90%. From an analysis of the flow dependence of the degree of dissociation the total dissociation rate coefficients of CF4, CF2Cl2, CF3Cl and C2F4Cl2 have been calculated.

Simultaneous detection of neuropeptides and messenger RNA in the magnocellular hypothalamo-neurohypophysial system by a combination of non-radioactive in situ hybridization histochemistry and immunohistochemistry

A protocol was developed combining non-radioactive in situ hybridization histochemistry with enzyme based immunohistochemistry, detect the expression of mRNA in phenotypically defined neurons. Free-floating brain sections were hybridized with the oligonucleotide probes which have been 3'-end labelled with biotin-11-dUTP. The hybridized probe was visualized by a combined avidin-biotin bridge method, anti-avidin immunohistochemistry, and horseradish peroxidase detection using diaminobenzidine as a substrate. The in situ hybridization step yielded a very stable reaction product enabling subsequent immunohistochemical reactions using horseradish peroxidase and benzidine dihydrochloride as a chromogen. Magnocellular neurons of the hypothalamo-neurophypophysial system synthesize either vasopressin or oxytocin; water deprivation and chronic saline ingestion are potent stimuli for the expression of both of the genes encoding these neuropeptides. A number of other neuropeptides with putative transmitter action are synthesized in magnocellular neurons during such stimulation. Experiments were performed to explore whether neuropeptide Y immunoreactivity is present within magnocellular vasopressin mRNA-expressing neurons of the hypothalamo-neurophypophysial system. The results clearly demonstrated that neuropeptide Y-immunoreactive elements were present within a number of magnocellular vasopressin mRNA-containing cells. In addition, immunohistochemical detection of the neuropeptides ocytocin and cholecystokinin was carried out on sections hybridized non-radioactively for vasopressin; as expected vasopressin mRNA did not co-exist with cholecystokinin, whereas a few oxytocin immunoreactive neurons in osmotically stimulated animals also contained vasopressin mRNA. The developed method makes possible the immunohistochemical detection of intracellular antigens with concomitant detection of intracellular mRNA.