Loss Of Group Research Articles

Understanding of the mass spectrometric fragmentation pathways of a few potentially genotoxic haloaniline isomers in their protonated form by collision-induced dissociation

Collision-induced dissociation (CID) mass spectra of a few haloaniline isomers, (chloroanilines, dichloroanilines, difluoroanilines, chloro-fluoroanilines and bromo-fluoroanilines) were characterized. The mass spectral behaviour of difluoroanilines was different from those of the corresponding regioisomers of the other haloanilines. For all ortho regioisomers except difluoroanilines, CID mass spectra resulted in hydrogen halide as well as halogen radical loss. In the case of difluoroanilines, peaks corresponding to hydrogen fluoride loss were observed during the same process. Meta and para-haloanilines have the tendency to lose either ammonia or halogen radicals. Six regioisomers of dichloroanilines were subjected to hydrogen/deuterium exchange experiments in solution to determine the CID fragmentation pathways. From the experimental results we propose two fragmentation pathways for the dicholoroanilines: (a) formation of aza-biheterocyclic intermediate and (b) via heterolytic hydrogen transfer from the charged center. The demonstrated unique characteristics in CID mass fragmentation of haloanilines may be useful in identification and differentiation of isomers as impurities during chemical process development. A good use of the ortho effect is the significant differentiation between 2-chloro-4-fluoroaniline and 4-chloro-2-fluoroaniline by CID mass spectra.

Re‐examination of the anion derivatives of isoflavones by radical fragmentation in negative electrospray ionization tandem mass spectrometry: experimental and computational studies

This paper reports theoretical and experimental studies of gas-phase fragmentation reactions of four naturally occurring isoflavones. The samples were analyzed in negative ion mode by direct infusion in ESI-QqQ, ESI-QqTOF and ESI-Orbitrap systems. The MS/MS and MS(n) spectra are in agreement with the fragmentation proposals and high-resolution analyses have confirmed the formulae for each ion observed. As expected, compounds with methoxyl aromatic substitution have showed a radical elimination of •CH(3) as the main fragmentation pathway. A second radical loss (•H) occurs as previously observed for compounds which exhibit a previous homolytic •CH(3) cleavage (radical anion) and involves radical resonance to stabilize the anion formed. However, in this study we suggest another mechanism for the formation of the main ions, on the basis of the enthalpies for each species. Compounds without methoxy substituent dissociate at the highest energies and exhibit the deprotonated molecule as the most intense ion. Finally, energy-resolved experiments were carried out to give more details about the gas-phase dissociation reaction of the isoflavones and the results are in agreement with the theoretical approaches.

Effect of Solvents on the Decomposition of Hydroperoxides and Reactions of Phenoxyl Radical Loss

The influence of the properties of the medium on the rate of radical processes can be generalised by means of multiparameter equations taking into account the different properties of the solvents. The applicability of this approach is demonstrated for the case of the decomposition of tetralin hydroperoxide and lithium salt of tert-butyl hydroperoxide, and also the recombination of phenoxyl radicals. In all cases, an important role is played by the cohesive energy density of the solvents, i.e. their self-association.

Negative pressure wound therapy: an adjuvant to surgical reconstruction of large or difficult skin and soft tissue defects

Wide and deep wound defects are a challenge to surgeons, particularly when aggressive or more radical operations are unavailable. This article introduces refined techniques, indications and the clinical experience of the application of negative pressure wound therapy (NPWT) as an easy ancillary wound dressing method for the reconstruction of large or difficult skin and soft tissue defects. The authors used NPWT on 88 patients as an adjuvant therapy before reconstructive surgery since 2006. NPWT was applied for two different treatment strategies: as an adjuvant therapy to facilitate the formation of a healthy wound bed and to reduce the size and depth of a defect. NPWT was used as an alternative dressing method for skin grafting for infants or mobile skin surfaces such as the neck, penis, dorsum of the hand, knee joint, abdomen, etc. There were no typical wound complications. NPWT application, if used appropriately, produces successful surgical reconstructions for large, deep skin and soft tissue defects without extensive or radical flap surgery or loss of skin graft.

Gas Chromatography—Electron Ionization—Mass Spectrometry Analysis of O,O′-Dialkyl Methylphosphonites for Verification Analysis of the Chemical Weapons Convention

Gas chromatography-mass spectrometry (GC-MS) analysis of O,O'-dialkyl methylphosphonites (DAMPs) was carried out with a view to developing a database and understanding the mechanism of fragmentation. DAMPs are included in the list of schedule 2B4 chemicals of the Chemical Weapons Convention. GC-MS analysis of DAMPs and their deuterated analogs revealed that their fragmentations were dominated by α-cleavages, alkenyl radical loss and hydrogen rearrangements. Based on fragment ions of deuterated analogs and density functional theory calculations, the fragmentation routes were rationalized.

Interaction of gold and silver cluster cations with CH3Br: thermal and photoinduced reaction pathways

The reactions of free, size selected Au n + (n = 1–3, 5, 7) and Ag3 + clusters with CH3Br as well as the photodissociation of the resulting complexes at 266 nm were studied in a radio frequency ion trap under multiple collision conditions. CH3Br was found to interact more strongly with the gold clusters than with the silver clusters. All investigated metal clusters exhibited characteristic size dependent adsorbate coverages. Furthermore, the successive loss of methyl radicals was identified as a major thermal reaction channel of the adsorbed CH3Br molecules. Photodissociation experiments were performed with the product complexes of the trimer clusters and revealed the strongly preferred light-induced fragmentation of Au3Br3 + and Ag3Br3 +, respectively, over any other thermal reaction products. However, whereas in the case of the gold cluster complexes the bare Au3 + was exclusively re-formed through laser irradiation, considerable photoinduced metal cluster fragmentation occurred in the case of Ag3Br3 +.

Reactions of simple and peptidic alpha-carboxylate radical anions with dioxygen in the gas phase

α-Carboxylate radical anions are potential reactive intermediates in the free radical oxidation of biological molecules (e.g., fatty acids, peptides and proteins). We have synthesised well-defined α-carboxylate radical anions in the gas phase by UV laser photolysis of halogenated precursors in an ion-trap mass spectrometer. Reactions of isolated acetate (˙CH(2)CO(2)(-)) and 1-carboxylatobutyl (CH(3)CH(2)CH(2)˙CHCO(2)(-)) radical anions with dioxygen yield carbonate (CO(3)˙(-)) radical anions and this chemistry is shown to be a hallmark of oxidation in simple and alkyl-substituted cross-conjugated species. Previous solution phase studies have shown that C(α)-radicals in peptides, formed from free radical damage, combine with dioxygen to form peroxyl radicals that subsequently decompose into imine and keto acid products. Here, we demonstrate that a novel alternative pathway exists for two α-carboxylate C(α)-radical anions: the acetylglycinate radical anion (CH(3)C(O)NH˙CHCO(2)(-)) and the model peptide radical anion, YGGFG˙(-). Reaction of these radical anions with dioxygen results in concerted loss of carbon dioxide and hydroxyl radical. The reaction of the acetylglycinate radical anion with dioxygen reveals a two-stage process involving a slow, followed by a fast kinetic regime. Computational modelling suggests the reversible formation of the C(α) peroxyl radical facilitates proton transfer from the amide to the carboxylate group, a process reminiscent of, but distinctive from, classical proton-transfer catalysis. Interestingly, inclusion of this isomerization step in the RRKM/ME modelling of a G3SX level potential energy surface enables recapitulation of the experimentally observed two-stage kinetics.

Dissociative photoionization mechanism of methanol isotopologues (CH3OH, CD3OH, CH3OD and CD3OD) by iPEPICO: energetics, statistical and non-statistical kinetics and isotope effects

The dissociative photoionization of energy selected methanol isotopologue (CH(3)OH, CD(3)OH, CH(3)OD and CD(3)OD) cations was investigated using imaging Photoelectron Photoion Coincidence (iPEPICO) spectroscopy. The first dissociation is an H/D-atom loss from the carbon, also confirmed by partial deuteration. Somewhat above 12 eV, a parallel H(2)-loss channel weakly asserts itself. At photon energies above 15 eV, in a consecutive hydrogen molecule loss to the first H-atom loss, the formation of CHO(+)/CDO(+) dominates as opposed to COH(+)/COD(+) formation. We see little evidence for H-atom scrambling in these processes. In the photon energy range corresponding to the B[combining tilde] and C[combining tilde] ion states, a hydroxyl radical loss appears yielding CH(3)(+)/CD(3)(+). Based on the branching ratios, statistical considerations and ab initio calculations, this process is confirmed to take place on the first electronically excited Ã(2)A' ion state. Uncharacteristically, internal conversion is outcompeted by unimolecular dissociation due to the apparently weak Renner-Teller-like coupling between the X[combining tilde] and the à ion states. The experimental 0 K appearance energies of the ions CH(2)OH(+), CD(2)OH(+), CH(2)OD(+) and CD(2)OD(+) are measured to be 11.646 ± 0.003 eV, 11.739 ± 0.003 eV, 11.642 ± 0.003 eV and 11.737 ± 0.003 eV, respectively. The E(0)(CH(2)OH(+)) = 11.6454 ± 0.0017 eV was obtained based on the independently measured isotopologue results and calculated zero point effects. The 0 K heat of formation of CH(2)OH(+), protonated formaldehyde, was determined to be 717.7 ± 0.7 kJ mol(-1). This yields a 0 K heat of formation of CH(2)OH of -11.1 ± 0.9 kJ mol(-1) and an experimental 298 K proton affinity of formaldehyde of 711.6 ± 0.8 kJ mol(-1). The reverse barrier to homonuclear H(2)-loss from CH(3)OH(+) is determined to be 36 kJ mol(-1), whereas for heteronuclear H(2)-loss from CH(2)OH(+) it is found to be 210 kJ mol(-1).

Kinetics of reduction of tyrosine phenoxyl radicals by glutathione

Modification of tyrosine (TyrOH) is used as a marker of oxidative and nitrosative stress. 3,3′-Dityrosine formation, in particular, reflects oxidative damage and results from the combination of two tyrosyl phenoxyl radicals (TyrO ). This reaction is in competition with reductive processes in the cell which ‘repair’ tyrosyl radicals: possible reductants include thiols and ascorbate. In this study, a rate constant of 2 × 10 6 M −1 s −1 was estimated for the reaction between tyrosyl radicals and glutathione (GSH) at pH 7.15, generating the radicals by pulse radiolysis and monitoring the tyrosyl radical by kinetic spectrophotometry. Earlier measurements have suggested that this ‘repair’ reaction could be an equilibrium, and to investigate this possibility the reduction (electrode) potential of the (TyrO ,H +/TyrOH) couple was reinvestigated by observing the fast redox equilibrium with the indicator 2,2′-azinobis(3-ethylbenzothiazoline-6-sulphonate). Extrapolation of the reduction potential of TyrO measured at pH 9–11 indicated the mid-point reduction potential of the tyrosyl radical at pH 7, E m7(TyrO ,H +/TyrOH) = 0.93 ± 0.02 V. This is close to the reported reduction potential of the glutathione thiyl radical, E m7 = 0.94 ± 0.03 V, confirming the ‘repair’ equilibrium constant is of the order of unity and suggesting that efficient reduction of TyrO by GSH might require removal of thiyl radicals to move the equilibrium in the direction of repair. Loss of thiyl radicals, facilitating repair of TyrO , can arise either via conjugation of thiyl with thiol/thiolate or oxygen, or unimolecular transformation, the latter important at low concentrations of thiols and oxygen.

Peroxy radical partitioning during the AMMA radical intercomparison exercise

Abstract. Peroxy radicals were measured onboard two scientific aircrafts during the AMMA (African Monsoon Multidisciplinary Analysis) campaign in summer 2006. This paper reports results from the flight on 16 August 2006 during which measurements of HO2 by laser induced fluorescence spectroscopy at low pressure (LIF-FAGE) and total peroxy radicals (RO2* = HO2+ΣRO2, R = organic chain) by two similar instruments based on the peroxy radical chemical amplification (PeRCA) technique were subject of a blind intercomparison. The German DLR-Falcon and the British FAAM-BAe-146 flew wing tip to wing tip for about 30 min making concurrent measurements on 2 horizontal level runs at 697 and 485 hPa over the same geographical area in Burkina Faso. A full set of supporting measurements comprising photolysis frequencies, and relevant trace gases like CO, NO, NO2, NOy, O3 and a wider range of VOCs were collected simultaneously. Results are discussed on the basis of the characteristics and limitations of the different instruments used. Generally, no data bias are identified and the RO2* data available agree quite reasonably within the instrumental errors. The [RO2*]/[HO2] ratios, which vary between 1:1 and 3:1, as well as the peroxy radical variability, concur with variations in photolysis rates and in other potential radical precursors. Model results provide additional information about dominant radical formation and loss processes.

The influence of fragment size and intermediate barriers on competing near-identical simple cleavage reactions: A variational RRKM study

The fragmentation reactions of almost-symmetrical radical cations exhibit remarkable energy-dependent selectivity. The simple cleavage of the N-propylbutylamine radical cation and its N-methyl analog results in loss of ethyl or propyl radicals. The energy required to expel the ethyl radical (the smaller alkyl fragment) is lower, and ethyl loss is the preferred reaction of ions with low internal energy. The branching ratio is reversed for ions with higher energy, which can be accounted for with variational transition state theory. The competition between the two concurrent cleavage reactions is not governed by product-like orbiting transition states, but by relatively tight transition states; however, these are not equally tight. The transition state for loss of ethyl from higher energy reactant ions lies earlier on the reaction coordinate than does the transition state for loss of propyl, and the latter will in turn be slightly more product-like and hence more loose. k( E) vs E for loss of propyl radicals therefore rises more steeply with increasing internal energy, and this reaction becomes the more rapid process for higher-energy reactants such as the molecular ions formed in mass spectrometers with 70 eV electron ionization. The transition states for cleavage reactions exhibit energy-dependent shifting also when moderate intermediate barriers are present.

Binuclear Oxidative Addition of Aryl Halides

Reaction of LCoCH2SiMe3 (L = 2,6-bis[2,6-dimethylphenyliminoethyl]pyridine) with H2 produces LCo(N2), presumably via intermediate LCoH. Reaction of LCo(N2) (prepared in this way or via reaction of LCoCl2 with Na/Hg) with aryl halides ArX (X = Cl, Br, I) produces LCoAr and LCoX in a ratio depending on the nature of Ar and X. For X = Cl, the reaction is slowest but also produces the largest amount of LCoAr. Electron-withdrawing substituents both accelerate the reaction and improve the yield of LCoAr. Computational studies support a radical mechanism for this reaction, involving displacement of N2 to give LCo(XAr) followed by loss of the Ar radical, which then binds to a second Co(0) moiety.

Conformality of Plasma-Assisted ALD: Physical Processes and Modeling

For plasma-assisted atomic layer deposition (ALD), reaching conformal deposition in high aspect ratio structures is less straightforward than for thermal ALD due to surface recombination loss of plasma radicals. To obtain a detailed insight into the consequences of this additional radical loss, the physical processes in plasma-assisted ALD affecting conformality were identified and investigated through Monte Carlo simulations. The conformality was dictated by the recombination probability , the reaction probability , and the diffusion rate of particles. When recombination losses play a role, the saturation dose depended strongly on the value of . For the deposition profiles, a minimum at the bottom of trench structures was observed (before reaching saturation), which was more pronounced with larger values of . In turn, three deposition regimes could be identified, i.e., a reaction-limited regime, a diffusion-limited regime, and a new regime that is recombination-limited. For low values of , conformal deposition in high aspect ratio structures can still be achieved, as observed for several metal oxides, even for aspect ratios as large as 30. For high surface recombination loss probabilities, as appears to be the case for many metals, achieving a reasonable conformality becomes challenging, especially for aspect ratios .

Magnetic field effects on photochemical reaction III. Mechanistic study of the photodecarboxylation of arylmethyl esters in micellar solutions

Photolysis of benzyl phenylacetate (ACO2A), benzyl p-methylphenylacetate (BCO2A) and 1-naphthyl phenylacetate (ACO2Np) in homogeneous and micellar solutions results in products typical of radical coupling reactions and loss of carbon dioxide. The cage effects upon direct photolysis of these esters in acetonitrile are significantly greater than zero and those in SDS or HDTBr micellar solutions are even greater. In the triplet sensitization reaction of ACO2Np in micellar solutions the cage effects are smaller than those obtained upon direct irradiation. However, the application of a 0.05 or 0.2 T external magnetic field in the photolysis in micellar solutions decreases the cage effects dramatically. All these results suggest that the photoextrusions of carbon dioxide from these esters proceed via radical intermediates, and upon direct photolysis the reactive states for the cleavage reaction involve both singlet and triplet excited states.

Discriminating Hodgdon Pyrodex® and Triple Seven® Using Gas Chromatography-Mass Spectrometry

Pyrodex(®) and Triple Seven(®) are black powder substitutes that often find use as fillers in improvised explosive devices, such as pipe bombs. These propellants have essentially the same overall appearance and oxidizers, but different fuels. For example, Pyrodex(®) contains sulfur, sodium benzoate, and dicyandiamide (DCDA), whereas Triple Seven(®) lacks sulfur but also contains 3-nitrobenzoic acid. In this method, intact particles and postblast solid residues were reacted with bis(trimethylsilyl)trifluoroacetamide + 1% trimethylchlorosilane in acetonitrile for 30 min at 60°C. The resultant trimethylsilyl derivatives of the organic fuels were then analyzed by gas chromatography-mass spectrometry. Each derivative was clearly resolved from other components, and high-quality mass spectra were obtained. In addition, characteristic fragments resulting from loss of a methyl radical from the molecular ion (m/z 163 for sulfur, m/z 171 for DCDA, m/z 179 for benzoic acid, and m/z 224 for nitrobenzoic acid) were able to be monitored.

Macroradical reaction in ultra-high molecular weight polyethylene in the presence of vitamin E

Free radical measurements in compression molded ultra-high molecular weight polyethylene (UHMWPE), which contained vitamin E (α-tocopherol (α-T)), was performed using electron spin resonance (ESR) technique in air at room temperature following gamma irradiation (25–32 kGy) in N 2. The vitamin E was incorporated into one set of samples by blending UHMWPE resin with vitamin E (1 and 10 wt%), then compression molded into a solid and then irradiated. Another set of samples had vitamin E incorporated into them by diffusing vitamin E at 100 °C for 2 h after irradiation. Compared to a control (with no vitamin E), the vitamin E-containing UHMWPE (α-TPE) samples suffered a partial loss of PE radicals, but this loss only occurred during or immediately after irradiation (before exposure to air). Subsequently, when all blended samples were exposed to air, the remaining radicals in each sample decayed to the well-known OIR, R1 (− CH–[CH=CH−] m −) and R2 ( OCH–[CH=CH−] m −) radicals. However, because of the initial loss or partial quenching, α-TPE produced a lower concentration of OIR (measured over a four-year period), but no difference was found between 1% and 10% α-TPEs. In the diffused α-TPE, similar OIR was also found when tested after four months of post-treatment exposure to air.

Sources and Cycling of Tropospheric Hydroxyl Radicals – An Overview

Abstract The hydroxyl radical is the principal oxidizing agent in the troposphere. It controls and determines the oxidizing power of the atmosphere and thus governs the atmospheric lifetime of many species, and their potential to contribute to climate change, air pollution and ozone formation. Owing to the development of new measurement techniques and the discovery of new reaction mechanisms, the understanding of the OH chemistry has witnessed significant advances during the last decades. This article presents a comprehensive review of the current understanding of OH radical chemistry with an outlook to future work in this active research area of atmospheric chemistry.Among the different OH radical initiation sources, e.g. O3 photolysis, HCHO photolysis, ozonolysis of alkenes and the photolysis of nitrous acid (HONO), the latter has been shown in recent field and model studies to make a major contribution to tropospheric OH levels. New photochemical sources of HONO have been discovered in the laboratory which can explain unexpectedly high concentrations of HONO observed in the atmosphere during daytime. A detailed analysis of HONO sources in the atmosphere is presented. OH radicals react with volatile organic compounds (VOCs) in a complex cycle regenerating OH radicals in the propagation cycle. Recent field and model studies show that the OH radical propagation cycle is balanced for high NOx conditions. Thus, radical losses by reactions with VOCs are balanced by OH formation through the reaction HO2+NO. However, studies at low NO and high VOC levels imply that unknown radical sources of OH exist in rural and forested areas. Possible sources of OH radicals under these conditions are discussed.

ChemInform Abstract: Mn(III)-Promoted Sulfur-Directed 4-exo-Trig Radical Cyclization of Enamides to β-Lactams.

Abstract The synthesis of β-lactams vinylated at C-4 from N-(3-phenylthio-1-alkenyl)amides was carried out by Mn(III)-promoted 4-exo-trig radical cyclization followed by β-fragmentative loss of phenylthiyl radical. The effects on the reaction course of different substituents both on enamidic nitrogen atom or double bond were analyzed. The overall reaction was stereoselective, leading to trans azetidinones.

Natural Structural Motifs that Suppress Peptide Ion Fragmentation after Electron Capture

Series of doubly and triply protonated diarginated peptide molecules with different number of glutamic acid (E) and asparagine (N) residues were analyzed under ECD conditions. ECD spectra of doubly-protonated peptides show a strong dependence on the number of E and N residues. Both the backbone cleavages and hydrogen radical (H*) loss from the charge-reduced precursor ions ([M+2H](+*)) were suppressed as the number of E and N residues increases. A strong inhibition of the backbone cleavages and H* loss from [M+2H](+*) was found for peptides with 6E residues (or 4E + 2N residues). The results obtained using these model peptides were re-confirmed by analyzing N-arginated Fibrinopeptide-B (i.e., REGVNDNEEGFFSAR). In contrast to the N-arginated peptide, ECD of the doubly-protonated Fibrinopeptide-B and its analogues show extensive backbone cleavages leading to series of c- and z-ions ( approximately 80% sequence coverage). Based on these results, it is believed that peptide ions with all surplus protons sequestered in arginine-residues would show enhanced stability under ECD conditions as the number of acid-residue increases. The suppression of backbone cleavages and H* loss from [M+2H](+*) are presumably attributed to the low reactivity of the charge-reduced precursor ions. One of the possible hypothesis is that diarginated E-rich peptides may contain hydrogen bonds between carbonyl oxygen of E side chains and backbone amide hydrogen. These hydrogen bonds would provide extra stabilization for [M+2H](+*). This is the first demonstration of natural structural motifs in peptides that would inhibit the backbone fragmentation of the charge-reduced peptide ions under ECD conditions.

Variable Denticity in Carboxylate Binding to the Uranyl Coordination Complexes

Tris-carboxylate complexes of uranyl [UO(2)](2+) with acetate and benzoate were generated using electrospray ionization mass spectrometry, and then isolated in a Fourier transform ion cyclotron resonance mass spectrometer. Wavelength-selective infrared multiple photon dissociation (IRMPD) of the tris-acetato uranyl anion resulted in a redox elimination of an acetate radical, which was used to generate an IR spectrum that consisted of six prominent absorption bands. These were interpreted with the aid of density functional theory calculations in terms of symmetric and antisymmetric -CO(2) stretches of the monodentate and bidentate acetate, CH(3) bending and umbrella vibrations, and a uranyl O-U-O asymmetric stretch. The comparison of the calculated and measured IR spectra indicated that the predominant conformer of the tris-acetate complex contained two acetate ligands bound in a bidentate fashion, while the third acetate was monodentate. In similar fashion, the tris-benzoate uranyl anion was formed and photodissociated by loss of a benzoate radical, enabling measurement of the infrared spectrum that was in close agreement with that calculated for a structure containing one monodentate and two bidentate benzoate ligands.