Oxidation of glycated phosphatidylethanolamines: evidence of oxidation in glycated polar head identified by LC-MS/MS

Phosphatidylethanolamine glycation occurs in diabetic patients and was found to be related with oxidative stress and with diabetic complications. Glycated phosphatidylethanolamines seem to increase oxidation of other molecules; however, the reason why is not understood. In this work, we have studied the oxidation of glycated phosphatidylethanolamines (1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylethanolamine (PLPE) and 1,2-dipalmitoyl-sn-glycero-3-phosphatidylethanolamine (dPPE)) using a Fenton system. Liquid chromatography–electrospray ionization (ESI)–mass spectrometry and ESI–tandem mass spectrometry in both positive and negative modes were used for detecting and identifying the oxidation products. We were able to identify several oxidation products with oxidation in unsaturated sn-2 acyl chain of PLPE, as long- and short-chain products with main oxidation sites on C-7, C-8, C-9, and C-12 carbons. Other products were identified in both glycated PLPE and glycated dPPE, revealing that oxidation also occurs in the glycated polar head. This fact has not been reported before. These products may be generated from oxidation of glycated phosphatidylethanolamines (PE) as Schiff base, leading to short-chain product without the amine moiety, due to cleavage of glycated polar head and long-chain product with two keto groups linked to the glycated polar head or from glycated PE as Amadori product, short-chain products with –NHCHO and –NHCHOHCHO terminal in polar head. Oxidation of glycated phosphatidylethanolamines occurred more quickly than the oxidation of non-glycated phosphatidylethanolamines probably because of the existence of more oxidation sites derived from glycation of polar head group. Monitoring glycated polar head oxidation could be important to evaluate oxidative stress modifications that occur in diabetic patients.     

The selective catalytic oxidation of toluene

It was found for the first time that the selectivity of toluene transformations into benzaldehyde and benzoic acid decreased and into maleic anhydride and deep oxidation products increased as the ability of vanadium-containing catalysts of toluene oxidation to generate the singlet form of molecular oxygen grew. A scheme of the formation of the products of toluene oxidation with oxygen was suggested. Quinones were shown to be final rather than intermediate oxidation products. The selectivity of the reaction with respect to mild oxidation products in the presence of V₂O₅, MoO₃, and V₂O₅ · MoO₃ could be increased by changing the temperature of catalyst preparation from 400 to 500°C.     

Identification of intact oxidation products of glycerophospholipids in vitro and in vivo using negative ion electrospray iontrap mass spectrometry

Free radical‐induced oxidation products of polyunsaturated fatty acids esterified to phospholipids have been implicated in a number of human diseases including atherosclerosis and neurodegenerative diseases. Some of these phospholipid oxidation products have potent biological activities and likely contribute to human pathophysiological conditions. Oxidation products have also been used as markers of oxidative stress in vivo. Identification and quantification of phospholipid oxidation products are often performed by analyzing the oxidized free fatty acid moieties after hydrolysis from the phospholipids head groups by gas chromatography–mass spectrometry (GC–MS) or liquid chromatography–mass spectrometry (LC–MS). We now describe the definitive identification of intact oxidized products of glycerophospholipids including glycerophosphatidylcholine (GPC), glycerophosphatidylethanolamine (GPE), and glycerophosphatidylserine (GPS) in vitro and in vivo using iontrap MS. For these analyses, the negative ions of the oxidation products of phospholipids are fragmented to MSⁿ and unequivocal structural characterization is obtained based on collision‐induced dissociation (CID) of the sn‐2 carboxylate ion. This technique overcomes the need to hydrolyze fatty acids from phospholipids in the analysis. The method has been used to identify a number of oxidation products of glycerophospholipids including hydroxyeicosatetraenoates (HETEs) and isoprostanes (IsoPs) esterified to different classes of glycerophospholipids in vitro and in vivo. These studies thus provide a new approach to identify the intact oxidation products of glycerolphospholipids. Copyright © 2009 John Wiley & Sons, Ltd.     

Study of Electrochemical Oxidation of Xanthohumol by Ultra-Performance Liquid Chromatography Coupled to High Resolution Tandem Mass Spectrometry and Ion Mobility Mass Spectrometry

Electrochemically assisted oxidation off-line combined with UPLC/ESI–MS and ion mobility mass spectrometry enabled us to gain insight into the oxidation mechanisms of xanthohumol. Several types of monomeric oxidation products were identified, i.e., monohydroxylated and dehydrogenated derivatives and related quinones. Besides, high contents of dimers were observed. The structures of four main oxidative condensation products of two xanthohumol molecules were proposed based on combination of retention time, exact mass measurement, fragmentation pattern, data from on-line ion mobility mass spectrometric experiments and with the support of independent electrochemical experiments. To the best of our knowledge, this is the first evidence on formation of xanthohumol dimers. The effect of the pH on the generation of oxidation products was further investigated. The monomeric and dimeric oxidation products are favored at pH of 5.5 and 4.5, respectively.

     

The heterogeneous catalytic oxidation of propylene in the presence of singlet oxygen in the reaction mixture

Three methods for the introduction of singlet oxygen into the reaction mixture were tested, including thermal generation of singlet oxygen on the catalyst itself, the introduction of singlet oxygen from an external source, and photogeneration of singlet oxygen on the catalyst. Zeolites with admixtures of Mo, Bi, V, and Ni and SiO₂ with deposited Mo, V, and Bi were used. Common to all reactions was an increase in the yield of deep oxidation products in the presence of singlet oxygen. A sharp increase in the yield of mild oxidation products was observed in the oxidation of propylene on a Bi/SiO₂ catalyst. The generation of singlet oxygen under irradiation at 240–260 nm was found to cause deep oxidation only. Mild oxidation products could only form under the action of total mercury lamp light.     

ToF‐SIMS characterization of glyoxal surface oxidation products by hydrogen peroxide: A comparison between dry and liquid samples

As one of the simplest volatile organic compounds, glyoxal and its oxidation products were considered to be important precursors to aqueous secondary organic aerosol formation. Herein, we analyzed products from glyoxal oxidation by hydrogen peroxide in dry and liquid samples using time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). ToF‐SIMS spectra and spectral principal component analysis (PCA) were used to investigate surface oxidation products. Dry samples were prepared on clean silicon wafers. Liquid samples consisting of glyoxal and hydrogen peroxide (H₂O₂) were introduced to a vacuum compatible microfluidic reactor prior to UV illumination or dark aging followed by in situ liquid SIMS analysis. A number of reaction products were observed in both dry and liquid samples; different oligomers and carboxylic acids could be formed depending on reaction conditions. In addition, hydrolyzed products were observed in the liquid samples, but not in the dry samples. Although dry samples reveal some products of the aqueous process, they are not fully representative as results from those of the aqueous samples. Our findings suggest that the ability to characterize the liquid surface reaction products provides more realistic information of the reaction products associated with aqueous secondary organic aerosol formation in the atmosphere. Meanwhile, the high mass resolution spectra from the dry sample SIMS measurement are helpful to identify oxidation products in the liquid samples.     

Capillary-HPLC-ESI-MS/MS method for the determination of acidic products from the oxidation of monoterpenes in atmospheric aerosol samples

A method is presented for the determination of acidic products from terpene oxidation in filter samples of the atmospheric particle phase. Oxidation products of monoterpenes are believed to add a large fraction to the secondary organic aerosol (SOA) in the troposphere. Those products with structures containing one or more carboxylic acid groups have especially low vapour pressures and therefore they are believed to contribute substantially to the particle phase. Although many experiments were performed in simulation chambers to study the SOA generation by oxidation of terpenes, concentration measurements of products in the atmospheric particle phase are still rare. This is especially true for oxidation products of terpenes other than α- and β-pinene. Therefore, we developed a method for the quantification of acidic products from terpene oxidation in atmospheric aerosol samples. After passing a PM 2.5 (PM = particulate matter) pre-separator to remove coarse particles, fine atmospheric particles were collected onto quartz fibre filters. A backup filter was placed behind the first filter to estimate possible sampling artifacts. The filters were extracted in an ultrasonic bath using methanol. After enrichment and re-dissolving in water the samples were analysed using a capillary-HPLC-ESI(−)-MSⁿ set-up. The ion trap mass spectrometer could be used to gain structural information about the analytes and to enhance the selectivity of the measurements by using its MS/MS capability. A variety of products from different terpenes could be identified and quantified in samples of the ambient atmosphere using reference data from chamber experiments. Due to strong matrix effects quantification of samples from the real atmosphere had to be done by the standard addition method.     

Studies of the soluble part of oxidised coals

Soluble products obtained from the oxidation of four types of coal, each characterised by different degree of coalification and different degree of sulphur content, are studied. The coals are oxidised with peracetic acid (PAA) and nitric acid. Analyses are performed by Atmospheric Pressure-Temperature Programmed Reduction (AP-TPR) and Fourier Transform Infrared Spectroscopy (FTIR). The soluble products contain much more sulphur than the insoluble products of oxidation. The products obtained from the reaction with HNO₃ contain higher amounts of inorganic sulphur compounds, while those obtained from the reaction with PAA are characterised by an increased content of organic sulphur species.     

High Performance Liquid Chromatography/Electrochemistry/High Resolution Electrospray Ionization‐Mass Spectrometry (HPLC/EC/HR ESI‐MS) Characterization of Selected Cytokinins Oxidation Products

Electrochemistry combined with mass spectrometry represents an emerging analytical technique used to study the oxidation pathway of various drugs and in vivo occurring compounds, continuously showing a capability to generate many known metabolites or new oxidation products. An on‐line HPLC/EC/HR ESI‐MS method had been used to investigate the oxidation of selected cytokinin compounds. This setup allowed rapid identification and general structure elucidation of the obtained products. An electrochemical oxidation of isopentenyladenine resulted in five products, including hydroxylated and dehydrogenated products, which correlates very well with its in vivo metabolism. Electrochemical conversion of trans‐zeatin revealed six products, with two dehydrogenation products corresponding to its in vivo occurring metabolites. cis‐Zeatin oxidation in the electrochemical cell gave rise to eight products, resembling similarity to trans‐zeatin oxidation. All three compounds underwent a complete turnover mainly through two oxidation reactions occurring in the electrochemical cell? dehydrogenation and a less typical aliphatic hydroxylation. The resulting products are in correlation with their known in vivo metabolism.     

Oxidation of Polyisoprene Popcorn Polymer. III. Further Studies on Water and Carbon Dioxide Production

The volatile products resulting from polyisoprene popcorn polymer oxidation were analyzed quantitatively for carbon dioxide and water, and semiquantitatively for formaldehyde. The production of these three products was a linear function of the amount of oxygen consumed in the reaction. For every mole of oxygen reacted, 0.098 mole of water, 0.038 mole of carbon dioxide, and > 0.016 mole of formaldehyde were formed. Twenty-four products were detected after extensive oxidation; the major ones being water, carbon dioxide, formaldehyde, formic acid, 2,5-hexanedione, and acetic acid. No levulin-aldehyde was identified in the products. A tentative oxidation mechanism is discussed.     

Inhibited oxidation of a polyethylene melt

The oxidation of linear polyethylene inhibited by an effective antioxidant of the phenolic type, viz. 2,2′-methylene-bis (4-methyl-6-tert. butylphenol), has been studied over the temperature range 170–210. There is a critical concentration of antioxidant above which the rate of its consumption is directly proportional to its concentration. Deviations from this dependence are due to chain initiation by direct oxidation of the polymer and to reactions involving the products derived from the antioxidant: these products can also retard the oxidation. Similar results have been obtained using the amine type antioxidant, N-phenyl-N′-cyclohexyl-p-phenylenediamine. Some features of the mechanism of polyolefin oxidation are discussed.     

Desaturase reactions complicate the use of norcarane as a mechanistic probe. Unraveling the mixture of twenty-plus products formed in enzyme-catalyzed oxidations of norcarane

Norcarane, bicyclo[4.1.0]heptane, has been widely used as a mechanistic probe in studies of oxidations catalyzed by several iron-containing enzymes. We report here that, in addition to oxygenated products, norcarane is also oxidized by iron-containing enzymes in desaturase reactions that give 2-norcarene and 3-norcarene. Furthermore, secondary products from further oxidation reactions of the norcarenes are produced in yields that are comparable to those of the minor products from oxidation of the norcarane. We studied oxidations catalyzed by a representative spectrum of iron-containing enzymes including four cytochrome P450 enzymes, CYP2B1, CYPDelta2B4, CYPDelta2E1, and CYPDelta2E1 T303A, and three diiron enzymes, soluble methane monooxygenase (sMMO) from Methylococcus capsulatus (Bath), toluene monooxygenase (ToMO) from Pseudomonas stutzeri OX1, and phenol hydroxylase (PH) from Pseudomonas stutzeri OX1. 2-Norcarene and 3-norcarene and their oxidation products were found in all reaction mixtures, accounting for up to half of the oxidation products in some cases. In total, more than 20 oxidation products were identified from the enzyme-catalyzed reactions of norcarane. The putative radical-derived product from the oxidation of norcarane, 3-hydroxymethylcyclohexene (21), and the putative cation-derived product from the oxidation of norcarane, cyclohept-3-enol (22), coelute with other oxidation products on low-polarity GC columns. The yields of product 21 found in this study are smaller than those previously reported for the same or similar enzymes in studies where the products from norcarene oxidations were ignored, and therefore, the limiting values for lifetimes of radical intermediates produced in the enzyme-catalyzed oxidation reactions are shorter than those previously reported.     

Oxidation of Plasmalogen,Low‐Density Lipoprotein and RAW 264.7 Cells by Photoactivatable Atomic Oxygen Precursors

The oxidation of lipids by endogenous or environmental reactive oxygen species (ROS) generates a myriad of different lipid oxidation products that have important roles in disease pathology. The lipid oxidation products obtained in these reactions are dependent upon the identity of the reacting ROS. The photoinduced deoxygenation of various aromatic heterocyclic oxides has been suggested to generate ground state atomic oxygen (O[³P]) as an oxidant; however, very little is known about reactions between lipids and O(³P). To identify lipid oxidation products arising from the reaction of lipids with O(³P), photoactivatable precursors of O(³P) were irradiated in the presence of lysoplasmenylcholine, low‐density lipoprotein and RAW 264.7 cells under aerobic and anaerobic conditions. Four different aldehyde products consistent with the oxidation of plasmalogens were observed. The four aldehydes were: tetradecanal, pentadecanal, 2‐hexadecenal and hexadecanal. Depending upon the conditions, either pentadecanal or 2‐hexadecenal was the major product. Increased amounts of the aldehyde products were observed in aerobic conditions.     

Reactivity of Tyr–Leu and Leu–Tyr dipeptides: identification of oxidation products by liquid chromatography–tandem mass spectrometry

The exposure of peptides and proteins to reactive hydroxyl radicals results in covalent modifications of amino acid side‐chains and protein backbone. In this study we have investigated the oxidation the isomeric peptides tyrosine–leucine (YL) and leucine–tyrosine (LY), by the hydroxyl radical formed under Fenton reaction (Fe²⁺/H₂O₂). Through mass spectrometry (MS), high‐performance liquid chromatography (HPLC‐MS) and electrospray tandem mass spectrometry (HPLC‐MSⁿ) measurements, we have identified and characterized the oxidation products of these two dipeptides. This approach allowed observing and identifying a wide variety of oxidation products, including isomeric forms of the oxidized dipeptides. We detected oxidation products with 1, 2, 3 and 4 oxygen atoms for both peptides; however, oxidation products with 5 oxygen atoms were only present in LY. LY dipeptide oxidation leads to more isomers with 1 and 2 oxygen atoms than YL (3 vs 5 and 4 vs 5, respectively). Formation of the peroxy group occurred preferentially in the C‐terminal residue. We have also detected oxidation products with double bonds or keto groups, dimers (YL–YL and LY–LY) and other products as a result of cross‐linking. Both amino acids in the dipeptides were oxidized although the peptides showed different oxidation products. Also, amino acid residues have shown different oxidation products depending on the relative position on the dipeptide. Results suggest that amino acids in the C‐terminal position are more prone to oxidation. Copyright © 2009 John Wiley & Sons, Ltd.     

Thermo- and photooxidation of polyisobutylene. I. Evolutions at temperatures above 50°C

Polyisobutylene films (PIB) were submitted to a thermal oxidation at 100°C and to a photooxidation by exposure to long-wavelength radiations (λ ≥ 300 nm) at 60°C. The modifications of the chemical structure resulting from the oxidation were determined by FT-IR analysis of the polymer films, coupled to chemical treatments that converted specifically the oxidation products. Dissolution of oxidized samples permitted analysis of the polymer by ¹³C- and ¹H-NMR. The structure of the volatile products was determined by mass spectroscopy analysis of the gas phase. Identification of the numerous products formed permitted the proposal of a scheme that accounts for the oxidation of PIB. When the irradiations are carried out at a temperature above 50°C, the depolymerization is favored and the mechanism involves two main routes of oxidation. A direct oxidation starts with the oxidation of radicals obtained by homolysis of the C C bonds on the main chain, and an induced oxidation involves hydrogen abstraction on the methylene and methyl groups by radicals formed by the direct oxidation of the polymer. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 1689–1701, 1997     

Polarographic oxidation of some 5,10-dihydrophenazine derivatives

The polarographic anode oxidation of 5-substituted dihydrophenazines in an aprotic medium and an aqueous acetone solution has one-electron or two-electron character. It was shown by means of the electronic and ESR spectra that the intermediates in the anode oxidation of dihydrophenazine and 5-methyldihydrophenazine are cation radicals and that the products of one-electron oxidation of 1,3-dinitro-5-aryl-substituted dihydrophenazines are phenazyl radicals. The final products of anode oxidation are phenazinium salts.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 548–551, April, 1976.     

Unambiguous NMR Structural Determination of (+)-Catechin—Laccase Dimeric Reaction Products as Potential Markers of Grape and Wine Oxidation

(+)-Catechin—laccase oxidation dimeric standards were hemi-synthesized using laccase from Trametes versicolor in a water-ethanol solution at pH 3.6. Eight fractions corresponding to eight potential oxidation dimeric products were detected. The fractions profiles were compared with profiles obtained with two other oxidoreductases: polyphenoloxidase extracted from grapes and laccase from Botrytis cinerea. The profiles were very similar, although some minor differences suggested possible dissimilarities in the reactivity of these enzymes. Five fractions were then isolated and analyzed by 1D and 2D NMR spectroscopy. The addition of traces of cadmium nitrate in the samples solubilized in acetone-d₆ led to fully resolved NMR signals of phenolic protons, allowing the unambiguous structural determination of six reaction products, one of the fractions containing two enantiomers. These products can further be used as oxidation markers to investigate their presence and evolution in wine during winemaking and wine ageing.     

Identification of hydroxyl radical oxidation products of N‐hexanoyl‐homoserine lactone by reversed‐phase high‐performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry

A reversed‐phase high‐performance liquid chromatography/electrospray tandem mass spectrometry method was developed for the characterization of hydroxyl radical oxidation products of N‐hexanoyl‐homoserine lactone (C6‐HSL), a member of the N‐acylhomoserine lactone (AHL) class of microbial quorum‐sensing signaling molecules identified in many Gram‐negative strains of bacteria. Six products were identified: four with molecular weight (MW) of 213 and two with MW of 260. The characteristic product ions formed through collision‐induced dissociation (CID) provided diagnostic structural information. One of the photolysis products was determined to be N‐(3‐oxohexanoyl)homoserine lactone (3OC6‐HSL), a highly active quorum‐sensing signal, by comparison with a reference standard. Three structural isomers with the same mass as 3OC6‐HSL were identified as acyl side chain oxidized C6‐HSL (keto/enol functionalized) by accurate mass measurement and the structures of these products were proposed from CID spectral interpretation. Two structural isomers formed from concurrent oxidation and nitration of C6‐HSL were also observed and their structures were postulated based on CID spectra. In addition to the six hydroxyl radical oxidation products formed from the C6‐HSL precursor, five additional compounds generated from combined oxidation and lactonolysis of C6‐HSL were identified and structures were postulated. Copyright © 2009 John Wiley & Sons, Ltd.     

Oxidation mechanism of Ni−P alloys

The powders of Ni?P alloys containing 13% of phosphorus were obtained by precipitation from the solution. The oxidation of Ni?P alloys in polythermal conditions was studied. It was found that oxidation of Ni?P alloys goes through stages and that intermediate products of the oxidation are: Ni₂P and Ni₂P₂O₇. The final products of oxidation process are NiO and Ni₃(PO₄)₂. The sequence of chemical reactions describing the oxidation of Ni?P alloys was proposed.     

Catalytic oxidation of fluorobenzene by molecular oxygen

Kinetic and thermal-desorption methods have been applied to the oxidation of fluorobenzene by molecular oxygen. The catalytic oxidation mechanism for fluorobenzene is similar to that for benzene. A suggestion is made on why there is no fluoromaleic anhydride in the oxidation products from fluorobenzene.Translated from, Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 1, pp. 116–119, January–February, 1989.