Major Oxidation Products Research Articles

Characterization of skin sensitizers from autoxidized citronellol – impact of the terpene structure on the autoxidation process

Citronellol is a frequently used fragrance compound in consumer products. It is present in fragrance mix II, which is used for screening of contact allergy to fragrances. Because of its chemical structure, citronellol could be susceptible to autoxidation. To compare the behaviour of citronellol with that of the structurally similar compounds linalool and geraniol, in terms of ability to autoxidize, the products formed, and the sensitization potencies of these. Citronellol was exposed to air, and autoxidation was followed by gas chromatography-mass spectrometry (GC-MS) analysis after derivatization of thermolabile compounds. The sensitizing potencies of the oxidation mixture and its major oxidation compounds were examined with the local lymph node assay. The concentration of citronellol decreased while the sensitization potency increased in air-exposed samples over time, with hydroperoxides being identified as the major oxidation products and main skin sensitizers. The present study shows the impact of the absence of the 2,3-double bond in the citronellol structure on the oxidation pathways for formation of oxidation products. The study also shows the usefulness of our new GC-MS method for quantification of the citronellol oxidation products, especially the hydroperoxides. The investigated citronellol hydroperoxides could be important allergens, owing to the high concentrations detected and frequent exposure to citronellol in the population.

Detection of phosphatidylserine with a modified polar head group in human keratinocytes exposed to the radical generator AAPH

Phosphatidylserine (PS) is preferentially located in the inner leaflet of the cell membrane, and translocation of PS oxidized in fatty acyl chains to the outside of membrane has been reported as signaling to macrophage receptors to clear apoptotic cells. It was recently shown that PS can be oxidized in serine moiety of polar head-group. In the present work, a targeted lipidomic approach was applied to detecting OxPS modified at the polar head-group in keratinocytes that were exposed to the radical generator AAPH. Glycerophosphoacetic acid derivatives (GPAA) were found to be the major oxidation products of OxPS modified at the polar head-group during oxidation induced by AAPH-generated radicals, similarly to previous observations for the oxidation induced by OH radical. The neutral loss scan of 58Da and a novel precursor ion scan of m/z 137.1 (HOPO3CH2COOH) allowed the recognition of GPAA derivatives in the total lipid extracts obtained from HaCaT cells treated with AAPH. The positive identification of serine head group oxidation products in cells under controlled oxidative conditions opens new perspectives and justifies further studies in other cellular environments in order to understand fully the role of PS polar head-group oxidation in cell homeostasis and disease.

Using total OH reactivity to assess isoprene photooxidation via measurement and model

The Tropics provide a reactive atmospheric environment with high levels of biogenic emissions, rapidly growing anthropogenic influence, high solar radiation and temperature levels. The major reactive biogenic emission is isoprene which reacts rapidly with the primary daytime oxidant OH, the hydroxyl radical. This key photooxidation process has recently been the focus of several experimental and computational studies. A novel isoprene degradation mechanism was recently proposed (MIME) supplementing the commonly used MCM 3.2 scheme.This study examined the photooxidation of isoprene in the controlled conditions of the Valencia atmospheric reaction chamber, EUPHORE (EUropean PHOtoREactor). Besides the detection of isoprene and its major oxidation products formaldehyde, methyl vinyl ketone (MVK) and methacrolein (MACR), the total loss rate of OH (total OH reactivity) was measured. The total OH reactivity was compared to the individual measurements of isoprene and its oxidation products to assess the significant contributors to the overall OH loss rate. Measured total OH reactivity showed excellent agreement to the calculation based on individual compounds detected by a Proton-Transfer-Reaction-Time-Of-Flight-Mass-Spectrometer (PTR-TOF-MS). On average 97% of the measured total OH reactivity could be explained by isoprene and its major oxidation products.Total OH reactivity was also compared to various isoprene degradation schemes to evaluate known mechanisms. The MCM 3.2 isoprene mechanism reproduced the temporal degradation of total OH reactivity (and isoprene) reasonably well with a 57% (and 95%) agreement within the model uncertainties and a linear curve fit slope of 0.69 (and 1.02) for a model to measurement correlation. Large discrepancies between modeled values and all observed compounds were found for the recent isoprene oxidation scheme in MIME. Possible mechanistic reasons are discussed and improvements proposed. The subsequently modified version of MIME differed from the measured total OH reactivity only about 12% at the end of the experiment and represented best the overall temporal profile (linear curve fit slope of correlation: 0.95).

Analysis of guanine oxidation products in double-stranded DNA and proposed guanine oxidation pathways in single-stranded, double-stranded or quadruplex DNA.

Guanine is the most easily oxidized among the four DNA bases, and some guanine-rich sequences can form quadruplex structures. In a previous study using 6-mer DNA d(TGGGGT), which is the shortest oligomer capable of forming quadruplex structures, we demonstrated that guanine oxidation products of quadruplex DNA differ from those of single-stranded DNA. Therefore, the hotooxidation products of double-stranded DNA (dsDNA) may also differ from that of quadruplex or single-stranded DNA, with the difference likely explaining the influence of DNA structures on guanine oxidation pathways. In this study, the guanine oxidation products of the dsDNA d(TGGGGT)/d(ACCCCA) were analyzed using HPLC and electrospray ionization-mass spectrometry (ESI-MS). As a result, the oxidation products in this dsDNA were identified as 2,5-diamino-4H-imidazol-4-one (Iz), 8-oxo-7,8-dihydroguanine (8oxoG), dehydroguanidinohydantoin (Ghox), and guanidinohydantoin (Gh). The major oxidation products in dsDNA were consistent with a combination of each major oxidation product observed in single-stranded and quadruplex DNA. We previously reported that the kinds of the oxidation products in single-stranded or quadruplex DNA depend on the ease of deprotonation of the guanine radical cation (G•+) at the N1 proton. Similarly, this mechanism was also involved in dsDNA. Deprotonation in dsDNA is easier than in quadruplex DNA and more difficult in single-stranded DNA, which can explain the formation of the four oxidation products in dsDNA.

Mechanism of O2 Activation and Methanol Production by (Di(2-pyridyl)methanesulfonate)PtIIMe(OHn)(2–n)– Complex from Theory with Validation from Experiment

The mechanism of the (dpms)Pt(II)Me(OH(n))((2-n)-) oxidation in water to form (dpms)Pt(IV)Me(OH)2 and (dpms)Pt(IV)Me2(OH) complexes was analyzed using DFT calculations. At pH < 10, (dpms)Pt(II)Me(OH(n))((2-n)-) reacts with O2 to form a methyl Pt(IV)-OOH species with the methyl group trans to the pyridine nitrogen, which then reacts with (dpms)Pt(II)Me(OH(n))((2-n)-) to form 2 equiv of (dpms)Pt(IV)Me(OH)2, the major oxidation product. Both the O2 activation and the O-O bond cleavage are pH dependent. At higher pH, O-O cleavage is inhibited whereas the Pt-to-Pt methyl transfer is not slowed down, so making the latter reaction predominant at pH > 12. The pH-independent Pt-to-Pt methyl transfer involves the isomeric methyl Pt(IV)-OOH species with the methyl group trans to the sulfonate. This methyl Pt(IV)-OOH complex is more stable and more reactive in the Pt-to-Pt methyl-transfer reaction as compared to its isomer with the methyl group trans to the pyridine nitrogen. A similar structure-reactivity relationship is also observed for the S(N)2 functionalization to form methanol by two isomeric (dpms)Pt(IV)Me(OH)2 complexes, one featuring the methyl ligand trans to the sulfonate group and another with the methyl trans to the pyridine nitrogen. The barrier to functionalize the former isomer with the CH3 group trans to the sulfonate group is 2-9 kcal/mol lower. The possibility of the involvement of Pt(III) species in the reactions studied was found to correspond to high-barrier reactions and is hence not viable. It is concluded that the dpms ligand facilitates Pt(II) oxidation both enthalpically and entropically.

Analysis of organic aerosols using a micro-orifice volatilization impactor coupled to an atmospheric-pressure chemical ionization mass spectrometer.

We present the development and characterization of a combination of a micro-orifice volatilization impactor (MOVI) and an ion trap mass spectrometer (IT/MS) with an atmospheric-pressure chemical ionization (APCI) source. The MOVI is a multi-jet impactor with 100 nozzles, allowing the collection of aerosol particles by inertial impaction on a deposition plate. The pressure drop behind the nozzles is approximately 5%, resulting in a pressure of 96kPa on the collection surface for ambient pressures of 101.3 kPa. The cut-point diameter (diameter of 50% collection efficiency) is at 0.13 microm for a sampling flow rate of 10 L min(-1). After the collection step, aerosol particles are evaporated by heating the impaction surface and transferred into the APCI-IT/MS for detection of the analytes. APCI was used in the negative ion mode to detect predominantly mono- and dicarboxylic acids, which are major oxidation products of biogenic terpenes. The MOVI-APCI-IT/MS instrument was used for the analysis of laboratory-generated secondary organic aerosol (SOA), which was generated by ozonolysis of alpha-pinene in a 100 L continuous-flow reactor under dark and dry conditions. The combination of the MOVI with an APCI-IT/MS improved the detection Limits for small dicarboxylic acids, such as pinic acid, compared to online measurements by APCI-IT/MS. The Limits of detection and quantification for pinic acid were determined by external calibration to 4.4 ng and 13.2 ng, respectively. During a field campaign in the southern Rocky Mountains (USA) in summer 2011 (BEACHON-RoMBAS), the MOVI-APCI-IT/MS was applied for the analysis of ambient organic aerosols and the quantification of individual biogenic SOA marker compounds. Based on a measurement frequency of approximately 5 h, a diurnal cycle for pinic acid in the sampled aerosol particles was found with maximum concentrations at night (median: 10.1 ngm(-3)) and minimum concentrations during the day (median: 8.2 ng m(-3)), which is likely due to the partitioning behavior of pinic acid and the changing phase state of the organic aerosol particles with changing relative humidity.

Oxidation of 2,4-dinitrotoluene with ozone in the presence of a stop reagent

Liquid-phase oxidation of 2,4-dinitrotoluene with ozone in the presence of acetic anhydride as stop reagent and manganese(II) acetate as catalyst was studied. The major products of oxidation with ozone in acetic anhydride are 2,4-dinitrobenzyl alcohol (65.8%) and 2,4-dinitrobenzaldehyde (18.8%) in the form of the corresponding acetates. A reaction scheme accounting for the results obtained is considered.

Diurnal profiles of isoprene, methacrolein and methyl vinyl ketone at an urban site in Hong Kong

Methacrolein (MACR) and methyl vinyl ketone (MVK) are major oxidation products of isoprene, but they also have primary emissions in urban environments, for example from fuel use. To examine whether MACR and MVK could be used as a direct measurement of the oxidation rate of isoprene in an urban setting, the diurnal variations of isoprene, MACR and MVK were characterized at an urban site in Hong Kong from September to November, 2010. Ozone (O3), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen oxides (NOx) were simultaneously monitored. The average isoprene mixing ratio was 252 ± 204 pptv, with a bell-shaped distribution observed on most sampling days. Higher levels of isoprene were recorded in the beginning of the sampling period, when the temperature was higher. The average mixing ratios of MACR and MVK were 101 ± 85 pptv and 175 ± 131 pptv, respectively. While isoprene, MACR and MVK experienced peak concentrations from 11 a.m. to 3 p.m., increased levels of MACR and MVK during the morning rush hour did not coincide with isoprene. The low associations between isoprene and MACR/MVK suggest that either MACR/MVK were not formed from local isoprene oxidation and/or they could partly originate from primary emissions such as fuel evaporation or combustion. Statistical analyses of linear regression and positive matrix factorization revealed that approximately 20–29% of the measured MACR and MVK was associated with biogenic emissions, and 55–71% was impacted by vehicular emissions, particularly during morning rush hours. Since MACR and MVK originated from both primary emissions and biogenic emissions at this urban site, they can therefore overestimate the actual rate of isoprene oxidation and its contribution to O3 production in urban areas with strong primary emissions.

Accelerated clinical course of prion disease in mice compromised in repair of oxidative DNA damage

The detailed mechanisms of prion-induced neurotoxicity are largely unknown. Here, we have studied the role of DNA damage caused by reactive oxygen species in a mouse scrapie model by characterizing prion disease in the ogg1−/−mutyh−/− double knockout, which is compromised in oxidative DNA base excision repair. Ogg1 initiates removal of the major oxidation product 8-oxoguanine (8-oxoG) in DNA, and Mutyh initiates removal of adenine that has been misincorporated opposite 8-oxoG. Our data show that the onset of clinical signs appeared unaffected by Mutyh and Ogg1 expression. However, the ogg1−/−mutyh−/− mice displayed a significantly shorter clinical phase of the disease. Thus, accumulation of oxidative DNA damage might be of particular importance in the terminal clinical phase of prion disease. The prion-induced pathology and lesion profile were similar between knockout mice and controls. The fragmentation pattern of protease-resistant PrP as revealed in Western blots was also identical between the groups. Our data show that the fundamentals of prion propagation and pathological manifestation are not influenced by the oxidative DNA damage repair mechanisms studied here, but that progressive accumulation of oxidative lesions may accelerate the final toxic phase of prion disease.

Exploring the Substrate Specificity and Enantioselectivity of a Baeyer–Villiger Monooxygenase from Dietzia sp. D5: Oxidation of Sulfides and Aldehydes

Baeyer–Villiger monooxygenases (BVMOs) are valuable enzymes for specific oxyfunctionalization chemistry. They catalyze the oxidation of ketones to esters, but are also capable of oxidizing other chemical functions, namely aldehydes and heteroatoms such as sulfur, nitrogen, selenium and boron. The oxidation specificity and enantioselectivity of a newly characterized BVMO (BVMO4) from a strain of Dietzia towards sulfide- and aldehyde substrates have been studied. BVMO4 could react with sulfides containing an aromatic group. The presence of a substituent on the aromatic group was tolerated when they were in the meta- and para position and the oxidations yielded predominantly the (R)-sulfoxides. Similarly, BVMO4 displayed a higher activity for aldehydes containing a phenyl group, but long aliphatic aldehydes, namely octanal and decanal, were also accepted as substrate by this enzyme. The major oxidation products of the aldehyde substrates were the respective carboxylic acids in contrast to formate ester that was obtained in most of the previous reports. The Baeyer–Villiger oxidation of the substrate 2-phenylpropionaldehyde was studied in further detail and the corresponding acid product was obtained with good regio- and enantioselectivity. This is a unique feature for BVMO4 and is of great interest for further exploration of an alternative biocatalytic process.

Monopersulfate oxidation of tetrabromobisphenol A by biomimetic catalysts supported on cation- and anion-exchange resins

Abstract Iron(III)-porphyrin complexes, regarded as biomimetic models for oxidative enzymes, catalyze the degradation of a tetrabromobisphenol A (TBBPA) derivative that is distributed in landfills. To suppress the deactivation of the catalyst via self-degradation and dimerization, iron(III)-tetrakis(1-methylpyridinium-4-yl)porphyrin (FeTMPyP) and iron(III)-tetrakis(4-sulfonatephenyl)porphyrin (FeTPPS) were supported on cation- (FeTMPyP/CER) and anion-exchange (FeTPPS/AER) resins, respectively, and their catalytic activity and durability were examined in the absence and presence of humic acid (HA), which is major component of landfill leachates. The FeTPPS/AER catalyst had a fast reaction rate for TBBPA degradation, but decolorized quickly and had no reusability. In contrast, the FeTMPyP/CER catalyst was highly durable, catalyzing the degradation of over 90% of the TBBPA and no bleaching was observed in the FeTMPyP/CER catalyst after ten recyclings. Thus, the FeTMPyP/CER catalyst can be considered to be a biomimetic model of oxidative enzymes in landfills. 4-(2-Hydroxyisopropyl)-2,6-dibromophenol (2HIP-2,6DBP) was the major oxidation product, both in the absence and presence of HA. In the absence of HA, the coupling compound between a TBBPA radical species and 2,6-dibromophenol, which is more toxic than the parent compound, was detected. However, this formation was suppressed in the presence of HA.

DNA Damage by the Direct Effect of Ionizing Radiation: Products Produced by Two Sequential One-Electron Oxidations

It has long been assumed that the population of radicals trapped in irradiated DNA (that is, the radicals escaping recombination) would quantitatively account for the lesions observed in DNA. Recent results indicate that this is not the case. The yield of DNA lesions exceed the yield of trappable radicals. To account for a portion of this shortfall, it is thought that some of the initially formed 2'-deoxyribose radicals undergo a second oxidation by nearby base cation radicals to form 2'-carbocations. The carbocations react to give strand breaks and free base release. Schemes are presented to account for the major oxidation products observed including 8-oxoGua, 8-oxoAde, 5-OHMeUra, and free base release. Theoretical calculations were performed to ascertain the likelihood of the second oxidation step in these reaction pathways actually occurring, and to account for base sequence dependence and various levels of hydration.

N-acetyl lysyltyrosylcysteine amide inhibits myeloperoxidase, a novel tripeptide inhibitor

Myeloperoxidase (MPO) plays important roles in disease by increasing oxidative and nitrosative stress and oxidizing lipoproteins. Here we report N-acetyl lysyltyrosylcysteine amide (KYC) is an effective inhibitor of MPO activity. We show KYC inhibits MPO-mediated hypochlorous acid (HOCl) formation and nitration/oxidation of LDL. Disulfide is the major product of MPO-mediated KYC oxidation. KYC (≤4,000 μM) does not induce cytotoxicity in bovine aortic endothelial cells (BAECs). KYC inhibits HOCl generation by phorbol myristate acetate (PMA)-stimulated neutrophils and human promyelocytic leukemia (HL-60) cells but not superoxide generation by PMA-stimulated HL-60 cells. KYC inhibits MPO-mediated HOCl formation in BAEC culture and protects BAECs from MPO-induced injury. KYC inhibits MPO-mediated lipid peroxidation of LDL whereas tyrosine (Tyr) and tryptophan (Trp) enhance oxidation. KYC is unique as its isomers do not inhibit MPO activity, or are much less effective. Ultraviolet-visible spectral studies indicate KYC binds to the active site of MPO and reacts with compounds I and II. Docking studies show the Tyr of KYC rests just above the heme of MPO. Interestingly, KYC increases MPO-dependent H₂O₂ consumption. These data indicate KYC is a novel and specific inhibitor of MPO activity that is nontoxic to endothelial cell cultures. Accordingly, KYC may be useful for treating MPO-mediated vascular disease.

Protein oxidation: identification and utilisation of molecular markers to differentiate singlet oxygen and hydroxyl radical-mediated oxidative pathways

The effect of reactive oxidation species (ROS) on tryptophan or tyrosine was investigated by qualitatively determining the major detectable oxidation products generated by hydroxyl radicals, produced by the Fenton process, or singlet oxygen, generated by exposure to green light in the presence of Rose Bengal, on these photosensitive amino acids in synthetic pentapeptides. Based on mass spectrometric analysis it would appear that the hydroxyl radical favours a pathway leading to the formation of tryptophandione-based products from tryptophan. In contrast singlet oxygen attack appears to favour the formation of kynurenine-type products from tryptophan. Specific oxidative products observed proteomically are therefore potentially able to discriminate between predominant ROS-mediated pathways. To validate these findings, a keratin-enriched extract was exposed to UVB light under aqueous conditions. The observation of the conversion of tryptophan to hydroxytryptophan in marker peptides, and the absence of singlet-oxygen specific modifications, suggested that under these conditions oxidative degradation occurred primarily via hydroxyl radical attack. These observations provide the first direct proteomic evidence of the dominant photodegradation pathways in wet wool.

Mechanistic Investigation of the Oxidative Cleavage of the Carbon–Carbon Double Bond in α,β‐Unsaturated Compounds by Hexachloroiridate(IV) in Acetate Buffer

ABSTRACTThe hexachloroiridate(IV) oxidation of α,β‐unsaturated compounds such as acrylic acid, acrylamide, and acrylonitrile (CH2=CHX; X = –COOH, –CONH2, and –CN) was carried out in NaOAc‐AcOH buffer medium. The reaction follows complex kinetics, being first order in [IrIV] and complex order in [CH2=CHX]. H+ ion has no effect on the reaction rate in the pH range 3.42–4.63. The pseudo–first‐order rate constant decreases with a decrease in the dielectric constant and with a decrease of ionic strength of the medium. The oxidation rate follows the sequence: acrylonitrile &gt; acrylamide &gt; acrylic acid. A mechanism is proposed involving the formation of an unstable intermediate complex between the substrate and the oxidant which is transformed to the radical cation in a slow rate‐determining step with the concomitant reduction of Ir(IV) to Ir(III). The radical cation subsequently decomposes to the aldehyde that appears as the ultimate product of the carbon–carbon double bond cleavage. The major product of oxidation was identified as HCHO by 1H NMR. Activation parameters for the slow rate‐determining step and thermodynamic parameters associated with the equilibrium step of the proposed mechanism have been evaluated. The enthalpy of activation is linearly related to the entropy of activation, and this linear relationship confirms that the oxidation of all the α,β‐unsaturated compounds follows a common mechanism.

Forced degradation behavior of epidepride and development of a stability-indicating method based on liquid chromatography–mass spectrometry

A reversed-phase high-performance liquid chromatography (HPLC) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was applied to study the forced degradation behavior and stability of epidepride. 123I radioisotope-labeled epidepride, [(-)-N-{[(2S)-1-ethylpyrrolidin-2-yl]methyl}-5-iodo-2,3- dimethoxybenzamide] is a radiotracer with a high affinity for dopamine D2 receptors in the brain and has been used as an imaging agent for single-photon emission computed tomography. HPLC studies were performed using 127I-epidepride (the nonradioactive compound), instead of 123I-epidepride, with an RP-18 column using a mobile phase consisting of methanol, acetonitrile, and ammonium acetate (pH 7.0, 10 mM). The eluent flow rate and the wavelength for HPLC detection were 0.5 mL/min and 210 nm, respectively. The ligand was exposed to acid (1 N HCl) and alkaline (1 N NaOH) media and was subjected to oxidative decomposition at room temperature using 3% H2O2 and to thermal decomposition at 50°C. After various reaction times (30 minutes, 1 hour, 2 hours, 8 hours, and 24 hours), the substances were investigated by HPLC and LC-MS/MS. Although no decomposition products were observed after the acidic, alkaline, and thermal treatments, >80% of the initial amount of 127I-epidepride was oxidized within 24 hours in the presence of H2O2. Only one major oxidation product with an m/z value of 435 was observed, in addition to the 127I-epidepride species (m/z 419). The product was characterized by LC-MS/MS fragmentation, and the deteriorated type and fragmentation pathways were proposed for epidepride.

Oxidative stability of functional phytosterol-enriched dark chocolate

A dark chocolate containing phytosterols (PS) esters was developed to reduce cholesterol in individuals. However, oxidative instability during chocolate processing and storage could reduce the PS bioactivity. Chocolate bars were prepared containing palm oil (CONT) or 2.2 g of PS (PHYT). All samples were stored at 20 °C and 30 °C during 5 months. A peak of hydroperoxides formation was observed after 60 days at 20 °C and after 30 days at 30 °C. PS-enriched samples presented higher values of hydroperoxides than control samples, which could be attributed to the higher level of alpha-linolenic acid present in the PHYT samples. All chocolate bars became lighter and softer after 90 days of storage. However, these physical changes did not reduce their sensory acceptability. In addition, PS bioactivity was kept during the storage, since no significant alterations in the PS esters were observed up to 5 months. However, some PS oxidation occurred in the PHYT bars, being sitostanetriol, 6-ketositosterol, 6β-hydroxycampesterol and 7-ketocampesterol the major phytosterol oxidation products (POPs). The POPs/PS ratio was low (0.001). Therefore, the dark chocolate bars developed in this study kept their potential functionality after 5 months of storage at room temperature, representing an option as a functional food.

Cholesterol oxidation and astaxanthin degradation in shrimp during sun drying and storage

Dried salted shrimps are made from raw shrimps, which are cooked and dried under direct sunlight. The preparation and storage include treatments and conditions that can promote oxidative changes in different components. The aim of this study was to monitor the formation of major cholesterol oxidation products and the changes in the astaxanthin content and fatty acid profile in dried salted shrimp during cooking, sun drying and storage. During sun drying, most of the astaxanthin (75%) was degraded in cooked shrimp, while cholesterol oxidation products (COPs) showed a dramatic increase (8.6-fold), reaching a total concentration of 372.9±16.3μg/g of lipids. Further storage favoured both astaxanthin degradation (83%) and COPs formation (886.6±97.9μg/g of lipids after 90days of storage). The high degradation of astaxanthin and the elevated formation of COPs during sun drying and storage indicate the necessity to re-evaluate the processing and storage conditions of salted dried shrimp.

Chemical insights, explicit chemistry, and yields of secondary organic aerosol from OH radical oxidation of methylglyoxal and glyoxal in the aqueous phase

Abstract. Atmospherically abundant, volatile water-soluble organic compounds formed through gas-phase chemistry (e.g., glyoxal (C2), methylglyoxal (C3), and acetic acid) have great potential to form secondary organic aerosol (SOA) via aqueous chemistry in clouds, fogs, and wet aerosols. This paper (1) provides chemical insights into aqueous-phase OH-radical-initiated reactions leading to SOA formation from methylglyoxal and (2) uses this and a previously published glyoxal mechanism (Lim et al., 2010) to provide SOA yields for use in chemical transport models. Detailed reaction mechanisms including peroxy radical chemistry and a full kinetic model for aqueous photochemistry of acetic acid and methylglyoxal are developed and validated by comparing simulations with the experimental results from previous studies (Tan et al., 2010, 2012). This new methylglyoxal model is then combined with the previous glyoxal model (Lim et al., 2010), and is used to simulate the profiles of products and to estimate SOA yields. At cloud-relevant concentrations (~ 10−6 − ~ 10−3 M; Munger et al., 1995) of glyoxal and methylglyoxal, the major photooxidation products are oxalic acid and pyruvic acid, and simulated SOA yields (by mass) are ~ 120% for glyoxal and ~ 80% for methylglyoxal. During droplet evaporation oligomerization of unreacted methylglyoxal/glyoxal that did not undergo aqueous photooxidation could enhance yields. In wet aerosols, where total dissolved organics are present at much higher concentrations (~ 10 M), the major oxidation products are oligomers formed via organic radical–radical reactions, and simulated SOA yields (by mass) are ~ 90% for both glyoxal and methylglyoxal. Non-radical reactions (e.g., with ammonium) could enhance yields.

New tracer compounds for secondary organic aerosol formation from β-caryophyllene oxidation

Five products from β-caryophyllene oxidation (β-caryophyllonic acid (I), 3,3-dimethyl-2-(3-oxobutyl)cyclobutanecarboxylic acid (βCA198) (II), β-nocaryophyllonic acid (III), β-caryophyllinic acid (IV), and 2-(2-carboxyethyl)-3,3-dimethylcyclobutanecarboxylic acid (βCA200) (V)) were synthesized and their structures confirmed by nuclear magnetic resonance spectroscopy. Reaction chamber experiments with β-caryophyllene at two different ozone mixing ratios were performed and the carboxylic acid oxidation products in the particle phase were characterized by APCI–MS and HPLC–ESI–MS. All five synthesized acids were found as β-caryophyllene oxidation products in the reaction chamber aerosol. The main oxidation products of the reaction chamber experiments were β-14-hydroxynocaryophyllonic acid, β-nocaryophyllonic acid (III) and βCA198 (II). Product yields of the acids were estimated based on the chamber experiments and the application of the atmospheric chemistry box model CAABA/MECCA. Finally, ambient aerosol samples taken during the HUMPPA campaign in Hyytiälä, Finland in summer 2010 were analysed for the carboxylic acid β-caryophyllene oxidation products. All five synthesized compounds were detected and were quantified in the ambient aerosol samples. The major β-caryophyllene carboxylic acid oxidation products in the ambient air samples were β-nocaryophyllonic acid (III) and βCA198 (II) with concentrations in the range of about 0.2–14 ng m−3 and 0.8–6.8 ng m−3. The fact that the concentrations of these two acids in ambient aerosol are generally higher than the concentration of β-caryophyllinic acid (IV) (often used in previous studies as oxidation tracer) with a concentration of about 0.16 ng m−3 leads to the conclusion that these two acids are better suited as tracer compounds for β-caryophyllene secondary organic aerosol formation.