Transform Ion Cyclotron Resonance Electrospray Research Articles

Formation mechanism of secondary organic aerosol in aerosol liquid water: A review

Secondary organic aerosol (SOA) is the main component of PM2.5, with great impact on regional air quality and global climate. The traditional view that SOA forms through the partitioning of photochemical processing involving volatile organic compounds (VOCs) cannot fully explain measured SOA concentrations. It has been increasing recognized that SOA can form through aqueous reactions in recent years. Besides cloud/fog aqueous chemistry, aqueous SOA (aqSOA) formation in aerosol liquid water has become one of the frontier scientific problems of atmospheric chemistry. AqSOA precursors enter into aqueous phase through uptake to wet aerosol particles, participate in reactions inside aerosol particles, and then form aqSOA such as organic sulfur compounds and organic nitrogen compounds. This paper provides an overview of the uptake of aqSOA precursors, aqSOA formation mechanism and current research methods of aqSOA. AqSOA precursors include atmospheric oxidants (OH, HO2, O3, etc.), anthropogenic and biogenic VOCs and related gas-phase oxidation products. Aerosol liquid water can influence the uptake of aqSOA precursors on wet aerosols, but related researches are limited. OH uptake coefficient ( γ OH) on different kinds of aerosols varies from 0.02 to 2.41, depending on chemical composition of aerosols and relative humidity. For the uptake of VOCs, take methylglyoxal as an example, measured and theoretical methylglyoxal uptake coefficient ( γ MGLY) differ by 4 to 5 orders of magnitude. Aerosol liquid water may change ionic strength, diffusion limitation and viscosity of wet aerosols, but how these affect the uptake process of aqSOA precursors remain poorly understood. Based on previous analyses of aqueous chemistry, aqueous-phase reactions can be divided into radical reactions and non-radical reactions. Aqueous-phase radical reactions resemble gas-phase reactions in general. However, there are also OH radical reactions unique to the aqueous phase: efficient conversion of aldehydes to carboxylic acids, rapid OH oxidation of carboxylate, and radical induced oligomerization. Recent studies also pay increasing attention to the role of other oxidants in the aqueous radical chemistry, like singlet oxygen, peroxyl radicals, peroxides, molecular oxygen (1O2*), and triplet excited states of organic compounds (3C*). Non-radical reactions include hemiacetal formation, aldol condensation, imine formation and other types of reactions (anhydride formation, organosulfate formation, etc.). Most non-radical reactions lead to the formation of high molecular weight compounds. Although a lot of investigations have been taken to explore aqSOA formation mechanism, the majority are laboratory studies, because of the limit of technology in field measurements. Laboratory simulation includes bulk solution simulation and reaction chamber experiments. Bulk solution cannot simulate typical ambient wet aerosols well, so nowadays reaction chamber is used more widely. There are two types of reaction chambers: smog chamber and flow tube, differing in the volume of reaction chamber and simulated atmospheric oxidation timescale. However, the appropriate application of these laboratory results into field observations and model framework needs further efforts. Moreover, one key factor that has enabled great progress in aqSOA chemistry studies is the development of mass-spectrometric methods, mainly including electrospray ionization-mass spectrometry (ESI-MS), Fourier transform ion cyclotron resonance electrospray ionization mass spectrometry (FTICR-MS), chemical ionization mass spectrometry (CIMS) and extractive electrospray ionization-mass spectrometry (EESI-MS). These techniques can realize accurate molecular level identification of complex compounds. But quantification remains a thorny issue, owing to the absence of available authentic standards. Finally, possible future directions regarding aqSOA chemistry studies are discussed.

Laboratory evidence of organic peroxide and peroxyhemiacetal formation in the aqueous phase and implications for aqueous OH

Abstract. Aqueous chemistry in atmospheric waters (e.g., cloud droplets or wet aerosols) is considered a potentially important atmospheric pathway to produce secondary organic aerosol (SOAaq). Water-soluble organic compounds with small carbon numbers (C2–C3) are precursors for SOAaq; products include organic acids, organic sulfates, and high-molecular-weight compounds/oligomers. Fenton reactions and the uptake of gas-phase OH radicals are considered to be the major oxidant sources for aqueous organic chemistry. However, the sources and availability of oxidants in atmospheric waters are not well understood. The degree to which OH is produced in the aqueous phase affects the balance of radical and non-radical aqueous chemistry, the properties of the resulting aerosol, and likely its atmospheric behavior. This paper demonstrates organic peroxide formation during aqueous photooxidation of methylglyoxal using ultra-high-resolution Fourier transform ion cyclotron resonance electrospray ionization mass spectrometry (FTICR-MS). Organic peroxides are known to form through gas-phase oxidation of volatile organic compounds. They contribute secondary organic aerosol (SOA) formation directly by forming peroxyhemiacetals and epoxides (i.e., IEPOX), and indirectly by enhancing gas-phase oxidation through OH recycling. We provide simulation results of organic peroxide/peroxyhemiacetal formation in clouds and wet aerosols and discuss organic peroxides as a source of condensed-phase OH radicals and as a contributor to aqueous SOA.

The Crystal Structure of Shiga Toxin Type 2 with Bound Disaccharide Guides the Design of a Heterobifunctional Toxin Inhibitor

Shiga toxin type 2 (Stx2a) is clinically most closely associated with enterohemorrhagic E. coli O157:H7-mediated hemorrhagic colitis that sometimes progresses to hemolytic-uremic syndrome. The ability to express the toxin has been acquired by other Escherichia coli strains, and outbreaks of food poisoning have caused significant mortality rates as, for example, in the 2011 outbreak in northern Germany. Stx2a, an AB5 toxin, gains entry into human cells via the glycosphingolipid receptor Gb3. We have determined the first crystal structure of a disaccharide analog of Gb3 bound to the B5 pentamer of Stx2a holotoxin. In this Gb3 analog,-GalNAc replaces the terminal-Gal residue. This co-crystal structure confirms previous inferences that two of the primary binding sites identified in theB5 pentamer of Stx1 are also functional in Stx2a. This knowledge provides a rationale for the synthesis and evaluation of heterobifunctional antagonists for E. coli toxins that target Stx2a. Incorporation of GalNAc Gb3 trisaccharide in a heterobifunctional ligand with an attached pyruvate acetal, a ligand for human amyloid P component, and conjugation to poly[acrylamide-co-(3-azidopropylmethacrylamide)] produced a polymer that neutralized Stx2a in a mouse model of Shigatoxemia.

Effects of sample properties and mass spectroscopic parameters on electrospray ionization mass spectra of size-fractions from a soil humic acid

Size-fractions from a soil humic acid were separated by preparative size-exclusion chromatography (SEC), desalted, and concentrated by ultrafiltration and vacuum centrifugation without being subjected to any freeze-drying process. After having assessed the lack of formation of any multiple-charged ions by high-resolution Fourier transform ion cyclotron resonance electrospray ionization (ESI) mass spectrometry (MS), the size-fractions were used by direct infusion to compare the molecular ion distribution by both atmospheric pressure chemical ionization (APCI)- and ESI-MS in negative mode. The weight- (Mw) and number-averaged (Mn) molecular weight obtained by ESI-MS were invariably larger than by APCI-MS for all size-fractions, thereby indicating that ESI is more efficient than APCI to evaluate the molecular mass distribution of humic samples. No substantial difference was observed when concentration and pH of unfreeze-dried humic size-fractions were varied. The negative mode was applied to assess the effect of cone voltage from -20 to -60 V on ESI of the humic size-fractions further separated through an on-line SEC column. The resulting mass spectra and Mw and Mn values suggested that the variation of cone voltage in ESI-MS affects the ionization potential of associated humic molecules more in solution rather than their fragmentation. These findings agree with previous observations which indicated a limitation of ESI in providing consistent mass detection for a complex mixture of heterogeneous humic molecules, especially when they are aggregated by a freeze-drying process.

Characterization of novel hydrolysis products of carbapenems by electrospray ionization mass spectrometry

Carbapenems, including meropenem and imipenem, exhibit low stability against acid or base reagents. The fragmentation behavior of meropenem and its acid hydrolysis products was investigated by Fourier transform ion cyclotron resonance electrospray ionization tandem mass spectrometry and ion trap tandem multi-stage mass spectrometry in both positive and negative ion mode. Only one neutral loss of CO(2) was observed from the precursor ion to the MS(4) product ions for the acid hydrolysis product and this behavior did not correspond to that expected for the previously accepted 1-pyrroline or 2-pyrroline structure with two carbonyl acid units. The unknown product was then proposed to be 2-(4-(5-(dimethylcarbamoyl)pyrrolidin-3-ylthio)-5-imino-3-methyl-6-oxotetrahydro-2H-pyran-2-yl)-3-hydroxybutanoic acid on the basis of the multi-stage mass spectrometric and accurate mass data. A similar acid hydrolysis product of imipenem was also identified by mass spectrometry, confirming that these carbapenems had the same acid hydrolysis behavior. The proposed structures were further confirmed by NMR experiments.

Study of the non‐covalent interaction between amyloid‐β‐peptide and melatonin using electrospray ionization mass spectrometry

Oxidative stress and unregulated immune response are believed to play a key role in the processes inherent to Alzheimer's disease (AD). The fact that free radicals can result in neurodegeneration suggests that actions against reactive oxygen species may be beneficial in treating and preventing AD. In the light of the suggested link between oxidative stress and AD, it is proposed that antioxidants and, even more, endogenous antioxidants may offer a therapeutic regime for protection against the risk of this disease. For this reason, the formation of non-covalent complexes between amyloid-beta-peptide (A beta) or its oxidized forms and melatonin was studied by quadrupole and Fourier transform ion cyclotron resonance electrospray ionization mass spectrometry. The stability of the non-covalent complex was examined under several experimental conditions, such as orifice voltage, pH, presence of organic modifier, concentration and time. Two different digestion protocols combined with mass spectrometric analysis of the resulting peptide fragments were employed in order to locate the binding site of melatonin in A beta.

Low- and high-energy collision-induced dissociation of pyridinoline and deoxypyridinoline ions using Fourier transform ion cyclotron resonance electrospray and liquid secondary-ion magnetic sector mass spectrometry

The trifunctional collagen cross-link molecules pyridinoline and deoxypyridinoline have been structurally characterised by (1) low-energy collision-induced dissociation (CID) ( E COM ≈ 15 eV) using Fourier transform ion cyclotron resonance electrospray ionisation mass spectrometry (FT-ICR-ESI-MS), and (2) high-energy collision-induced dissociation ( E COM ≈ 400 eV) using liquid secondary-ion mass spectrometry (LSIMS) on a four-sector mass spectrometer. Both ESI and LSIMS ionisation readily produce the naturally occurring parent cations, although the LSIMS ionisation method is somewhat hampered by background ions from the matrix. Low-energy or high-energy collision-induced dissociation produces structurally informative fragmentations. The observed fragmentation patterns in the low- and high-energy CID spectra are, however, strikingly different. Both low- and high-energy CID are structurally informative and appear to be complementary in the characterisation of collagen cross-link molecules.