Particle-phase accretion forms dimer esters in pinene secondary organic aerosol.

Secondary organic aerosol (SOA) is ubiquitous in the atmosphere and plays a pivotal role in climate, air quality, and health. The production of low-volatility dimeric compounds through accretion reactions is a key aspect of SOA formation. However, despite extensive study, the structures and thus the formation mechanisms of dimers in SOA remain largely uncharacterized. In this work, we elucidate the structures of several major dimer esters in SOA from ozonolysis of α-pinene and ß-pinene-substantial global SOA sources-through independent synthesis of authentic standards. We show that these dimer esters are formed in the particle phase and propose a mechanism of nucleophilic addition of alcohols to a cyclic acylperoxyhemiacetal. This chemistry likely represents a general pathway to dimeric compounds in ambient SOA.

Synthesis of Carboxylic Acid and Dimer Ester Surrogates to Constrain the Abundance and Distribution of Molecular Products in α-Pinene and ß-Pinene Secondary Organic Aerosol.

Liquid chromatography/negative electrospray ionization mass spectrometry [LC/(-)ESI-MS] is routinely employed to characterize the identity and abundance of molecular products in secondary organic aerosol (SOA) derived from monoterpene oxidation. Due to a lack of authentic standards, however, commercial terpenoic acids (e.g., cis-pinonic acid) are typically used as surrogates to quantify both monomeric and dimeric SOA constituents. Here, we synthesize a series of enantiopure, pinene-derived carboxylic acid and dimer ester homologues. We find that the (-)ESI efficiencies of the dimer esters are 19-36 times higher than that of cis-pinonic acid, demonstrating that the mass contribution of dimers to monoterpene SOA has been significantly overestimated in past studies. Using the measured (-)ESI efficiencies of the carboxylic acids and dimer esters as more representative surrogates, we determine that molecular products measureable by LC/(-)ESI-MS account for only 21.8 ± 2.6% and 18.9 ± 3.2% of the mass of SOA formed from ozonolysis of α-pinene and ß-pinene, respectively. The 28-36 identified monomers (C7-10H10-18O3-6) constitute 15.6-20.5% of total SOA mass, whereas only 1.3-3.3% of the SOA mass is attributable to the 46-62 identified dimers (C15-19H24-32O4-11). The distribution of identified α-pinene and ß-pinene SOA molecular products is examined as a function of carbon number (nC), average carbon oxidation state (OS¯C), and volatility (C*). The observed order-of-magnitude difference in (-)ESI efficiency between monomers and dimers is expected to be broadly applicable to other biogenic and anthropogenic SOA systems analyzed via (-) or (+) LC/ESI-MS under various LC conditions, and demonstrates that the use of unrepresentative surrogates can lead to substantial systematic errors in quantitative LC/ESI-MS analyses of SOA.

Coupling Filter-Based Thermal Desorption Chemical Ionization Mass Spectrometry with Liquid Chromatography/Electrospray Ionization Mass Spectrometry for Molecular Analysis of Secondary Organic Aerosol.

Filter-based thermal desorption (F-TD) techniques, such as the filter inlet for gases and aerosols, are widely employed to investigate the molecular composition and physicochemical properties of secondary organic aerosol (SOA). Here, we introduce an enhanced capability of F-TD through the combination of a customized F-TD inlet with chemical ionization mass spectrometry (CIMS) and ultraperformance liquid chromatography/electrospray ionization mass spectrometry (UPLC/ESI-MS). The utility of F-TD/CIMS + UPLC/ESI-MS is demonstrated by application to α-pinene ozonolysis SOA for which increased filter aerosol mass loading is shown to slow the evaporation rates of deposited compounds. Evidence for oligomer decomposition producing multimode F-TD/CIMS thermograms is provided by the measurement of the mass fraction remaining of monomeric and dimeric α-pinene oxidation products on the filter via UPLC/ESI-MS. In situ evaporation of aerosol particles suggests that α-pinene-derived hydroperoxides are thermally labile; thus, analysis of particle-phase (hydro)peroxides via F-TD may not be appropriate. A synthesized pinene-derived dimer ester (C20H32O5) is found to be thermally stable up to 200 °C, whereas particle-phase dimers (C19H30O5) are observed to form during F-TD analysis via thermally induced condensation of synthesized pinene-derived alcohols and diacids. The complementary F-TD/CIMS + UPLC/ESI-MS method offers previously inaccessible insight into the molecular composition and thermal desorption behavior of SOA that both clarifies and expands on analysis via traditional F-TD techniques.

Stable Isotope Analysis of Intact Oxyanions Using Electrospray Quadrupole-Orbitrap Mass Spectrometry.

The stable isotopes of sulfate, nitrate, and phosphate are frequently used to study geobiological processes of the atmosphere, ocean, as well as land. Conventionally, the isotopes of these and other oxyanions are measured by isotope-ratio sector mass spectrometers after conversion into gases. Such methods are prone to various limitations on sensitivity, sample throughput, or precision. In addition, there is no general tool that can analyze several oxyanions or all the chemical elements they contain. Here, we describe a new approach that can potentially overcome some of these limitations based on electrospray hyphenated with Quadrupole Orbitrap mass spectrometry. This technique yields an average accuracy of 1-2‰ for sulfate δ34S and δ18O and nitrate δ15N and δ18O, based on in-house and international standards. Less abundant variants such as δ17O, δ33S, and δ36S, and the 34S-18O "clumped" sulfate can be quantified simultaneously. The observed precision of isotope ratios is limited by the number of ions counted. The counting of rare ions can be accelerated by removing abundant ions with the quadrupole mass filter. Electrospray mass spectrometry (ESMS) exhibits high-throughput and sufficient sensitivity. For example, less than 1 nmol sulfate is required to determine 18O/34S ratios with 0.2‰ precision within minutes. A purification step is recommended for environmental samples as our proposed technique is susceptible to matrix effects. Building upon these initial provisions, new features of the isotopic anatomy of mineral ions can now be explored with ESMS instruments that are increasingly available to bioanalytical laboratories.

Towards measuring growth rates of pathogens during infections by D2 O-labeling lipidomics.

RATIONALE: Microbial growth rate is an important physiological parameter that is challenging to measure in situ, partly because microbes grow slowly in many environments. Recently, it has been demonstrated that generation times of S. aureus in cystic fibrosis (CF) infections can be determined by D2 O-labeling of actively synthesized fatty acids. To improve species specificity and allow growth rate monitoring for a greater range of pathogens during the treatment of infections, it is desirable to accurately quantify trace incorporation of deuterium into phospholipids. METHODS: Lipid extracts of D2 O-treated E. coli cultures were measured on liquid chromatography/electrospray ionization mass spectrometry (LC/ESI-MS) instruments equipped with time-of-flight (TOF) and orbitrap mass analyzers, and used for comparison with the analysis of fatty acids by isotope-ratio gas chromatography (GC)/MS. We then developed an approach to enable tracking of lipid labeling, by following the transition from stationary into exponential growth in pure cultures. Lastly, we applied D2 O-labeling lipidomics to clinical samples from CF patients with chronic lung infections. RESULTS: Lipidomics facilitates deuterium quantification in lipids at levels that are useful for many labeling applications (>0.03 at% D). In the E. coli cultures, labeling dynamics of phospholipids depend largely on their acyl chains and between phospholipids we notice differences that are not obvious from absolute concentrations alone. For example, cyclopropyl-containing lipids reflect the regulation of cyclopropane fatty acid synthase, which is predominantly expressed at the beginning of stationary phase. The deuterium incorporation into a lipid that is specific for S. aureus in CF sputum indicates an average generation time of the pathogen on the order of one cell doubling per day. CONCLUSIONS: This study demonstrates how trace level measurement of stable isotopes in intact lipids can be used to quantify lipid metabolism in pure cultures and provides guidelines that enable growth rate measurements in microbiome samples after incubation with a low percentage of D2 O.

Synergistic O3 + OH oxidation pathway to extremely low-volatility dimers revealed in ß-pinene secondary organic aerosol.

Dimeric compounds contribute significantly to the formation and growth of atmospheric secondary organic aerosol (SOA) derived from monoterpene oxidation. However, the mechanisms of dimer production, in particular the relevance of gas- vs. particle-phase chemistry, remain unclear. Here, through a combination of mass spectrometric, chromatographic, and synthetic techniques, we identify a suite of dimeric compounds (C15-19H24-32O5-11) formed from concerted O3 and OH oxidation of ß-pinene (i.e., accretion of O3- and OH-derived products/intermediates). These dimers account for an appreciable fraction (5.9-25.4%) of the ß-pinene SOA mass and are designated as extremely low-volatility organic compounds. Certain dimers, characterized as covalent dimer esters, are conclusively shown to form through heterogeneous chemistry, while evidence of dimer production via gas-phase reactions is also presented. The formation of dimers through synergistic O3 + OH oxidation represents a potentially significant, heretofore-unidentified source of low-volatility monoterpene SOA. This reactivity also suggests that the current treatment of SOA formation as a sum of products originating from the isolated oxidation of individual precursors fails to accurately reflect the complexity of oxidation pathways at play in the real atmosphere. Accounting for the role of synergistic oxidation in ambient SOA formation could help to resolve the discrepancy between the measured atmospheric burden of SOA and that predicted by regional air quality and global climate models.

Rapid Aqueous-Phase Hydrolysis of Ester Hydroperoxides Arising from Criegee Intermediates and Organic Acids.

Stabilized Criegee intermediates react with organic acids in the gas phase and at the air-water interface to form a class of ester hydroperoxides, α-acyloxyalkyl hydroperoxides (αAAHPs). A number of recent studies have proposed the importance of αAAHPs to the formation and growth of secondary organic aerosol (SOA). The chemistry of αAAHPs has not been investigated due to a lack of commercially available chemical standards. In this work, the behavior of αAAHPs in condensed phases is investigated for the first time. Experiments were performed with two synthesized αAAHP species. αAAHPs decomposed rapidly in the aqueous phase, with the rate highly dependent on the solvent, temperature, solution pH, and other compounds present in the solution. The measured 1st-order decomposition rate coefficient varied between 10-3 and 10-5 s-1 under the conditions examined in this work. Elucidation of the reaction mechanism is complicated by byproducts arising from the synthetic procedure, but observations are consistent with a base-catalyzed hydrolysis of αAAHPs. The rapid hydrolysis of αAAHPs observed in this work implies their short lifetimes in ambient cloud and fog waters. Decomposition of αAAHPs likely gives rise to smaller peroxides, such as H2O2. The loss of αAAHPs is also relevant to filter extraction, which is commonly practiced in laboratory experiments, potentially explaining contradictory results reported in the existing literature regarding the importance of αAAHPs in SOA.

Iodometry-Assisted Liquid Chromatography Electrospray Ionization Mass Spectrometry for Analysis of Organic Peroxides: An Application to Atmospheric Secondary Organic Aerosol.

Organic peroxides comprise a significant fraction of atmospheric secondary organic aerosol (SOA). Detection and quantification of particle-phase organic peroxides are highly challenging, and current efforts rely significantly on filter extraction and offline mass spectrometry (MS). Here, a novel technique, iodometry-assisted liquid chromatography electrospray ionization mass spectrometry (iodometry-assisted LC-ESI-MS), is developed and evaluated with a class of atmospherically relevant organic peroxides, α-acyloxyalkyl hydroperoxides, synthesized via liquid ozonolysis. Iodometry-assisted LC-ESI-MS unambiguously distinguishes organic peroxides, compensating for the lack of functional group information that can be obtained with MS. This technique can be versatile for a wide spectrum of environmental analytical applications for which a molecular-level identification of organic peroxide is required. Here, iodometry-assisted LC-ESI-MS is applied to the water-soluble organic carbon (WSOC) of α-pinene SOA. Unexpectedly, a limited number of detectable compounds in WSOC appear to be organic peroxides, despite the fact that spectroscopy-based iodometry indicates 15% of WSOC mass is associated with organic peroxides. This observation would be consistent with decomposition of multifunctional organic peroxides to small peroxides that can be quantified by spectroscopy-based iodometry but not by LC-ESI-MS. Overall, this study raises concerns regarding filter extraction-based studies, showing that assignment of organic peroxides solely on the basis of MS signatures can be misleading.

Starvation and recovery in the deep-sea methanotroph Methyloprofundus sedimenti.

In the deep ocean, the conversion of methane into derived carbon and energy drives the establishment of diverse faunal communities. Yet specific biological mechanisms underlying the introduction of methane-derived carbon into the food web remain poorly described, due to a lack of cultured representative deep-sea methanotrophic prokaryotes. Here, the response of the deep-sea aerobic methanotroph Methyloprofundus sedimenti to methane starvation and recovery was characterized. By combining lipid analysis, RNA analysis, and electron cryotomography, it was shown that M. sedimenti undergoes discrete cellular shifts in response to methane starvation, including changes in headgroup-specific fatty acid saturation levels, and reductions in cytoplasmic storage granules. Methane starvation is associated with a significant increase in the abundance of gene transcripts pertinent to methane oxidation. Methane reintroduction to starved cells stimulates a rapid, transient extracellular accumulation of methanol, revealing a way in which methane-derived carbon may be routed to community members. This study provides new understanding of methanotrophic responses to methane starvation and recovery, and lays the initial groundwork to develop Methyloprofundus as a model chemosynthesizing bacterium from the deep sea.

Identification and differential production of ubiquinone-8 in the bacterial predator Bdellovibrio bacteriovorus.

Bdellovibrio bacteriovorus 109J, a predatory bacterium with potential as a bacterial control agent, can exist in several lifestyles that differ both in predatory capacity and color. We determined that levels of ubiquinone-8 contribute to the distinctive but variable yellow color of different types of Bdellovibrio cells. Steady-state ubiquinone-8 concentrations did not differ markedly between conventional predatory and host-independent B. bacteriovorus despite upregulation of a suite of ubiquinone-8 synthesis genes in host-independent cells. In contrast, in spatially organized B. bacteriovorus films, the yellow inner regions contain significantly higher ubiquinone-8 concentrations than the off-white outer regions. Correspondingly, RT-PCR analysis reveals that the inner region, previously shown to consist primarily of active predators, clearly expresses two ubiquinone biosynthesis genes, while the outer region, composed mainly of quiescent or stalled bdelloplasts, expresses those genes weakly or not at all. Moreover, B. bacteriovorus cells in the inner region of week-old interfacial films, which are phenotypically attack-phase, have much higher UQ8 levels than regular attack-phase bdellovibrios, most likely because their "trapped" state prevents a high expenditure of energy to power flagellar motion.

Formation and evolution of molecular products in α-pinene secondary organic aerosol.

Much of our understanding of atmospheric secondary organic aerosol (SOA) formation from volatile organic compounds derives from laboratory chamber measurements, including mass yield and elemental composition. These measurements alone are insufficient to identify the chemical mechanisms of SOA production. We present here a comprehensive dataset on the molecular identity, abundance, and kinetics of α-pinene SOA, a canonical system that has received much attention owing to its importance as an organic aerosol source in the pristine atmosphere. Identified organic species account for ∼58-72% of the α-pinene SOA mass, and are characterized as semivolatile/low-volatility monomers and extremely low volatility dimers, which exhibit comparable oxidation states yet different functionalities. Features of the α-pinene SOA formation process are revealed for the first time, to our knowledge, from the dynamics of individual particle-phase components. Although monomeric products dominate the overall aerosol mass, rapid production of dimers plays a key role in initiating particle growth. Continuous production of monomers is observed after the parent α-pinene is consumed, which cannot be explained solely by gas-phase photochemical production. Additionally, distinct responses of monomers and dimers to α-pinene oxidation by ozone vs. hydroxyl radicals, temperature, and relative humidity are observed. Gas-phase radical combination reactions together with condensed phase rearrangement of labile molecules potentially explain the newly characterized SOA features, thereby opening up further avenues for understanding formation and evolution mechanisms of α-pinene SOA.

Secondary organic aerosol composition from C12 alkanes.

The effects of structure, NOx conditions, relative humidity, and aerosol acidity on the chemical composition of secondary organic aerosol (SOA) are reported for the photooxidation of three C12 alkanes: n-dodecane, cyclododecane, and hexylcyclohexane. Acidity was modified through seed particle composition: NaCl, (NH4)2SO4, and (NH4)2SO4 + H2SO4. Off-line analysis of SOA was carried out by solvent extraction and gas chromatography-mass spectrometry (GC/MS) and direct analysis in real-time mass spectrometry. We report here 750 individual masses of SOA products identified from these three alkane systems and 324 isomers resolved by GC/MS analysis. The chemical compositions for each alkane system provide compelling evidence of particle-phase chemistry, including reactions leading to oligomer formation. Major oligomeric species for alkane SOA are peroxyhemiacetals, hemiacetals, esters, and aldol condensation products. Furans, dihydrofurans, hydroxycarbonyls, and their corresponding imine analogues are important participants in these oligomer-producing reactions. Imines are formed in the particle phase from the reaction of the ammonium sulfate seed aerosol with carbonyl-bearing compounds present in all the SOA systems. Under high-NO conditions, organonitrate products can lead to an increase of aerosol volume concentration by up to a factor of 5 over that in low-NO conditions. Structure was found to play a key role in determining the degree of functionalization and fragmentation of the parent alkane, influencing the mean molecular weight of the SOA produced and the mean atomic O:C ratio.

Fosmidomycin decreases membrane hopanoids and potentiates the effects of colistin on Burkholderia multivorans clinical isolates.

Burkholderia cepacia complex (Bcc) pulmonary infections in people living with cystic fibrosis (CF) are difficult to treat because of the extreme intrinsic resistance of most isolates to a broad range of antimicrobials. Fosmidomycin is an antibacterial and antiparasitic agent that disrupts the isoprenoid biosynthesis pathway, a precursor to hopanoid biosynthesis. Hopanoids are involved in membrane stability and contribute to polymyxin resistance in Bcc bacteria. Checkerboard MIC assays determined that although isolates of the Bcc species B. multivorans were highly resistant to treatment with fosmidomycin or colistin (polymyxin E), antimicrobial synergy was observed in certain isolates when the antimicrobials were used in combination. Treatment with fosmidomycin decreased the MIC of colistin for isolates as much as 64-fold to as low as 8 µg/ml, a concentration achievable with colistin inhalation therapy. A liquid chromatography-tandem mass spectrometry technique was developed for the accurate quantitative determination of underivatized hopanoids in total lipid extracts, and bacteriohopanetetrol cyclitol ether (BHT-CE) was found to be the dominant hopanoid made by B. multivorans. The amount of BHT-CE made was significantly reduced upon fosmidomycin treatment of the bacteria. Uptake assays with 1-N-phenylnaphthylamine were used to determine that dual treatment with fosmidomycin and colistin increases membrane permeability, while binding assays with boron-dipyrromethene-conjugated polymyxin B illustrated that the addition of fosmidomycin had no impact on polymyxin binding. This work indicates that pharmacological suppression of membrane hopanoids with fosmidomycin treatment can increase the susceptibility of certain clinical B. multivorans isolates to colistin, an agent currently in use to treat pulmonary infections in CF patients.

Branched polymeric media: boron-chelating resins from hyperbranched polyethylenimine.

Extraction of boron from aqueous solutions using selective resins is important in a variety of applications including desalination, ultrapure water production, and nuclear power generation. Today's commercial boron-selective resins are exclusively prepared by functionalization of styrene-divinylbenzene (STY-DVB) beads with N-methylglucamine to produce resins with boron-chelating groups. However, such boron-selective resins have a limited binding capacity with a maximum free base content of 0.7 eq/L, which corresponds to a sorption capacity of 1.16 ± 0.03 mMol/g in aqueous solutions with equilibrium boron concentration of ∼70 mM. In this article, we describe the synthesis and characterization of a new resin that can selectively extract boron from aqueous solutions. We show that branched polyethylenimine (PEI) beads obtained from an inverse suspension process can be reacted with glucono-1,5-D-lactone to afford a resin consisting of spherical beads with high density of boron-chelating groups. This resin has a sorption capacity of 1.93 ± 0.04 mMol/g in aqueous solution with equilibrium boron concentration of ∼70 mM, which is 66% percent larger than that of standard commercial STY-DVB resins. Our new boron-selective resin also shows excellent regeneration efficiency using a standard acid wash with a 1.0 M HCl solution followed by neutralization with a 0.1 M NaOH solution.

Reductive degradation of perfluoroalkyl compounds with aquated electrons generated from iodide photolysis at 254 nm.

The perfluoroalkyl compounds (PFCs), perfluoroalkyl sulfonates (PFXS) and perfluoroalkyl carboxylates (PFXA) are environmentally persistent and recalcitrant towards most conventional water treatment technologies. Here, we complete an in depth examination of the UV-254 nm production of aquated electrons during iodide photolysis for the reductive defluorination of six aquated perfluoroalkyl compounds (PFCs) of various headgroup and perfluorocarbon tail length. Cyclic voltammograms (CV) show that a potential of +2.0 V (vs. NHE) is required to induce PFC oxidation and -1.0 V is required to induce PFC reduction indicating that PFC reduction is the thermodynamically preferred process. However, PFCs are observed to degrade faster during UV(254 nm)/persulfate (S(2)O(8)(2-)) photolysis yielding sulfate radicals (E° = +2.4 V) as compared to UV(254 nm)/iodide (I(-)) photolysis yielding aquated electrons (E° = -2.9 V). Aquated electron scavenging by photoproduced triiodide (I(3)(-)), which achieved a steady-state concentration proportional to [PFOS](0), reduces the efficacy of the UV/iodide system towards PFC degradation. PFC photoreduction kinetics are observed to be dependent on PFC headgroup, perfluorocarbon chain length, initial PFC concentration, and iodide concentration. From 2 to 12, pH had no observable effect on PFC photoreduction kinetics, suggesting that the aquated electron was the predominant reductant with negligible contribution from the H-atom. A large number of gaseous fluorocarbon intermediates were semi-quantitatively identified and determined to account for ∼25% of the initial PFOS carbon and fluorine. Reaction mechanisms that are consistent with kinetic observations are discussed.

Cerebrospinal fluid sodium rhythms.

BACKGROUND: Cerebrospinal fluid (CSF) sodium levels have been reported to rise during episodic migraine. Since migraine frequently starts in early morning or late afternoon, we hypothesized that natural sodium chronobiology may predispose susceptible persons when extracellular CSF sodium increases. Since no mammalian brain sodium rhythms are known, we designed a study of healthy humans to test if cation rhythms exist in CSF. METHODS: Lumbar CSF was collected every ten minutes at 0.1 mL/min for 24 h from six healthy participants. CSF sodium and potassium concentrations were measured by ion chromatography, total protein by fluorescent spectrometry, and osmolarity by freezing point depression. We analyzed cation and protein distributions over the 24 h period and spectral and permutation tests to identify significant rhythms. We applied the False Discovery Rate method to adjust significance levels for multiple tests and Spearman correlations to compare sodium fluctuations with potassium, protein, and osmolarity. RESULTS: The distribution of sodium varied much more than potassium, and there were statistically significant rhythms at 12 and 1.65 h periods. Curve fitting to the average time course of the mean sodium of all six subjects revealed the lowest sodium levels at 03.20 h and highest at 08.00 h, a second nadir at 09.50 h and a second peak at 18.10 h. Sodium levels were not correlated with potassium or protein concentration, or with osmolarity. CONCLUSION: These CSF rhythms are the first reports of sodium chronobiology in the human nervous system. The results are consistent with our hypothesis that rising levels of extracellular sodium may contribute to the timing of migraine onset. The physiological importance of sodium in the nervous system suggests that these rhythms may have additional repercussions on ultradian functions.

Partitioning of poly(amidoamine) dendrimers between n-octanol and water.

Dendritic nanomaterials are emerging as key building blocks for a variety of nanoscale materials and technologies. Poly(amidoamine) (PAMAM) dendrimers were the first class of dendritic nanomaterials to be commercialized. Despite numerous investigations, the environmental fate, transport, and toxicity of PAMAM dendrimers is still not well understood. As a first step toward the characterization of the environmental behavior of dendrimers in aquatic systems, we measured the octanol-water partition coefficients (logK(ow)) of a homologous series of PAMAM dendrimers as a function of dendrimer generation (size), terminal group and core chemistry. We find that the logK(ow) of PAMAM dendrimers depend primarily on their size and terminal group chemistry. For G1-G5 PAMAM dendrimers with terminal NH2 groups, the negative values of their logK(ow) indicate that they prefer to remain in the water phase. Conversely, the formation of stable emulsions at the octanol-water (O/ W) interface in the presence of G6-NH2 and G8-NH2 PAMAM dendrimers suggest they prefer to partition at the O/W interface. In all cases, published studies of the cytotoxicity of Gx-NH2 PAMAM dendrimers show they strongly interact with the lipid bilayers of cells. These results suggest that the logK(ow) of a PAMAM dendrimer may not be a good predictor of its affinity with natural organic media such as the lipid bilayers of cell membranes.