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1. Effect of aromatic ring substituents on the ability of catechol to produce brown carbon in iron( iii )-catalyzed reactions

   https://doi.org/10.1039/d0ea00007h  

   Chin, Henry; Hopstock, Katherine S.; Fleming, Lauren T.; Nizkorodov, Sergey A.; Al-Abadleh, Hind A. (February 2021, Environmental Science: Atmospheres)

   null (Ed.)

   Our previous work demonstrated formation of highly insoluble and strongly light-absorbing organic particles in reactions between catechol or guaiacol with Fe( iii ) under pH = 3 conditions characteristic of aerosol liquid water. This work extends these measurements to reactions of Fe( iii ) with 2,4-dinitrophenol, 4-nitrocatechol, 4-methylcatechol, 1,2,4-benzenetriol, 1,2,3-benzenetriol (pyrogallol) and coniferaldehyde to better understand the mechanism of particle formation catalyzed by Fe( iii ). Particles were observed after 2 h of reactions of catechol (43 ± 1% mass yield), 1,2,4-benzenetriol (32 ± 3%), pyrogallol (27 ± 2%) and coniferaldehyde (35 ± 4%), while reactions of 2,4-dinitrophenol and 4-nitrocatechol did not produce any insoluble products. No particles were observed in reaction of 4-methylcatechol after 2 h, however, insoluble products appeared after a 24 h reaction time. Irradiation of a catechol + Fe( iii ) mixture by 405 nm light was found to reduce (but not fully suppress) the particle yield due to a competition between photodegradation and Fe( iii )-catalyzed oligomerization. Particles produced from precursors + Fe( iii ) solutions were dissolved in organic solvents and analyzed with ultra performance liquid chromatography coupled to a photodiode array spectrophotometer and a high resolution mass spectrometer. Major separated chromophores were identified as dimeric, trimeric, and tetrameric products of precursor molecules. Purpurogallin was identified as a major reaction product of pyrogallol reaction with Fe( iii ). To test whether this chemistry can occur in more realistic atmospheric aerosols, reactions of biomass burning organic aerosol (BBOA) extracts with Fe( iii ) were also examined. Two BBOA samples collected under flaming conditions produced no particles, whereas a BBOA sample produced under smoldering conditions resulted in particle formation under both dark and 405 nm irradiation conditions. The results suggest that Fe( iii )-catalyzed chemistry can take place in aging BBOA plumes resulting from smoldering fires and make aerosol particles more light-absorbing. 

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2. Catalytic reduction of aryl trialkylammonium salts to aryl silanes and arenes

   https://doi.org/10.1039/c9sc01083a  

   Rand, Alexander W.; Montgomery, John (January 2019, Chemical Science)

   A new approach for the reduction of aryl ammonium salts to arenes or aryl silanes using nickel catalysis is reported. This method displays excellent ligand-controlled selectivity based on the N-heterocyclic carbene (NHC) ligand employed. Utilizing a large NHC in non-polar solvents generates aryl silanes, while small NHCs in polar solvents promote reduction to arenes. Several classes of aryl silanes can be accessed from simple aniline building blocks, including those useful for cross-couplings, oxidations, and halogenations. The reaction conditions are mild, functional group tolerant, and provide efficient access to a variety of benzene derivatives. 

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3. Formation of insoluble brown carbon through iron-catalyzed reaction of biomass burning organics

   https://doi.org/10.1039/d2ea00141a  

   Hopstock, Katherine S.; Carpenter, Brooke P.; Patterson, Joseph P.; Al-Abadleh, Hind A.; Nizkorodov, Sergey A. (January 2023, Environmental Science: Atmospheres)

   Biomass burning organic aerosol (BBOA) is one of the largest sources of organics in the atmosphere. Mineral dust and biomass burning smoke frequently co-exist in the same atmospheric environment. Common biomass burning compounds, such as dihydroxybenzenes and their derivatives, are known to produce light-absorbing, water-insoluble polymeric particles upon reaction with soluble Fe( iii ) under conditions characteristic of aerosol liquid water. However, such reactions have not been tested in realistic mixtures of BBOA compounds. In this study, model organic aerosol (OA), meant to replicate BBOA from smoldering fires, was generated through the pyrolysis of Canary Island pine needles in a tube furnace at 300, 400, 500, 600, 700, and 800 °C in nitrogen gas, and the water-soluble fractions were reacted with iron chloride under dark, acidic conditions. We utilized spectrophotometry to monitor the reaction progress. For OA samples produced at lower temperatures (300 and 400 °C), particles (P300 and P400) formed in solution, were syringe filtered, and extracted in organic solvents. Analysis was conducted with ultrahigh pressure liquid chromatography coupled to a photodiode array spectrophotometer and a high-resolution mass spectrometer (UHPLC-PDA-HRMS). For OA samples formed at higher pyrolysis temperatures (500–800 °C), water-insoluble, black particles (P500–800) formed in solution. In contrast to P300 and P400, P500–800 were not soluble in common solvents. Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and transmission electron microscopy (TEM) were used to image P600 and determine bulk elemental composition. Electron microscopy revealed that P600 had fractal morphology, reminiscent of soot particles, and contained no detectable iron. These results suggest that light-absorbing aerosol particles can be produced from Fe( iii )-catalyzed reactions in aging BBOA plumes produced from smoldering combustion in the absence of any photochemistry. This result has important implications for understanding the direct and indirect effects of aged BBOA on climate. 

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4. Quantification of dissolved metabolites in environmental samples through cation‐exchange solid‐phase extraction paired with liquid chromatography–mass spectrometry

   https://doi.org/10.1002/lom3.10513  

   Sacks, Joshua S; Heal, Katherine R; Boysen, Angela K; Carlson, Laura T; Ingalls, Anitra E (November 2022, Limnology and Oceanography: Methods)

   Abstract Small, biologically produced, organic molecules called metabolites play key roles in microbial systems where they directly mediate exchanges of nutrients, energy, and information. However, the study of dissolved polar metabolites in seawater and other environmental matrices has been hampered by analytical challenges including high inorganic ion concentrations, low analyte concentrations, and high chemical diversity. Here we show that a cation‐exchange solid‐phase extraction (CX‐SPE) sample preparation approach separates positively charged and zwitterionic metabolites from seawater and freshwater samples, allowing their analysis by liquid chromatography–mass spectrometry. We successfully extracted 69 known compounds from an in‐house compound collection and evaluated the performance of the method by establishing extraction efficiencies (EEs) and limits of detection (pM to low nM range) for these compounds. CX‐SPE extracted a range of compounds including amino acids and compatible solutes, resulted in very low matrix effects, and performed robustly across large variations in salinity and dissolved organic matter concentration. We compared CX‐SPE to an established SPE procedure (PPL‐SPE) and demonstrate that these two methods extract fundamentally different fractions of the dissolved metabolite pool with CX‐SPE extracting compounds that are on average smaller and more polar. We use CX‐SPE to analyze four environmental samples from distinct aquatic biomes, producing some of the first CX‐SPE dissolved metabolomes. Quantified compounds ranged in concentration from 0.0093 to 49 nM and were composed primarily of amino acids (0.15–16 nM) and compatible solutes such as trimethylamine N‐oxide (0.89–49 nM) and glycine betaine (2.8–5.2 nM). 

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5. Negative-Ion Electron Capture Dissociation of MALDI-Generated Peptide Anions

   https://doi.org/10.1021/acs.analchem.4c01292  

   DeFiglia, Steven A; Szot, Carson W; Håkansson, Kristina (May 2024, Analytical Chemistry)

   Negative-ion electron capture dissociation (niECD) is an anion MS/MS technique that provides fragmentation analogous to conventional ECD, including high peptide sequence coverage and retention of labile post-translational modifications (PTMs). niECD has been proposed to be the most efficient for salt-bridged zwitterionic precursor ion structures. Several important PTMs, e.g., sulfation and phosphorylation, are acidic and can, therefore, be challenging to characterize in the positive-ion mode. Furthermore, PTM-friendly techniques, such as ECD, require multiple precursor ion-positive charges. By contrast, singly charged ions, refractory to ECD, are most compatible with niECD. Because electrospray ionization (ESI) typically yields multiply charged ions, we sought to explore matrix-assisted laser desorption/ionization (MALDI) in combination with niECD. However, the requirement for zwitterionic gaseous structures may preclude efficient niECD of MALDI-generated anions. Unexpectedly, we found that niECD of anions from MALDI is not only possible but proceeds with similar or higher efficiency compared with ESI-generated anions. Matrix selection did not appear to have a major effect. With MALDI, niECD is demonstrated up to m/z ∼4300. For such larger analytes, multiple electron captures are observed, resulting in triply charged fragments from singly charged precursor ions. Such charge-increased fragments show improved detectability. Furthermore, significantly improved (∼20-fold signal-to-noise increase) niECD spectral quality is achieved with equivalent sample amounts from MALDI vs ESI. Overall, the reported combination with MALDI significantly boosts the analytical utility of niECD. 

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