33,713 research outputs found
How Phenol and Ξ±-Tocopherol React with Ambient Ozone at Gas/Liquid Interfaces
The exceptional ability of Ξ±-tocopherol (Ξ±-TOH) for scavenging free radicals is believed to also underlie its protective functions in respiratory epithelia. Phenols, however, can scavenge other reactive species. Herein, we report that Ξ±-TOH/Ξ±-TO^β reacts with closed-shell O_3(g) on the surface of inert solvent microdroplets in <1 ms to produce persistent Ξ±-TOβO_n^β(n = 1β4) adducts detectable by online thermospray ionization mass spectrometry. The prototype phenolate PhO^β, in contrast, undergoes electron transfer under identical conditions. These reactions are deemed to occur at the gas/liquid interface because their rates: (1) depend on pH, (2) are several orders of magnitude faster than within microdroplets saturated with O_3(g). They also fail to incorporate solvent into the products: the same Ξ±-TOβOn^β species are formed on acetonitrile or nucleophilic methanol microdroplets. Ξ±-TOβO_n(=1β3)^β signals initially evolve with [O_3(g)] as expected from first-generation species, but Ξ±-TOβO^β reacts further with O_3(g) and undergoes collisionally induced dissociation into a C_(19)H_(40) fragment (vs C_(19)H_(38) from Ξ±-TO^β) carrying the phytyl side chain, whereas the higher Ξ±-TOβO_(nβ₯2)^β homologues are unreactive toward O_3(g) and split CO_2 instead. On this basis, Ξ±-TOβO^β is assigned to a chroman-6-ol (4a, 8a)-ene oxide, Ξ±-TOβO2^β to an endoperoxide, and Ξ±-TOβO3^β to a secondary ozonide. The atmospheric degradation of the substituted phenols detected in combustion emissions is therefore expected to produce related oxidants on the aerosol particles present in the air we breathe
Absorption of Inhaled NO_2
Nitrogen dioxide (NO_2), a sparingly water-soluble Ο-radical gas, is a criteria air pollutant that induces adverse health effects. How is inhaled NO_2(g) incorporated into the fluid microfilms lining respiratory airways remains an open issue because its exceedingly small uptake coefficient (Ξ³ 10^(β7)β10^(β8)) limits physical dissolution on neat water. Here, we investigate whether the biological antioxidants present in these fluids enhance NO_2(g) dissolution by monitoring the surface of aqueous ascorbate, urate, and glutathione microdroplets exposed to NO_2(g) for 1 ms via online thermospray ionization mass spectrometry. We found that antioxidants catalyze the hydrolytic disproportionation of NO_2(g), 2NO_2(g) + H_2O(l) = NO_3^β(aq) + H^+(aq) + HONO, but are not consumed in the process. Because this function will be largely performed by chloride, the major anion in airway lining fluids, we infer that inhaled NO_2(g) delivers H^+, HONO, and NO_3^β as primary transducers of toxic action without antioxidant participation
Proton Availability at the Air/Water Interface
The acidity of the water surface sensed by a colliding gas is determined in experiments in which the protonation of gaseous trimethylamine (TMA) on aqueous microjets is monitored by online electrospray mass spectrometry as a function of the pH of the bulk liquid (pH_(BLK)). TMAH^+ signal intensities describe a titration curve whose equivalence point at pH_(BLK) 3.8 is dramatically smaller than the acidity constant of trimethylammonium in bulk solution, pK_A(TMAH^+) = 9.8. Notably, the degree of TMA protonation above pH_(BLK) 4 is enhanced hundred-fold by submillimolar LiCl or NaCl and weakly inhibited at larger concentrations. Protonation enhancements are associated with the onset of significant direct kinetic solvent hydrogen isotope effects. Since TMA(g) can be protonated by H_2O itself only upon extensive solvent participation, we infer that H3O^+ emerges at the surface of neat water below pH_(BLK) 4
Prompt Formation of Organic Acids in Pulse Ozonation of Terpenes on Aqueous Surfaces
A major atmospheric process, the gas-phase ozonation of terpenes yields suites of products via a cascade of chemically activated intermediates that ranges from primary ozonides to dioxiranes. If a similar mechanism operated in water, as it is generally assumed, such intermediates would be deactivated within picoseconds and, henceforth, be unable to produce carboxylic acids in microseconds. Herein, we report the online electrospray mass spectrometric detection of (M + 2O β H^+) and (M + 3O β H^+) carboxylates on the surface of aqueous Ξ²-caryophyllene (C_(15)H_(24), M = 204 Da) microjets exposed to a few ppmv of O_3(g) for < 10 ΞΌs. Since neither species is formed on dry solvent microjets and both incorporate deuterium from D_2O, we infer that carboxylates ensue from the interaction of nascent intermediates with interfacial water via heretofore unreported processes. These interfacial events proceed much faster than those in bulk liquids saturated with ozone
Ozone Oxidizes Glutathione to a Sulfonic Acid
Biosurfaces are universally covered with fluid microfilms containing reduced glutathione (GSH) and other antioxidants whose putative roles include the detoxification of ambient ozone (O_3). It is generally believed that O_3 accepts an electron from the thiolate GS^(2-) function [pK_a(GS^-) = 8.8] of GSH to produce thiyl GS^(β’-) radicals en route to the disulfide GSSG. Here, we report novel electrospray mass spectrometry experiments showing that sulfonates (GSO_3^-/GSO_3^(2-)), not GSSG, are the exclusive final products on the surface of aqueous GSH microdroplets exposed to dilute O_3(g) for ~1 ms. The higher reactivity of the thiolate GS^(2-) toward O_3(g) over the thiol GS^- is kinetically resolved in this time frame due to slow GS^- acid dissociation. However, our experiments also show that O_3 will be largely scavenged by the more reactive ascorbate coantioxidant in typical interfacial biofilms. The presence of GSSG and the absence of GSO_3^-/GSO_3^(2-) in extracellular lining fluids are therefore evidence of GSH oxidation by species other than O_3
Confocal Fluorescence Microscopy of the Morphology and Composition of Interstitial Fluids in Freezing Electrolyte Solutions
Ice rheology, the integrity of polar ice core records, and iceβatmosphere interactions are among the phenomena controlled by the morphology and composition of interstitial fluids threading polycrystalline ice. Herein, we investigate how ionic impurities affect such features via time-resolved confocal fluorescence microscopy of freezing electrolyte solutions doped with a pH probe. We find that the 10 ΞΌM probe accumulates into 12 ΞΌm thick glassy channels in frozen water, but it is incorporated into randomly distributed <1 ΞΌm diameter inclusions in freezing 1 mM NaCl. We infer that morphology is largely determined by the dynamic instabilities generated upon advancing ice by the rejected solute, rather than by thermodynamics. The protracted alkalinization of the fluid inclusions reveals that the excess negative charge generated by the preferential incorporation of Cl^β over Na^+ in ice is neutralized by the seepage of the OH^β slowly produced via H_2O β H^+ + OH^β thermal dissociation
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