OH-Radical Oxidation of Lung Surfactant Protein B on Aqueous Surfaces

Air pollutants generate reactive oxygen species on lung surfaces. Here we report how hydroxyl radicals (·OH) injected on the surface of water react with SP-B_(1–25), a 25-residue polypeptide surrogate of human lung surfactant protein B. Our experiments consist of intersecting microjets of aqueous SP-B_(1–25) solutions with O_3/O_2/H_2O/N_2(g) gas streams that are photolyzed into ·OH(g) in situ by 266 nm laser nanosecond pulses. Surface-sensitive mass spectrometry enables us to monitor the prompt (<10 μs) and simultaneous formation of primary O_n-containing products/intermediates (n ≤ 5) triggered by the reaction of ·OH with interfacial SP-B_(1–25). We found that O-atoms from both O_3 and ·OH are incorporated into the reactive cysteine Cys_8 and Cys_(11) and tryptophan Trp_9 components of the hydrophobic N-terminus of SP-B_(1–25) that lies at the topmost layers of the air–liquid interface. Remarkably, these processes are initiated by ·OH additions rather than by H-atom abstractions from S–H, C–H, or N–H groups. By increasing the hydrophilicity of the N-terminus region of SP-B_(1–25), these transformations will impair its role as a surfactant

Can the pH at the air/water interface be different from the pH of bulk water?

A recent article in PNAS (1) reports stable pH gradients in aqueous phosphate buffer microdroplets based on the pH dependence of surface-enhanced Raman spectroscopy (SERS) signals from 4-mercaptobenzoic acid–functionalized gold nanoparticle pH probes

Long-range specific ion-ion interactions in hydrogen-bonded liquid films

Anions populate fluid interfaces specifically. Here, we report experiments showing that on hydrogen-bonded interfaces anions interact specifically over unexpectedly long distances. The composition of binary electrolyte (Na+, X−/Y−) films was investigated as a function of solvent, film thickness, and third ion additions in free-standing films produced by blowing up drops with a high-speed gas. These films soon fragment into charged sub-micrometer droplets carrying excess anions detectable in situ by online electrospray ionization mass spectrometry. We found that (1) the larger anions are enriched in the thinner (nanoscopic air-liquid-air) films produced at higher gas velocities in all (water, methanol, 2-propanol, and acetonitrile) tested solvents, (2) third ions (beginning at sub-μM levels) specifically perturb X−/Y− ratios in water and methanol but have no effect in acetonitrile or 2-propanol. Thus, among these polar organic liquids (of similar viscosities but much smaller surface tensions and dielectric permittivities than water) only on methanol do anions interact specifically over long, viz.: ⟨ri − rj⟩/nm = 150 (c/μM)^(−1/3), distances. Our findings point to the extended hydrogen-bond networks of water and methanol as likely conduits for such interactions

Stability of Monoterpene-Derived α-Hydroxyalkyl-Hydroperoxides in Aqueous Organic Media: Relevance to the Fate of Hydroperoxides in Aerosol Particle Phases

The α-hydroxyalkyl-hydroperoxides [R–(H)C(−OH)(−OOH), α-HH] produced in the ozonolysis of unsaturated organic compounds may contribute to secondary organic aerosol (SOA) aging. α-HHs’ inherent instability, however, hampers their detection and a positive assessment of their actual role. Here we report, for the first time, the rates and products of the decomposition of the α-HHs generated in the ozonolysis of atmospherically important monoterpenes α-pinene (α-P), d-limonene (d-L), γ-terpinene (γ-Tn), and α-terpineol (α-Tp) in water/acetonitrile (W/AN) mixtures. We detect α-HHs and multifunctional decomposition products as chloride adducts by online electrospray ionization mass spectrometry. Experiments involving D₂O and H₂¹⁸O, instead of H₂¹⁶O, and an OH-radical scavenger show that α-HHs decompose into gem-diols + H₂O₂ rather than free radicals. α-HHs decay mono- or biexponentially depending on molecular structure and solvent composition. e-Fold times, τ_(1/e), in water-rich solvent mixtures range from τ_(1/e) = 15–45 min for monoterpene-derived α-HHs to τ_(1/e) > 10³ min for the α-Tp-derived α-HH. All τ_(1/e)’s dramatically increase in <20% (v/v) water. Decay rates of the α-Tp-derived α-HH in pure water increase at lower pH (2.3 ≤ pH ≤ 3.3). The hydroperoxides detected in day-old SOA samples may reflect their increased stability in water-poor media and/or the slow decomposition of α-HHs from functionalized terpenes

Superacid Chemistry on Mildly Acidic Water

The mechanism of proton transfer across water−hydrophobic media boundaries is investigated in experiments in which the protonation of gaseous n-hexanoic acid (PCOOH) upon collision with liquid water microjets is monitored by online electrospray mass spectrometry as a function of pH. Although PCOOH(aq) is a very weak base (pK_(BH+) < −3), PCOOH(g) is converted to PC(OH)_2^+ on pH < 4 water via a process that ostensibly retains some of the exoergicity of its gas-phase counterpart, PCOOH + H_3O^+ = PC(OH)_2^+ + H_2O, ΔG < −22 kcal mol^(−1). The large kinetic isotope effects observed on H_2O/D_2O microjets, PC(OH)_2^+/PC(OH)OD^+ = 88 and PC(OH)OD^+/PC(OD)_2^+ = 156 at pD = 2, and their inverse dependences on pH indicate that PCOOH(g) hydronation on water (1) involves tunneling, (2) is faster than H-isotope exchange, and (3) is progressively confined to the outermost layers as water becomes more acidic. Proton transfers across steep water density gradients appear to be promoted by both dynamic and thermodynamic factors

Can the pH at the air/water interface be different from the pH of bulk water?

A recent article in PNAS (1) reports stable pH gradients in aqueous phosphate buffer microdroplets based on the pH dependence of surface-enhanced Raman spectroscopy (SERS) signals from 4-mercaptobenzoic acid–functionalized gold nanoparticle pH probes

Extensive H-atom abstraction from benzoate by OH-radicals at the air–water interface

Much is known about OH-radical chemistry in the gas-phase and bulk water. Important atmospheric and biological processes, however, involve little investigated OH-radical reactions at aqueous interfaces with hydrophobic media. Here, we report the online mass-specific identification of the products and intermediates generated on the surface of aqueous (H_2O, D_2O) benzoate-h5 and -d5 microjets by ∼8 ns ˙OH(g) pulses in air at 1 atm. Isotopic labeling lets us unambiguously identify the phenylperoxyl radicals that ensue H-abstraction from the aromatic ring and establish a lower bound (>26%) to this process as it takes place in the interfacial water nanolayers probed by our experiments. The significant extent of H-abstraction vs. its negligible contribution both in the gas-phase and bulk water underscores the unique properties of the air–water interface as a reaction medium. The enhancement of H-atom abstraction in interfacial water is ascribed, in part, to the relative destabilization of a more polar transition state for OH-radical addition vs. H-abstraction due to incomplete hydration at the low water densities prevalent therein

In Situ Mass Spectrometric Detection of Interfacial Intermediates in the Oxidation of RCOOH(aq) by Gas-Phase OH-Radicals

Products and intermediates of the oxidation of aqueous alkanoic acids initiated by gas-phase hydroxyl radicals, ·OH(g), at the air–water interface were detected by mass spectrometry in a novel setup under various experimental conditions. Exposure of submillimolar RCOOH (R = methyl, n-pentyl, n-heptyl) aqueous microjets to ~10 ns ·OH(g) pulses from the 266 nm laser flash photolysis of O_3(g)/O_2(g)/H_2O(g) mixtures yielded an array of interfacial species that were unambiguously and simultaneously identified in situ by online electrospray mass spectrometry. We found that peroxyl radicals R(−H)(COO^–)OO· react within 50 μs to produce alcohols R(−H)(COO^–)OH and carbonyls R(−2H)(COO–)═O via competitive Russell and Bennett–Summers mechanisms. We confirmed the formation of hydroperoxides R(−H)(COO^–)OOH in experiments performed in D_2O. To our knowledge, this is the first report on the prompt and simultaneous detection of products and peroxyl/peroxide intermediates in the heterogeneous oxidation of aqueous organics initiated by ·OH(g)