Unraveling the Meaning of Effective Uptake Coefficients in Multiphase and Aerosol Chemistry.

ConspectusReactions of gas phase molecules with surfaces play key roles in atmospheric and environmental chemistry. Reactive uptake coefficients (γ), the fraction of gas-surface collisions that yield a reaction, are used to quantify the kinetics in these heterogeneous and multiphase systems. Unlike rate coefficients for homogeneous gas- or liquid-phase reactions, uptake coefficients are system- and observation-dependent quantities that depend upon a multitude of underlying elementary steps.

Balance of Unimolecular and Bimolecular Pathways Control the Temperature-Dependent Kinetics of Ozonolysis in Aerosols.

To better understand the key kinetic mechanisms controlling heterogeneous oxidation in organic aerosols, submicron particles composed of an alkene and a saturated carboxylic acid are exposed to ozone in a variable-temperature flow tube reactor. Effective uptake coefficients (γ) are obtained from the multiphase reaction kinetics, which are quantified by Vacuum Ultraviolet Photoionization Aerosol Mass Spectrometry. For aerosols composed of only of alkenes, γ doubles (from 6 × 10 to 1.

Direct Observation of Morphological and Chemical Changes during the Oxidation of Model Inorganic Ligand-Capped Particles.

Functionalization and volatilization are competing reactions during the oxidation of carbonaceous materials and are important processes in many different areas of science and technology. Here, we present a combined ambient pressure X-ray photoelectron spectroscopy (APXPS) and grazing incidence X-ray scattering (GIXS) investigation of the oxidation of oleic acid ligands surrounding NaYF nanoparticles (NPs) deposited onto SiO/Si substrates. While APXPS monitors the evolution of the oxidation products, GIXS provides insight into the morphology of the ligands and particles before and after the oxidation.

High-precision chemical quantum sensing in flowing monodisperse microdroplets.

A method is presented for high-precision chemical detection that integrates quantum sensing with droplet microfluidics. Using nanodiamonds (ND) with fluorescent nitrogen-vacancy (NV) centers as quantum sensors, rapidly flowing microdroplets containing analyte molecules are analyzed. A noise-suppressed mode of optically detected magnetic resonance is enabled by pairing controllable flow with microwave control of NV electronic spins, to detect analyte-induced signals of a few hundredths of a percent of the ND fluorescence.

Accelerated Zymonic Acid Formation from Pyruvic Acid at the Interface of Aqueous Nanodroplets.

To explore the role of the liquid interface in mediating reactivity in small compartments, the formation kinetics of zymonic acid (ZA) is measured in submicron aerosols (average radius = 240 nm) using mass spectrometry. The formation of ZA, from a condensation reaction of two pyruvic acid (PA) molecules, proceeds over days in bulk solutions, while in submicron aerosols, it occurs in minutes. The experimental results are replicated in a kinetic model using an apparent interfacial reaction rate coefficient of = (0.

Elucidating the Mechanism of Helium Evaporation from Liquid Water.

We investigate the evaporation of trace amounts of helium solvated in liquid water using molecular dynamics simulations and theory. Consistent with experimental observations, we find a super-Maxwellian distribution of kinetic energies of evaporated helium. This excess of kinetic energy over typical thermal expectations is explained by an effective continuum theory of evaporation based on a Fokker-Planck equation, parametrized molecularly by a potential of mean force and position-dependent friction.

A Detailed Reaction Mechanism for Thiosulfate Oxidation by Ozone in Aqueous Environments.

The ozone oxidation, or ozonation, of thiosulfate is an important reaction for wastewater processing, where it is used for remediation of mining effluents, and for studying aerosol chemistry, where its fast reaction rate makes it an excellent model reaction. Although thiosulfate ozonation has been studied since the 1950s, challenges remain in developing a realistic reaction mechanism that can satisfactorily account for all observed products with a sequence of elementary reaction steps. Here, we present novel measurements using trapped microdroplets to study the pH-dependent thiosulfate ozonation kinetics.

Distinguishing Surface and Bulk Reactivity: Concentration-Dependent Kinetics of Iodide Oxidation by Ozone in Microdroplets.

Iodine oxidation reactions play an important role in environmental, biological, and industrial contexts. The multiphase reaction between aqueous iodide and ozone is of particular interest due to its prevalence in the marine atmosphere and unique reactivity at the air-water interface. Here, we explore the concentration dependence of the I + O reaction in levitated microdroplets under both acidic and basic conditions.

Evaluating Possible Formation Mechanisms of Criegee Intermediates during the Heterogeneous Autoxidation of Squalene.

Organic molecules in the environment oxidatively degrade by a variety of free radical, microbial, and biogeochemical pathways. A significant pathway is heterogeneous autoxidation, in which degradation occurs via a network of carbon and oxygen centered free radicals. Recently, we found evidence for a new heterogeneous autoxidation mechanism of squalene that is initiated by hydroxyl (OH) radical addition to a carbon-carbon double bond and apparently propagated through pathways involving Criegee Intermediates (CI) produced from β-hydroxy peroxy radicals (β-OH-RO•).

Surfactant Partitioning Dynamics in Freshly Generated Aerosol Droplets.

Aerosol droplets are unique microcompartments with relevance to areas as diverse as materials and chemical synthesis, atmospheric chemistry, and cloud formation. Observations of highly accelerated and unusual chemistry taking place in such droplets have challenged our understanding of chemical kinetics in these microscopic systems. Due to their large surface-area-to-volume ratios, interfacial processes can play a dominant role in governing chemical reactivity and other processes in droplets.

Constitutive Flt3 signaling impacts conventional dendritic cell function.

The development of dendritic cells (DCs) depends on signaling via the FMS-like tyrosine kinase 3 (Flt3) receptor. How Flt3 signaling impacts terminally differentiated DC function is unknown. This is important given the increasing interest in exploiting Flt3 for vaccination and tumor immunotherapy.

On the Statistical Mechanics of Mass Accommodation at Liquid-Vapor Interfaces.

We propose a framework for describing the dynamics associated with the adsorption of small molecules to liquid-vapor interfaces using an intermediate resolution between traditional continuum theories that are bereft of molecular detail and molecular dynamics simulations that are replete with them. In particular, we develop an effective single particle equation of motion capable of describing the physical processes that determine thermal and mass accommodation probabilities. The effective equation is parametrized with quantities that vary through space away from the liquid-vapor interface.

Compression of a Stearic Acid Surfactant Layer on Water Investigated by Ambient Pressure X-ray Photoelectron Spectroscopy.

We present a combined Langmuir-Pockels trough and ambient pressure X-ray photoelectron spectroscopy (APXPS) study of the compression of stearic acid surfactant layers on neat water. Changes in the packing density of the molecules are directly determined from C 1s and O 1s APXPS data. The experimental data are fit with a 2D model for the stearic acid coverage.

GPR41 and GPR43 regulate CD8 T cell priming during herpes simplex virus type 1 infection.

Naïve CD8 T cells need to undergo a complex and coordinated differentiation program to gain the capacity to control virus infections. This not only involves the acquisition of effector functions, but also regulates the development of a subset of effector CD8 T cells into long-lived and protective memory cells. Microbiota-derived metabolites have recently gained interest for their influence on T cells, but much remains unclear about their role in CD8 T cell differentiation.

Interfacial Nanostructure and Hydrogen Bond Networks of Choline Chloride and Glycerol Mixtures Probed with X-ray and Vibrational Spectroscopies.

The molecular distribution at the liquid-vapor interface and evolution of the hydrogen bond interactions in mixtures of glycerol and choline chloride are investigated using X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Nanoscale depth profiles of supersaturated deep eutectic solvent (DES) mixtures up to ∼2 nm measured by ambient-pressure XPS show the enhancement of choline cation (Ch) concentration by a factor of 2 at the liquid-vapor interface compared to the bulk. In addition, Raman spectral analysis of a wide range of DES mixtures reveals the conversion of gauche-conformer Ch into the anti-conformer in relatively lower ChCl concentrations.

Chemical Kinetics in Microdroplets.

Micrometer-sized compartments play significant roles in driving heterogeneous transformations within atmospheric and biochemical systems as well as providing vehicles for drug delivery and novel reaction environments for the synthesis of industrial chemicals. Many reports now indicate that reaction kinetics are accelerated under microconfinement, for example, in sprays, thin films, droplets, aerosols, and emulsions. These observations are dramatic, posing a challenge to our understanding of chemical reaction mechanisms with potentially significant practical consequences for predicting the complex chemistry in natural systems.

Iodide oxidation by ozone at the surface of aqueous microdroplets.

The oxidation of iodide by ozone occurs at the sea-surface and within sea spray aerosol, influencing the overall ozone budget in the marine boundary layer and leading to the emission of reactive halogen gases. A detailed account of the surface mechanism has proven elusive, however, due to the difficulty in quantifying multiphase kinetics. To obtain a clearer understanding of this reaction mechanism at the air-water interface, we report pH-dependent oxidation kinetics of I in single levitated microdroplets as a function of [O] using a quadrupole electrodynamic trap and an open port sampling interface for mass spectrometry.

Spiers Memorial Lecture: Water at interfaces.

In this article we discuss current issues in the context of the four chosen subtopics for the meeting: dynamics and nano-rheology of interfacial water, electrified/charged aqueous interfaces, ice interfaces, and soft matter/water interfaces. We emphasize current advances in both theory and experiment, as well as important practical manifestations and areas of unresolved controversy.

pH jump kinetics in colliding microdroplets: accelerated synthesis of azamonardine from dopamine and resorcinol.

Recent studies report the dramatic acceleration of chemical reactions in micron-sized compartments. In the majority of these studies the exact acceleration mechanism is unknown but the droplet interface is thought to play a significant role. Dopamine reacts with resorcinol to form a fluorescent product azamonardine and is used as a model system to examine how droplet interfaces accelerate reaction kinetics.

Elucidating the Mechanism for the Reaction of -Phthalaldehyde with Primary Amines in the Presence of Thiols.

The use of -phthalaldehyde (OPA) in combination with a thiol reagent is a common method for detecting primary amines in amino acids, peptides, and proteins. Despite its widespread use, the exact reaction mechanism has been debated since the 1980s. Here, we measure the kinetics of the reaction between OPA, alanine, and a dithiol (1,4-dithiolthreitol, DTT) as a function of pH and reagent concentration.

A Kinetic Model for Predicting Trace Gas Uptake and Reaction.

A model is developed to describe trace gas uptake and reaction with applications to aerosols and microdroplets. Gas uptake by the liquid is formulated as a coupled equilibria that links gas, surface, and bulk regions of the droplet or solution. Previously, this framework was used in explicit stochastic reaction-diffusion simulations to predict the reactive uptake kinetics of ozone with droplets containing aqueous aconitic acid, maleic acid, and sodium nitrite.

Microbial Metabolites in the Maturation and Activation of Dendritic Cells and Their Relevance for Respiratory Immunity.

The respiratory tract is a gateway for viruses and bacteria from the external environment to invade the human body. Critical to the protection against these invaders are dendritic cells (DCs) - a group of highly specialized myeloid cells that monitors the lung microenvironment and relays contextual and antigenic information to T cells. Following the recognition of danger signals and/or pathogen molecular associated patterns in the lungs, DCs undergo activation.

Coupled Interfacial and Bulk Kinetics Govern the Timescales of Multiphase Ozonolysis Reactions.

Chemical transformations in aerosols impact the lifetime of particle phase species, the fate of atmospheric pollutants, and both climate- and health-relevant aerosol properties. Timescales for multiphase reactions of ozone in atmospheric aqueous phases are governed by coupled kinetic processes between the gas phase, the particle interface, and its bulk, which respond dynamically to reactive consumption of O. However, models of atmospheric aerosol reactivity often do not account for the coupled nature of multiphase processes.

Catalytic Coupling of Free Radical Oxidation and Electrophilic Chlorine Addition by Phase-Transfer Intermediates in Liquid Aerosols.

While examining the heterogeneous reaction of chlorine atoms with alkenes, in the presence of Cl, we have observed an unexpectedly large enhancement of reactivity and the predominance of chlorinated reaction products even under high O conditions, where Cl atom recycling is expected to be minimal. These observations cannot be explained by known free radical oxidation or cycling mechanisms, but rather we find evidence for the multiphase catalytic coupling of free radical oxidation with electrophilic Cl addition. The mechanism entails the production of oxygenated reaction intermediates, which act as gas-liquid phase-transfer catalysts (gl-PTCs) by promoting the accommodation of gas-phase Cl by the aerosol, thereby enhancing electrophilic addition.

Physiological substrates and ontogeny-specific expression of the ubiquitin ligases MARCH1 and MARCH8.

MARCH1 and MARCH8 are ubiquitin ligases that control the expression and trafficking of critical immunoreceptors. Understanding of their function is hampered by three major knowledge gaps: (i) it is unclear which cell types utilize these ligases; (ii) their level of redundancy is unknown; and (iii) most of their putative substrates have been described in cell lines, often overexpressing MARCH1 or MARCH8, and it is unclear which substrates are regulated by either ligase . Here we address these questions by systematically analyzing the immune cell repertoire of MARCH1- or MARCH8-deficient mice, and applying unbiased proteomic profiling of the plasma membrane of primary cells to identify MARCH1 and MARCH8 substrates.