Chemical Equilibria and Kinetics in Aqueous Solutions of Zymonic Acid

Investigation of pyruvic acid photolysis at the air-liquid interface as a source of aqueous secondary organic aerosols

Pyruvic acid (PA) is a ubiquitous 2-oxocarboxylic acid in the atmosphere. Its photochemical process at the air-liquid (a-l) interface has been suggested as an important source of aqueous secondary organic aerosols. We investigated the photochemical reaction pathways of PA at the a-l interface using synchrotron-based vacuum ultraviolet single-photon ionization mass spectrometry (VUV SPI-MS) coupled with the System for Analysis at the Liquid Vacuum Interface (SALVI) microreactor. Results from mass spectral analysis and the determination of appearance energies (AEs) indicate that photolysis of PA can generate radicals, then they recombine with carboxylic acids and simple molecular oligomers. Furthermore, the preliminary products could form larger oligomers via radical reaction or esterification in the presence of hydroxyl and carboxyl functional groups. Mass spectral comparison shows that most photochemical reactions would complete within 4 h. The expanded photochemistry-driven reaction flowchart of PA is proposed based on the newly discovered products. Our results reveal that the interfacial PA photochemical reactions have different mechanisms from the bulk liquid due to the interfacial properties, such as molecular density, composition, and ion concentration. Our findings show that in situ mass spectral analysis with bright photon ionization is useful to elucidate the contribution of a-l interfacial reactions leading to aqSOA formation.

Size-Dependent Sigmoidal Reaction Kinetics for Pyruvic Acid Condensation at the Air-Water Interface in Aqueous Microdroplets

The chemistry of pyruvic acid (PA) under thermal dark conditions is limited in bulk solutions, but in microdroplets it is shown to readily occur. Utilizing in situ micro-Raman spectroscopy as a probe, we investigated the chemistry of PA within aqueous microdroplets in a relative humidity- and temperature-controlled environmental cell. We found that PA undergoes a condensation reaction to yield mostly zymonic acid. Interestingly, the reaction follows a size-dependent sigmoidal kinetic profile, i.e., an induction period followed by reaction and then completion. The induction time is linearly proportional to the surface area (R2), and the maximum apparent reaction rate is proportional to the surface-to-volume ratio (1/R), showing that both the induction and reaction occur at the air-water interface. Furthermore, the droplet size is shown to be dynamic due to changes in droplet composition and re-equilibration with the relative humidity within the environmental cell as the reaction proceeds. Overall, the size-dependent sigmoidal kinetics, shown for the first time in microdroplets, demonstrates the complexity of the reaction mechanism and the importance of the air-water interface in the pyruvic acid condensation reaction.

Infrared Reflection-Absorption Spectroscopy of α-Keto Acids at the Air-Water Interface: Effects of Chain Length and Headgroup on Environmentally Relevant Surfactant Films

A variety of organic surfactants are found at air-water interfaces in natural environments, including on the surfaces of aqueous aerosols. The structure and morphology of these organic films can have profound impacts on material transfer between the gas and condensed phases, the optical properties of atmospheric aerosol, and chemical processing at air-water interfaces. Combined, these effects can have significant impacts on climate via radiative forcing, but our understanding of organic films at air-water interfaces is incomplete. Here, we examine the impact of the polar headgroup and alkyl tail length on the structure and morphology of organic monolayers at the air-water interfaces. First, we focus on the substituted carboxylic acids, α-keto acids, using Langmuir isotherms and infrared reflection absorption spectroscopy (IR-RAS) to elucidate key structures and phase behaviors of α-keto acids with a range of surface activities. We show that the structure of α-keto acids, both soluble and insoluble, at water surfaces is a compromise between van der Waals interactions of the hydrocarbon tail and hydrogen bonding interactions involving the polar headgroup. Then, we use this new data set regarding α-keto acid films at water surfaces to examine the role of the polar headgroup on organic films using a similar substituted carboxylic acid (α-hydroxystearic acid), an unsubstituted carboxylic acid (stearic acid), and an alcohol (stearyl alcohol). We show that the polar headgroup and its hydrogen bonding interactions can significantly affect the orientation of amphiphiles at air-water interfaces. Here, we provide side-by-side comparisons of Langmuir isotherms and IR-RA spectra for a set of environmentally relevant organic amphiphiles with a range of alkyl tail lengths and polar headgroup structures.

In Situ Ternary Adduct Formation of Yttrium Polyaminocarboxylates Leads to Small Molecule Capture and Activation

In this work the chemistry of yttrium complexes is exploited for small molecule capture and activation. Nuclear magnetic resonance (NMR) and density functional theory (DFT) studies were used to investigate the in situ formation of solution state ternary yttrium-acetate, yttrium-bicarbonate, and yttrium-pyruvate adducts with a range of polyaminocarboxylate chelates. These studies reveal that [Y(DO3A)(H₂ O)₂ ] (H₃ DO3A - 1,4,7,10-tetraazacyclododecane-1,4,7-tricarboxylic acid) and [Y(EDTA)(H₂ O)_q ]^- (H₄ EDTA - ethylenediaminetetraacetic acid, q = 2 and 3) are able to form ternary adducts with bicarbonate and pyruvate. In the latter, unusual decarboxylation of pyruvate to form acetic acid and CO₂ was observed and further studied using SABRE-hyperpolarised ¹³ C NMR (SABRE - signal amplification by reversible exchange) to provide information about the reaction timescale and lifetime of intermediates involved in this conversion. The work presented demonstrates that yttrium complexes can capture and activate small molecules, which may lead to novel and useful applications of this metal in catalysis and medical imaging.

Thermal Electron Attachment to Pyruvic Acid, Thermal Detachment from the Parent Anion, and the Electron Affinity of Pyruvic Acid

The kinetics of electron attachment to pyruvic acid (CH₃COCOOH) and thermal detachment from the resulting parent anion were measured from 300-515 K using a flowing afterglow─Langmuir probe apparatus. An adiabatic electron affinity (EA) for pyruvic acid was derived, 0.84 ± 0.02 eV. Electron attachment rate constants to pyruvic acid of 2.1 × 10^-8 and 1.2 × 10^-8 were measured at 300 and 400 K, respectively. Rate constants at higher temperatures are less well-defined due to possible contributions from attachment to zymonic open ketone, an endemic impurity in pyruvic acid. Similarly, unimolecular detachment rates are complicated by secondary proton transfer reaction of the pyruvic acid anion with pyruvic acid to yield an 87 Da anion. The possible contributions from these chemistries are considered, and in all cases the equilibrium constant between attachment and detachment remains well-defined, allowing for determination of the EA.

Lower degree of dissociation of pyruvic acid at water surfaces than in bulk

Understanding the acid/base behavior of environmentally relevant organic acids is of key relevance for accurate climate modelling. Here we investigate the effect of pH on the (de)protonation state of pyruvic acid at the air-water interface and in bulk by using the analytical techniques surface-specific vibrational sum frequency generation and attenuated total reflection spectroscopy. To provide a molecular interpretation of the observed behavior, simulations are carried out using a free energy perturbation approach in combination with electronic structure-based molecular dynamics. In both the experimental and theoretical results we observe that the protonated form of pyruvic acid is preferred at the air-water interface. The increased proton affinity is the result of the specific microsolvation at the interface.

Theoretical Chemistry and the Calculation of the Atmospheric State

Theoretical chemists have been actively engaged for some time in processes such as ozone photodissociation, overtone photodissociation in nitric acid, pernitric acid, sulphuric acid, clusters and in small organic acids. The last of these have shown very different behaviours in the gas phase, liquid phase and importantly at the air–water interface in aqueous aerosols. The founder of molecular dynamics, B J Alder, pointed out long ago that hydrodynamic behaviour emerged when the symmetry of a random, thermalised population of hard spheres—billiard balls—was broken by a flux of energetic molecules. Despite this, efforts over two centuries to solve turbulence by finding top-down solutions to the Navier–Stokes equation have failed. It is time for theoretical chemistry to try a bottom-up solution. Gibbs free energy that drives the circulation arises from the entropy difference between the incoming low-entropy beam of visible and ultraviolet photons and the outgoing higher-entropy flux of infrared photons over the whole 4π solid angle. The role of the most energetic molecules with the highest velocities will affect the rovibrational line shapes of water, carbon dioxide and ozone in the far wings, where there is the largest effect on radiative transfer and hence on calculations of atmospheric temperature. The atmospheric state is determined by the interaction of radiation, chemistry and fluid dynamics on the microscopic scale, with propagation through the mesoscale to the macroscale. It will take theoretical chemistry to simulate that accurately. A challenging programme of research for theoretical chemistry is proposed, involving ab initio simulation by molecular dynamics of an air volume, starting in the upper stratosphere. The aim is to obtain scaling exponents for turbulence, providing a physical method for upscaling in numerical models. Turbulence affects chemistry, radiation and fluid dynamics at a fundamental, molecular level and is thus of basic concern to theoretical chemistry as it applies to the atmosphere, which consists of molecules in motion.

Primary photodissociation mechanisms of pyruvic acid on S1: observation of methylhydroxycarbene and its chemical reaction in the gas phase

Following S₁ ← S₀ excitation at 351 nm, pyruvic acid dissociates mainly into methylhydroxycarbene (MHC) and CO₂. Some MHC molecules isomerize to more stable acetaldehyde and vinyl alcohol; the remaining MHC is stabilized and reacts bimolecularly.

Pyruvate Aldol Condensation Product: A Metabolite That Escaped Synthetic Preparation for Over a Century

The homoaldol condensation product of pyruvate, 2-methyl-4-oxopent-2-enedioic acid (OMPD), has been recently implicated as a catabolic intermediate in the bacterial degradation of lignin and previously identified from other biological sources in reports ranging over 60 years. Yet, while a preparation of the pyruvate homoaldol product precursor, 4-hydroxy-4-methyl-2-oxoglutaric acid (HMOG/Parapyruvate), was first reported in 1901, there has not been a complete published synthesis of OMPD. Analyses of reaction mixtures have helped identify zymonic acid, the lactone of HMOG, as the direct precursor to OMPD. The reaction appears to proceed through an acid- or base-mediated ring opening that does not involve formal lactone hydrolysis. In addition to a preparative protocol, we provide a proposed mechanism for the formation of methylsuccinic acid that arises from the nonoxidative decarboxylation of OMPD. Finally, we calculated the relative stability of the isomers of OMPD and found Z-OMPD to be the lowest in energy. These computations also support our observations that Z-OMPD is the most abundant isomer across a range of pH values.

Crystal engineering of nutraceutical phytosterols: new cocrystal solid solutions

A cocrystal screening of solid solutions of three phytosterols (β-sitosterol, campesterol, stigmasterol) reveals that cocrystal solid solutions are enriched with β-sitosterol with respect to stigmasterol, a natural product with cytotoxicity concerns.

Some Preformulation Studies of Pyruvic Acid and Other α-Keto Carboxylic Acids in Aqueous Solution: Pharmaceutical Formulation Implications for These Peroxide Scavengers

The purpose of this study is to assess some of the variables determining the aldol-like condensation of pyruvic acid (1), a peroxide scavenger, in aqueous solution to parapyruvic acid and higher oligomers. Its stability is compared to 3 other α-keto carboxylic acids, 2 with sterically hindered methylene groups alpha to the keto functionality (2-3) and phenylglyoxylic acid (4) with no methylene group. High-performance liquid chromatography, nuclear magnetic resonance, and liquid chromatography mass spectroscopy techniques are used in the kinetics and product analyses. 1 condensation is concentration dependent and base catalyzed above pH 7, consistent with the reaction mechanism proceeding through the attack of the fraction of the methylene group, alpha to the keto group, in its anionic form, at the keto group of a second molecule of 1. The major product is confirmed to be parapyruvic acid, but higher-order oligomers are also observed. All 3 of the other α-keto carboxylic acids 2-4 are considerably less reactive, with 4 being completely stable. Stable solutions of 1 can be prepared by the use of relatively dilute solutions maintained at slightly acidic pH values. 1 prevents the oxidation of methionine on addition of hydrogen peroxide.

In-cell determination of Lactate Dehydrogenase Activity in a Luminal Breast Cancer Model ⁻ ex vivo Investigation of Excised Xenograft Tumor Slices Using dDNP Hyperpolarized [1-13C]pyruvate

[1-¹³C]pyruvate, the most widely used compound in dissolution-dynamic nuclear polarization (dDNP) magnetic resonance (MR), enables the visualization of lactate dehydrogenase (LDH) activity. This activity had been demonstrated in a wide variety of cancer models, ranging from cultured cells, to xenograft models, to human tumors in situ. Here we quantified the LDH activity in precision cut tumor slices (PCTS) of breast cancer xenografts. The Michigan Cancer Foundation-7 (MCF7) cell-line was chosen as a model for the luminal breast cancer type which is hormone responsive and is highly prevalent. The LDH activity, which was manifested as [1-¹³C]lactate production in the tumor slices, ranged between 3.8 and 6.1 nmole/nmole adenosine tri-phosphate (ATP) in 1 min (average 4.6 ± 1.0) on three different experimental set-ups consisting of arrested vs. continuous perfusion and non-selective and selective RF pulsation schemes and combinations thereof. This rate was converted to an expected LDH activity in a mass ranging between 3.3 and 5.2 µmole/g in 1 min, using the ATP level of these tumors. This indicated the likely utility of this approach in clinical dDNP of the human breast and may be useful as guidance for treatment response assessment in a large number of tumor types and therapies ex vivo.

Scalable dissolution-dynamic nuclear polarization with rapid transfer of a polarized solid

In dissolution-dynamic nuclear polarization, nuclear spins are hyperpolarized at cryogenic temperatures using radicals and microwave irradiation. The hyperpolarized solid is dissolved with hot solvent and the solution is transferred to a secondary magnet where strongly enhanced magnetic resonance signals are observed. Here we present a method for transferring the hyperpolarized solid. A bullet containing the frozen, hyperpolarized sample is ejected using pressurized helium gas, and shot into a receiving structure in the secondary magnet, where the bullet is retained and the polarized solid is dissolved rapidly. The transfer takes approximately 70 ms. A solenoid, wound along the entire transfer path ensures adiabatic transfer and limits radical-induced low-field relaxation. The method is fast and scalable towards small volumes suitable for high-resolution nuclear magnetic resonance spectroscopy while maintaining high concentrations of the target molecule. Polarization levels of approximately 30% have been observed for 1-13C-labelled pyruvic acid in solution.

Environmental Processing of Lipids Driven by Aqueous Photochemistry of α-Keto Acids

Sunlight can initiate photochemical reactions of organic molecules though direct photolysis, photosensitization, and indirect processes, often leading to complex radical chemistry that can increase molecular complexity in the environment. α-Keto acids act as photoinitiators for organic species that are not themselves photoactive. Here, we demonstrate this capability through the reaction of two α-keto acids, pyruvic acid and 2-oxooctanoic acid, with a series of fatty acids and fatty alcohols. We show for five different cases that a cross-product between the photoinitiated α-keto acid and non-photoactive species is formed during photolysis in aqueous solution. Fatty acids and alcohols are relatively unreactive species, which suggests that α-keto acids are able to act as radical initiators for many atmospherically relevant molecules found in the sea surface microlayer and on atmospheric aerosol particles.

Photochemical Synthesis of Oligomeric Amphiphiles from Alkyl Oxoacids in Aqueous Environments

The aqueous phase photochemistry of a series of amphiphilic α-keto acids with differing linear alkyl chain lengths was investigated, demonstrating the ability of sunlight-initiated reactions to build molecular complexity under environmentally relevant conditions. We show that the photochemical reaction mechanisms for α-keto acids in aqueous solution are robust and generalizable across alkyl chain lengths. The organic radicals generated during photolysis are indiscriminate, leading to a large mixture of photoproducts that are observed using high-resolution electrospray ionization mass spectrometry, but these products are identifiable following literature photochemical mechanisms. The alkyl oxoacids under study here can undergo a Norrish Type II reaction to generate pyruvic acid, increasing the diversity of observed photoproducts. The major products of this photochemistry are covalently bonded dimers and trimers of the starting oxoacids, many of which are multi-tailed lipids. The properties of these oligomers are discussed, including their spontaneous self-assembly into aggregates.

Electrochemistry for the Generation of Renewable Chemicals: One-Pot Electrochemical Deoxygenation of Xylose to δ-Valerolactone

In this study, the electrochemical conversion of xylose to δ-valerolactone via carbonyl intermediates is demonstrated. The conversion was achieved in aqueous media and at ambient conditions. This study also demonstrates that the feedstock for production of renewable chemicals and biofuels through electrochemistry can be extended to primary carbohydrate molecules. This is the first report on a one-pot electrochemical deoxygenation of xylose to δ-valerolactone.