Overtone-Induced Decarboxylation: A Potential Sink for Atmospheric Diacids

Computational insights into the binding modes, keto-enol tautomerization and stereo-electronically controlled decarboxylation of oxaloacetate in the active site of macrophomate synthase

Oxaloacetic acid (OAA) is a β-ketocarboxylic acid, which plays an important role as an intermediate in some metabolic pathways, including the tricarboxylic acid cycle, gluconeogenesis and fatty acid biosynthesis. Animal studies have indicated that supplementing oxaloacetic acid shows an increase of lifespan and other substantial health benefits including mitochondrial DNA protection, and protection of retinal, neural and pancreatic tissues. Most of the chemical transformations of OAA in the metabolic pathways have been extensively studied; however, the understanding of decarboxylation of OAA at the atomic level is relatively lacking. Here, we carried out MD simulations and combined quantum mechanical/molecular mechanical (QM/MM) calculations as an example to systematically elucidate the binding modes, keto-enol tautomerization and decarboxylation of OAA in the active site of macrophomate synthase (MPS), which is a Mg(II)-dependent bifunctional enzyme that catalyzes both the decarboxylation of OAA and [4+2] cycloaddition of 2-pyrone with the decarboxylated intermediate of OAA (pyruvate enolate). On the basis of our calculations, it was found that the Mg2+-coordinated oxaloacetate may exist in enol forms and keto forms. The four keto forms can be transformed into each other by simply rotating the C2-C3 single bond, nevertheless, the keto-enol tautomerization strictly requires the assistance of pocket water molecules. In addition, the decarboxylation is stereo-electronically controlled, i.e., it is the relative orientation of the terminal carboxyl anion that determines the rate of decarboxylation. As such, the chemistry of oxaloacetate in the active site of MPS is complex. On one hand, the most stable binding mode (K-I) may undergo enol-keto tautomerization to isomerize to the enol form, which may further react with the second substrate; on the other hand, K-I may isomerize to another binding mode K-II to proceed decarboxylation to generate pyruvate enolate and CO2. Starting from K-I, the enol-keto tautomerization corresponds to a barrier of 16.2 kcal mol-1, whereas the decarboxylation is associated with an overall barrier of 19.7 kcal mol-1. These findings may provide useful information for understanding the chemistry of OAA and the catalysis of related enzymes, and they are basically in agreement with the available experimental kinetic data.

Binuclear Ru(III)-Containing Polyoxometalate with Efficient Photocatalytic Activity for Oxidative Coupling of Amines to Imines

A novel binuclear ruthenium-based polyoxometalate, K₆H[{Ru₂Cl(H₂O)(CH₃COO)₂}{WO(H₂O)}₂(PW₉O₃₄)₂]·14H₂O (1), was successfully synthesized by the conventional hydrothermal method. Compound 1 was well-characterized by single-crystal X-ray diffraction, X-ray powder diffraction (PXRD), infrared (IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), electrospray ionization-mass spectrometry (ESI-MS), thermogravimetric analyses (TGA), and elemental analysis. The structural unit of compound 1 contains two [A-α-PW₉O₃₄]^9- building blocks at the upper and lower positions connected by two W atoms and two Ru atoms, where the W atoms and Ru atoms are arranged in a trapezoidal arrangement and the Ru atoms are bridged by acetic acid. Furthermore, compound 1 features characteristic absorption bands in the visible region, which allows the investigation of its photocatalytic properties in visible light. Under simulated sunlight radiation (λ > 400 nm), compound 1 exhibits high photocatalytic activity and good circularity toward the oxidative coupling of amines to imines at room temperature with O₂ as the sole oxidant.

Interfacial Vibrational Dynamics of Ice Ih and Liquid Water

Insights into energy flow dynamics at ice surfaces are essential for understanding chemical dynamics relevant to atmospheric and geographical sciences. Here, employing ultrafast surface-specific spectroscopy, we report the interfacial vibrational dynamics of ice I_h. A comparison to liquid water surfaces reveals accelerated vibrational energy relaxation and dissipation at the ice surface for hydrogen-bonded OH groups. In contrast, free-OH groups sticking into the vapor phase exhibit substantially slower vibrational dynamics on ice. The acceleration and deceleration of vibrational dynamics of these different OH groups at the ice surface are attributed to enhanced intermolecular coupling and reduced rotational mobility, respectively. Our results highlight the unique properties of free-OH groups on ice, putatively linked to the high catalytic activities of ice surfaces.

Comparative Adsorption of Acetone on Water and Ice Surfaces

Small organic molecules on ice and water surfaces are ubiquitous in nature and play a crucial role in many environmentally relevant processes. Herein, we combine surface‐specific vibrational spectroscopy and a controllable flow cell apparatus to investigate the molecular adsorption of acetone onto the basal plane of single‐crystalline hexagonal ice with a large surface area. By comparing the adsorption of acetone on the ice/air and the water/air interface, we observed two different types of acetone adsorption, as apparent from the different responses of both the free O−H and the hydrogen‐bonded network vibrations for ice and liquid water. Adsorption on ice occurs preferentially through interactions with the free OH group, while the interaction of acetone with the surface of liquid water appears less specific.

Contrasting Effects of Water on the Barriers to Decarboxylation of Two Oxalic Acid Monohydrates: A Combined Rotational Spectroscopic and Ab Initio Study

Using rotational spectroscopy, we have observed two isomers of the monohydrate of oxalic acid, the most abundant dicarboxylic acid in the atmosphere. In the lowest-energy isomer, water hydrogen-bonds to both carboxylic acid groups, and the barrier to decarboxylation decreases. In the second isomer, water bonds to only one carboxylic acid group, and the barrier increases. Though the lower barrier in the former is not unequivocal evidence that water acts as a photocatalyst, the higher barrier in the latter indicates that water acts as an inhibitor in this topology. Oxalic acid is unique among dicarboxylic acids: for the higher homologues calculated, the inhibiting topology of the monohydrate is lowest in energy and most abundant under atmospheric conditions. Consequently, oxalic acid is the only dicarboxylic acid for which single-water catalysis of overtone-induced decarboxylation in the atmosphere is plausible.

The benzoic acid-water complex: a potential atmospheric nucleation precursor studied using microwave spectroscopy and ab initio calculations

The pure rotational, high-resolution spectrum of the benzoic acid-water complex was measured in the range of 4-14 GHz, using a cavity-based molecular beam Fourier-transform microwave spectrometer. In all, 40 a-type transitions and 2 b-type transitions were measured for benzoic acid-water, and 12 a-type transitions were measured for benzoic acid-D2O. The equilibrium geometry of benzoic acid-water was determined with ab initio calculations, at the B3LYP, M06-2X, and MP2 levels of theory, with the 6-311++G(2df,2pd) basis set. The experimental rotational spectrum is most consistent with the B3LYP-predicted geometry. Narrow splittings were observed in the b-type transitions, and possible tunnelling motions were investigated using the B3LYP/6-311++G(d,p) level of theory. Rotation of the water moiety about the lone electron pair hydrogen-bonded to benzoic acid, across a barrier of 7.0 kJ mol(-1), is the most likely cause for the splitting. Wagging of the unbound hydrogen atom of water is barrier-less, and this large amplitude motion results in the absence of c-type transitions. The interaction and spectroscopic dissociation energies calculated using B3LYP and MP2 are in good agreement, but those calculated using M06-2X indicate excess stabilization, possibly due to dispersive interactions being over-estimated. The equilibrium constant of hydration was calculated by statistical thermodynamics, using ab initio results and the experimental rotational constants. This allowed us to estimate the changes in percentage of hydrated benzoic acid with variations in the altitude, region, and season. Using monitoring data from Calgary, Alberta, and the MP2-predicted dissociation energy, a yearly average of 1% of benzoic acid is expected to be present in the form of benzoic acid-water. However, this percentage depends sensitively on the dissociation energy. For example, when using the M06-2X-predicted dissociation energy, we find it increases to 18%.

Fundamental and overtone vibrational spectra of gas-phase pyruvic acid

Pyruvic acid (CH(3)COCOOH) is an important keto acid present in the atmosphere. In this study, the vibrational spectroscopy of gas-phase pyruvic acid has been investigated with special emphasis on the overtone transitions of the OH-stretch, with Delta v(OH) = 2, 4, 5. Assignments were made to fundamental and combination bands in the mid-IR. The two lowest energy rotational conformers of pyruvic acid are clearly observed in the spectrum. The lowest energy conformer possesses an intramolecular hydrogen bond, while the next lowest rotational conformer does not. This difference is clearly seen in the spectra of the OH vibrational overtone transitions, and it is reflected in the anharmonicities of the OH-stretching modes for each conformer. The spectra of the OH-stretching vibration for both conformers were investigated to establish the effect of the hydrogen bond on frequency, intensity, and line width.

Fluorosulfonic Acid and Chlorosulfonic Acid: Possible Candidates for OH-Stretching Overtone-Induced Photodissociation

We have calculated the stationary points and internal reaction coordinate pathway for the dissociation of fluorosulfonic acid (FSO3H) and chlorosulfonic acid (ClSO3H). These sulfonic acids dissociate to sulfur trioxide and hydrogen fluoride and chloride, respectively. We have calculated the frequencies and intensities of the OH-stretching transitions of FSO3H and ClSO3H with an anharmonic oscillator local mode model. We find that excitation of the fourth and third OH-stretching overtones provide adequate energy for photodissociation of FSO3H and ClSO3H, respectively. We propose that experimental detection of the products of OH-stretching overtone-induced photodissociation of FSO3H and ClSO3H would be easier than the sulfuric acid (H2SO4) equivalent. The photodissociation of H2SO4 is thought to be important in the stratosphere. The FSO3H and ClSO3H experiment could be used in proxy to support the recently proposed OH-stretching overtone-induced photodissociation mechanism of H2SO4.

Vibrational overtone induced elimination reactions within hydrogen-bonded molecular clusters: the dynamics of water catalyzed reactions in CH2FOH·(H2O)n

The dynamics of the light initiated OH-overtone induced elimination reactions CH(2)FOH.(H(2)O)(n) + hnu--> HF + CH(2)O + n(H(2)O), n = 1-3, are studied using classical trajectory simulations where the ab initio potential energy surface is computed "on-the-fly". Hydrogen bonding to the water is found to lower the barrier to reaction by over 20 kcal mol(-1) and modifies the mechanism to a concerted multiple H-atom transfer process. The reaction process is found to occur on a rapid timescale, <100 fs, and involves the hydronium ion as an intermediate. An essential aspect of dynamics is the successful competition of reaction with energy dissipation through water evaporation from the cluster.

Determination of Low Molecular Weight Dicarboxylic and Ketocarboxylic Acids in Seawater Samples

We report a new method developed for the isolation and determination of low molecular weight dicarboxylic acids and related polar compounds (C2-C9) from seawater samples. The seawater sample was first acidified and then passed through an activated charcoal column to adsorb the dicarboxylic acids. They are then desorbed with NH4OH/methanol/water mixture and derivatized with BF3/1-butanol to dibutyl esters, which are determined using a capillary GC/FID. We tested different pH (0.5, 1, 1.5) and different amounts of charcoal (100, 200, 300, 500 mg) to maximize the recovery of dicarboxylic acids. Using 300 mg of activated charcoal for 100 mL of sodium chloride solution acidified at pH 1, the recoveries for C2, C3, C4, C5, C6, C7, C8, and C9 were 61, 49, 96, 84, 77, 67, 57, and 53%, respectively. We applied this method to real seawater samples collected from the northwestern Mediterranean Sea. Here, we report, for the first time, concentrations of a homologous series of C2-C9 dicarboxylic acids as well as ketocarboxylic acids including glyoxylic and 4-oxobutanoic acids in surface water and deep seawater samples, with glyoxylic acid being the dominant species (8-40 microg L(-1)). This method allows the detection of dicarbonyls such as glyoxal in the samples although its quantification is not possible.

Cluster Phase Chemistry: Gas-Phase Reactions of Anionic Sodium Salts of Dicarboxylic Acid Clusters with Water Molecules

A homologous series of anionic gas-phase clusters of dicarboxylic acids (oxalic acid, malonic acid, succinic acid, glutaric acid, and adipic acid) generated via electrospray ionization (ESI) are investigated using collision-induced dissociation (CID). Sodiated clusters with the composition (Na(+))(2)(n+1)(dicarboxylate(2-)(n+1) for singly charged anionic clusters, where n = 1-4, are observed as major gas-phase species. Isolation of the clusters followed by CID results mainly in sequential loss of disodium dicarboxylate moieties for the clusters of succinic acid, glutaric acid, and adipic acid (C4-C6). However, all oxalate (C2) and malonate (C3) clusters and dimers (n = 1) of succinate (C4) and glutarate (C5) exhibit more complex chemistry initiated by collision of the activated cluster with water molecules. For example, with water addition, malonate clusters dissociate to yield sodium acetate, carbon dioxide, and sodium hydroxide. More generally, water molecules serve as proton donors for reacting dicarboxylate anions in the cluster and introduce energetically favorable dissociation pathways not otherwise available. Density functional theory (DFT) calculations of the binding energy of the cluster correlate well with the cluster phase reactions of oxalate and malonate clusters. Clusters of larger dicarboxylate ions (C4-C6) are more weakly bound, facilitating the sequential loss of disodium dicarboxylate moieties. The more strongly bound small dicarboxylate anions (oxalate and malonate) preferentially react with water molecules rather than dissociate to lose disodium dicarboxylate monomers when collisionally activated. Implications of these results for the atmospheric aerosol chemistry of dicarboxylic acids are discussed.