Bulk versus Interfacial Aqueous Solvation of Dicarboxylate Dianions

Preferential Solvation Study of Rosuvastatin in the {PEG400 (1) + Water (2)} Cosolvent Mixture and GastroPlus Software-Based In Vivo Predictions

Rosuvastatin (RST) is a poorly water-soluble drug responsible for limited in vivo dissolution and subsequently low oral systemic absorption (poor bioavailability). The mole fraction solubility values of RST in various ratios of binary mixtures "{PEG400 (1) + water (2)}" at 298.15 K were employed to investigate the preferential solvation (PS) of RST (3) by the binary components. Moreover, the GastroPlus program predicted the drug dissolution/absorption rates, plasma drug concentration, and compartmental regional drug absorbed from a conventional tablet as compared to the RST-loaded (PEG400 + water) mixture (at x ₁ = 0.5) in healthy subjects (considering the fast condition). Fedors' method was adopted to estimate the values of molar volume (314.8 cm³·mol^-1) and Hildebrand solubility parameter (28.08 MPa^1/2) of RST. The results of inverse Kirkwood-Buff integrals showed the PS of RST by PEG400 as observed in all studied ratios of the binary mixture. The highest PS value (δx _1,3 = 1.65 × 10^-2) for RST by PEG400 was attained at x ₁ = 0.5. Finally, the GastroPlus program predicted the maximum dissolution rate [20 mg within 15 min as compared to pure RST (1.5 mg within 15 min)]. Moreover, the program predicted increased in vivo oral absorption (1.2 μg/mL) and enhanced regional absorption (95.3%) of RST from upper segments of the gastrointestinal tract for the RST-loaded PEG400 + water mixture in humans as compared to conventional tablets (87.5% as total regional absorption and 0.88 μg/mL as in vivo absorption). Hence, the present binary system ferrying RST can be a promising strategy to control systemic dyslipidemia after oral or subcutaneous administration.

Structure and mechanism of oxalate transporter OxlT in an oxalate-degrading bacterium in the gut microbiota

An oxalate-degrading bacterium in the gut microbiota absorbs food-derived oxalate to use this as a carbon and energy source, thereby reducing the risk of kidney stone formation in host animals. The bacterial oxalate transporter OxlT selectively uptakes oxalate from the gut to bacterial cells with a strict discrimination from other nutrient carboxylates. Here, we present crystal structures of oxalate-bound and ligand-free OxlT in two distinct conformations, occluded and outward-facing states. The ligand-binding pocket contains basic residues that form salt bridges with oxalate while preventing the conformational switch to the occluded state without an acidic substrate. The occluded pocket can accommodate oxalate but not larger dicarboxylates, such as metabolic intermediates. The permeation pathways from the pocket are completely blocked by extensive interdomain interactions, which can be opened solely by a flip of a single side chain neighbouring the substrate. This study shows the structural basis underlying metabolic interactions enabling favourable symbiosis.

Photoelectron Spectroscopy of OH−-Anion–Water Clusters Generated by Ultrasonic Nebulizer

Investigating molecules in the gas phase is the only way to discover their intrinsic molecular properties; however, it is challenging to produce the gaseous phase of large-molecule chemicals. Thermal evaporation is typically used to convert molecules into gases, but it is still challenging to study ionic molecules in solutions in the gas phase. Electrospray ionization is one of the best methods to generate molecules in the gas phase, and it is uniquely capable of studying large biomolecules, including proteins. However, the molecular temperature required to study the spectroscopic properties of the molecules is very high. In this study, we developed a new, simple evaporation method using an ultrasonic nebulizer to obtain gas-phase molecules. Using this new equipment, we observed OH⁻ anions and their water clusters in the gas phase and obtained their photoelectron spectra. We observed that the vertical electron-detachment energy (VDE) of OH⁻ was 1.90 ± 0.05 eV and the VDEs of its water clusters and OH⁻ (H₂O)_n (n = 1–2) decreased to 1.50 ± 0.05 eV (n = 1) and 1.30 ± 0.05 eV (n = 2), respectively.

The aqueous interaction of neodymium with two omni existent biomoieties - a mechanistic understanding by experimental and theoretical studies

Neodymium (Nd), a technologically important metal ion, has emerged as a major contaminant in aquatic systems in recent years owing to its surge in electrical and electronic applications as a permanent magnet. The chelating molecules present in hydro- and biospheres could substantially enhance its absorption and lead to transportation and migration of Nd from the source. The mechanistic understanding of the Nd interaction with naturally relevant biomoieties present in flora and fauna is of primitive importance to estimate the toxicological effects of the metal ion. The present studies aimed at understanding the aquatic interaction of Nd with two biomoieties namely pyrazine-2-carboxylic acid (P2C) and pyrazine-2,3-dicarboxylic acid (P23C) by multiple experimental determinations and theoretical estimations. Potentiometry and spectrophotometry were employed to determine the aquatic speciation and thermodynamic stability of the complexes. Both techniques supported the formation of ML_i (i = 1-4) complexes by Nd(III) with P2C and ML_i (i = 1-3) complexes with P23C. The Nd-P23C complexes are more stable than the Nd-P2C complexes for ML formation, while the opposite trend is observed for the ML₂ and ML₃ complexes. Titration calorimetry was used to determine the enthalpies of complexation which was found to be exothermic and majorly favored by entropy contributions. The formation of the Nd(III)-P2C complexes is more exothermic than that of the respective Nd(III)-P23C complexes. Density functional theory was employed for the geometry optimization of the predicted complexes and for the estimation of the bond distances and partial charges on the coordinating atoms in the optimized geometries. Experimental insights provide crucial inputs at the macro (thermodynamic) level and theoretical calculations help in understanding the complexation process at the molecular level.

Molecular Specificity and Proton Transfer Mechanisms in Aerosol Prenucleation Clusters Relevant to New Particle Formation

Atmospheric aerosol particles influence the Earth's radiative energy balance and cloud properties, thus impacting the air quality, human health, and Earth's climate change. Because of the important scientific and overarching practical implications of aerosols, the past two decades have seen extensive research efforts, with emphasis on the chemical compositions and underlying mechanisms of aerosol formation. It has been recognized that new particle formation (NPF) contributes up to 50% of atmospheric aerosols. Nowadays, the general consensus is that NPF proceeds via two distinct stages: the nucleation from gaseous precursors to form critical nuclei of sub-1-2 nm size, and the subsequent growth into large particles. However, a fundamental understanding of both the NPF process and molecular-level characterization of the critical size aerosol clusters is still largely missing, hampering the efforts in developing reliable and predictive aerosol nucleation and climate models.Both field measurements and laboratory experiments have gathered convincing evidence about the importance of volatile organic compounds (VOCs) in enhancing the nucleation and growth of aerosol particles. Numerous and abundant small clusters composed of sulfuric acid or bisulfate ion and organic molecules have been shown to exist in ∼2 nm sized aerosol particles. In particular, kinetic studies indicated the formation of clusters with one H₂SO₄ and one or two organics being the rate-limiting step.This Account discusses our effort in developing an integrated approach, which involves the laboratory cluster synthesis via electrospray ionization, size and composition analysis via mass spectrometry, photoelectron spectroscopic characterization, and quantum mechanics based theoretical modeling, to investigate the structures, energetics, and thermodynamics of the aerosol prenucleation clusters relevant to NPF. We have been focusing on the clusters formed between H₂SO₄ or HSO₄^- and the organics from oxidation of both biogenic and anthropogenic emissions. We illustrated the significant thermodynamic advantage by involving organic acids in the formation and growth of aerosol clusters. We revealed that the functional groups in the organics play critical roles in promoting NPF process. The enhanced roles were quantified explicitly for specific functional groups, establishing a Molecular Scale that ranks highly hierarchic intermolecular interactions critical to aerosol formation. The different cluster formation pathways, probably mimicking the various polluted industrial environments, that involve cis-pinonic and cis-pinic acids were unveiled as well. Furthermore, one intriguing fundamental phenomenon on the unusual protonation pattern, which violates the gas-phase acidity (proton affinity) prediction, was discovered to be common in sulfuric acid-organic clusters. The mechanism underlying the phenomenon has been rationalized by employing the temperature-dependent experiments of sulfuric acid-formate/halide model clusters, which could explain the high stability of the sulfuric acid containing aerosol clusters. Our work provides critical molecular-level information to shed light on the initial steps of nucleation of common atmospheric precursors and benchmarks critical data for large-scale theoretical modeling to further address problems of environmental interest.

Effects of Hydration on the Conformational Behavior of Flexible Molecules with Two Charge Centers

The hydration behavior of alkyl-diammonium di-cations and alkyl-dicarboxylate di-anions, of varying alkyl chain length, was examined using basin-hopping (BH) global optimization techniques. For every di-ion investigated, a conformational transition from linear to folded is observed at a critical hydration number, n*, specific to each di-ion. A stepwise hydration study has been undertaken for alkyl-dicarboxylate di-anions in finite water clusters containing 1-12 water molecules, and low-energy structures have been examined for larger water clusters. An even number of carbons in the alkyl chain gives rise to more stable conformations in unhydrated, implicitly solvated, and explicitly solvated conditions. This work provides valuable information on how the hydration of ammonium and carboxylate ions influence larger biomolecules' conformations.

Aquatic interaction of uranium with two naturally ubiquitous pyrazine compounds: Speciation studies by experiment and theory

The present studies interpret the speciation of uranyl (UO₂²⁺) with the most ubiquitous class of natural species named pyrazines in terms of stability, speciation and its identification, thermodynamics, spectral properties determined by a range of experimental techniques and further evidenced by theoretical insights. UO₂²⁺ forms ML and ML₂ kind of species with a qualitative detection of ML₃ species, while the ESI-MS identified the formation of all the complexes including ML₃. Both the ligands act as bidentate chelators with a difference in ring size and coordinating atoms in the complex formed. The ML₃ complexes involve the third ligand participation as monodentate via carboxylate only due to the restricted coordination number and space around the UO₂²⁺ ion to accommodate three ligand molecules in its primary coordination sphere. All the complexes are found to be endothermic and purely entropy driven formations. The complex formations showed redshift in the absorption spectra and the shift was further enhanced from ML to ML₂ formation. The UO₂²⁺ ion redox properties are used to explore the redox potential and heterogeneous electron-transfer kinetic parameters as a function of pH and concentration of UO₂²⁺ in presence of pyrazine carboxylates. Interestingly, the cyclic voltammograms identified the ligands also as redox sensitive. The theoretical calculation gave inputs to understand the complex formation at the molecular level with major emphasis on geometry optimization, energetics, bonding parameters, molecular orbital diagrams and bond critical point analyses. The experimental observations in combination with theoretical addendum provided detailed knowledge on the interaction of UO₂²⁺ with pyrazine-2-carboxylate and pyrazine-2,3-dicarboxylates.

Imaging Ion Pairs Forming Structural Arrangements in Interfacial Regions

A technique to image ion pairs in solution is reported. We investigated structural and dynamic properties of ion-pair distributions deposited on highly oriented pyrolytic graphite (HOPG) surfaces in electrolyte solutions. Atomic force microscopy images of HOPG immersed in NaCl and KCl solutions display regular arrangements on top of the hexagonal carbon rings forming the HOPG atomic structure. These arrangements are the result of the low value of the aqueous interfacial dielectric constant (ε_r ≈ 3-11). The measured ion-pair radius is a function of the salt present in the solution; for KCl, the ion-pair radius is equal or smaller than 0.42 nm; for NaCl, the ion-pair radius is 0.36 nm. A comparison of these values with their crystalline lattice dimensions indicates that both KCl and NaCl ion pairs in solution at the HOPG/solution interfacial region exist as tight contact ion pairs in quasistationary distributions. The NaCl ion-pair distribution forms an aligned arrangement, and the KCl distribution is formed by intercalated pairs.

Structural, luminescence, thermodynamic and theoretical studies on mononuclear complexes of Eu(III) with pyridine monocarboxylate-N-oxides in aqueous solution

The mononuclear complexes formed by Eu(III) with three isomeric pyridine monocarboxylate-N-oxides namely picolinic acid-N-oxide (PANO), nicotinic acid-N-oxide (NANO) and isonicotinic acid-N-oxide (IANO) in aqueous solutions were studied by potentiometry, luminescence spectroscopy and isothermal titration calorimetry (ITC) to determine the speciation, coordination, luminescence properties and thermodynamic parameters of the complexes formed during the course of the reaction. More stable six membered chelate complexes with stoichiometry (ML_i, i=1-4) are formed by Eu(III) with PANO while non chelating ML and ML₂ complexes are formed by NANO and IANO. The stability of Eu(III) complexes follow the order PANO>IANO>NANO. The ITC studies inferred an endothermic and innersphere complex formation of Eu(III)-PANO and Eu(III)-IANO whereas an exothermic and outer-sphere complex formation for Eu(III)-NANO. The luminescence life time data further supported the ITC results. Density functional theoretical calculations were carried out to optimize geometries of the complexes and to estimate the energies, structural parameters (bond distances, bond angles) and charges on individual atoms of the same. Theoretical approximations are found to be in good agreement with the experimental observations.

Cluster Model Studies of Anion and Molecular Specificities via Electrospray Ionization Photoelectron Spectroscopy

Ion specificity, a widely observed macroscopic phenomenon in condensed phases and at interfaces, is a fundamental chemical physics issue. Herein we report our recent studies of such effects using cluster models in an "atom-by-atom" and "molecule-by-molecule" fashion not possible with the condensed-phase methods. We use electrospray ionization (ESI) to generate molecular and ionic clusters to simulate key molecular entities involved in local binding regions and characterize them by employing negative ion photoelectron spectroscopy (NIPES). Inter- and intramolecular interactions and binding configurations are directly obtained as functions of the cluster size and composition, providing molecular-level descriptions and characterization over the local active sites that play crucial roles in determining the solution chemistry and condensed-phase phenomena. The topics covered in this article are relevant to a wide range of research fields from ion specific effects in electrolyte solutions, ion selectivity/recognition in normal functioning of life, to molecular specificity in aerosol particle formation, as well as in rational material design and synthesis.

Probing the early stages of solvation of cis-pinate dianions by water, acetonitrile, and methanol: a photoelectron spectroscopy and theoretical study

Photoelectron spectroscopy and theoretical studies indicate the coexistence of symmetric and asymmetric solvated clusters for cis-pinate dianions.

Succinic acid in aqueous solution: connecting microscopic surface composition and macroscopic surface tension

The water-vapor interface of aqueous solutions of succinic acid, where pH values and bulk concentrations were varied, has been studied using surface sensitive X-ray photoelectron spectroscopy (XPS) and molecular dynamics (MD) simulations. It was found that succinic acid has a considerably higher propensity to reside in the aqueous surface region than its deprotonated form, which is effectively depleted from the surface due to the two strongly hydrated carboxylate groups. From both XPS experiments and MD simulations a strongly increased concentration of the acid form in the surface region compared to the bulk concentration was found and quantified. Detailed analysis of the surface of succinic acid solutions at different bulk concentrations led to the conclusion that succinic acid saturates the aqueous surface at high bulk concentrations. With the aid of MD simulations the thickness of the surface layer could be estimated, which enabled the quantification of surface concentration of succinic acid as a multiple of the known bulk concentration. The obtained enrichment factors were successfully used to model the surface tension of these binary aqueous solutions using two different models that account for the surface enrichment. This underlines the close correlation of increased concentration at the surface relative to the bulk and reduced surface tension of aqueous solutions of succinic acid. The results of this study shed light on the microscopic origin of surface tension, a macroscopic property. Furthermore, the impact of the results from this study on atmospheric modeling is discussed.

Excited states of multiply-charged anions probed by photoelectron imaging: riding the repulsive Coulomb barrier

Recent progress towards understanding the repulsive Coulomb barrier in multiply-charged anion using photoelectron spectroscopy is discussed.

Solvent-mediated folding of dicarboxylate dianions: aliphatic chain length dependence and origin of the IR intensity quenching

We combine infrared photodissociation spectroscopy with quantum chemical calculations to characterize the hydration behavior of microsolvated dicarboxylate dianions, (CH2)m(COO(-))2·(H2O)n, as a function of the aliphatic chain length m. We find evidence for solvent-mediated folding transitions, signaled by the intensity quenching of the symmetric carboxylate stretching modes, for all three species studied (m = 2, 4, 8). The number of water molecules required to induce folding increases monotonically with the chain length and is n = 9-12, n = 13, and n = 18-19 for succinate (m = 2), adipate (m = 4), and sebacate (m = 8), respectively. In the special case of succinate, the structural transition is complicated by the possibility of bridging water molecules that bind to both carboxylates with merely minimal chain deformation. On the basis of vibrational calculations on a set of model systems, we identify the factors responsible for intensity quenching. In particular, we find that the effect of hydrogen bonds on the carboxylate stretching mode intensities is strongly orientation dependent.

Negative Ion Photoelectron Spectroscopy Reveals Thermodynamic Advantage of Organic Acids in Facilitating Formation of Bisulfate Ion Clusters: Atmospheric Implications

Recent lab and field measurements have indicated critical roles of organic acids in enhancing new atmospheric aerosol formation. Such findings have stimulated theoretical studies with the aim of understanding the interaction of organic acids with common aerosol nucleation precursors like bisulfate (HSO4(-)). We report a combined negative ion photoelectron spectroscopic and theoretical investigation of molecular clusters formed by HSO4(-) with succinic acid (SUA, HO2C(CH2)2CO2H), HSO4(-)(SUA)n (n = 0-2), along with HSO4(-)(H2O)n and HSO4(-)(H2SO4)n. It is found that one SUA molecule can stabilize HSO4(-) by ca. 39 kcal/mol, three times the corresponding value that one water molecule is capable of (ca. 13 kcal/mol). Molecular dynamics simulations and quantum chemical calculations reveal the most plausible structures of these clusters and attribute the stability of these clusters to the formation of strong hydrogen bonds. This work provides direct experimental evidence showing significant thermodynamic advantage by involving organic acid molecules to promote formation and growth in bisulfate clusters and aerosols.

Vibrational spectroscopy of microhydrated conjugate base anions

Conjugate-base anions are ubiquitous in aqueous solution. Understanding the hydration of these anions at the molecular level represents a long-standing goal in chemistry. A molecular-level perspective on ion hydration is also important for understanding the surface speciation and reactivity of aerosols, which are a central component of atmospheric and oceanic chemical cycles. In this Account, as a means of studying conjugate-base anions in water, we describe infrared multiple-photon dissociation spectroscopy on clusters in which the sulfate, nitrate, bicarbonate, and suberate anions are hydrated by a known number of water molecules. This spectral technique, used over the range of 550-1800 cm(-1), serves as a structural probe of these clusters. The experiments follow how the solvent network around the conjugate-base anion evolves, one water molecule at a time. We make structural assignments by comparing the experimental infrared spectra to those obtained from electronic structure calculations. Our results show how changes in anion structure, symmetry, and charge state have a profound effect on the structure of the solvent network. Conversely, they indicate how hydration can markedly affect the structure of the anion core in a microhydrated cluster. Some key results include the following. The first few water molecules bind to the anion terminal oxo groups in a bridging fashion, forming two anion-water hydrogen bonds. Each oxo group can form up to three hydrogen bonds; one structural result, for example, is the highly symmetric, fully coordinated SO(4)(2-)(H(2)O)(6) cluster, which only contains bridging water molecules. Adding more water molecules results in the formation of a solvent network comprising water-water hydrogen bonding in addition to hydrogen bonding to the anion. For the nitrate, bicarbonate, and suberate anions, fewer bridging sites are available, namely, three, two, and one (per carboxylate group), respectively. As a result, an earlier onset of water-water hydrogen bonding is observed. When there are more than three hydrating water molecules (n > 3), the formation of a particularly stable four-membered water ring is observed for hydrated nitrate and bicarbonate clusters. This ring binds in either a side-on (bicarbonate) or top-on (nitrate) fashion. In the case of bicarbonate, additional water molecules then add to this water ring rather than directly to the anion, indicating a preference for surface hydration. In contrast, doubly charged sulfate dianions are internally hydrated and characterized by the closing of the first hydration shell at n = 12. The situation is different for the (-)O(2)C(CH(2))(6)CO(2-) (suberate) dianion, which adapts to the hydration network by changing from a linear to a folded structure at n > 15. This change is driven by the formation of additional solute-solvent hydrogen bonds.

Electromigration oscillations occurring in ternary electrolyte systems with complex eigenmobilities, as predicted by theory and ascertained by capillary electrophoresis

Chemical oscillations are driven by a gradient of chemical potential and can only develop in systems where the substances are far from chemical equilibrium. We have discovered a new analogous type of oscillations in ternary electrolyte mixtures, which we call electromigration oscillations. They appear in liquid solutions of electrolytes and are associated with the electromigration movement of ions when conducting an electric current. These electromigration oscillations are driven by the electric potential gradient, while the system can be close to chemical equilibrium. The unequivocal criterion for the instability of the electrolyte solution and its ability to oscillate is the existence of complex system eigenmobilities. We show how to calculate the system eigenmobilities by utilizing the linear theory of electromigration and how to identify the complex system eigenmobilities to predict electromigration oscillations. To experimentally prove these electromigration oscillations, we employ a commercially available instrument for capillary electrophoresis. The oscillations start a certain period of time after switching on the driving electric current. The axial concentration profiles of the electrolytes in the capillary attain a nearly periodic pattern with a spatial period in the range of 1-4 mm, with almost constant amplitude. This periodic pattern moves in the electric field with mobility that is equal to the real part of the complex eigenmobility pair. We have found several ternary oscillating electrolytes composed of a base and two acids, of which at least one has higher valence than one in absolute value. All the systems have three system eigenmobilities: one is real and close to zero, and the two others form the complex conjugate pair, the real part of which is far from zero.

Microsolvation of the Dicyanamide Anion: [N(CN)2-](H2O)n (n = 0−12)

Photoelectron spectroscopy is combined with ab initio calculations to study the microsolvation of the dicyanamide anion, N(CN)(2)(-). Photoelectron spectra of [N(CN)(2)(-)](H2O)n (n = 0-12) have been measured at room temperature and also at low temperature for n = 0-4. Vibrationally resolved photoelectron spectra are obtained for N(CN)(2)(-), allowing the electron affinity of the N(CN)2 radical to be determined accurately as 4.135 +/- 0.010 eV. The electron binding energies and the spectral width of the hydrated clusters are observed to increase with the number of water molecules. The first five waters are observed to provide significant stabilization to the solute, whereas the stabilization becomes weaker for n > 5. The spectral width, which carries information about the solvent reorganization upon electron detachment in [N(CN)(2)(-)](H2O)n, levels off for n > 6. Theoretical calculations reveal several close-lying isomers for n = 1 and 2 due to the fact that the N(CN)(2)(-) anion possesses three almost equivalent hydration sites. In all the hydrated clusters, the most stable structures consist of a water cluster solvating one end of the N(CN)(2)(-) anion.

Water mediated attraction between repulsive ions: A cluster-based simulation approach

Could two like ions be attractive to each other in the presence of water? To address this question and to further interrogate the intriguing solvent effects at a molecular level on multiply charged species, a "bottom-up" simulation approach was formulated, from which the inter-ionic potential of mean force and other properties were monitored closely with the gradual addition of the water molecules. This approach was first tested on a commonly studied ion pair (namely, Na+ and Cl-), where excellent agreement with the published bulk-phase data was found. Further application of this approach to the like-ion pair indicated that an attractive interaction between two anions or two cations can be induced by the addition of an appropriate number of water molecules. This result corroborates a recent experimental report of an intriguing folding of a dianionic polymer into a more compact structure with the addition of water molecules in gas phase as well as previous theoretical findings of possible attraction between like-ion pairs in bulk aqueous phases.

Propensity for the Air/Water Interface and Ion Pairing in Magnesium Acetate vs Magnesium Nitrate Solutions: Molecular Dynamics Simulations and Surface Tension Measurements

Molecular dynamics simulations in slab geometry and surface tension measurements were performed for aqueous solutions of magnesium acetate and magnesium nitrate at various concentrations. The simulations reveal a strong affinity of acetate anions for the surface, while nitrate exhibits only a very weak surface propensity, and magnesium is per se strongly repelled from the air/water interface. CH3COO- also exhibits a much stronger tendency than NO3- for ion pairing with Mg2+ in the bulk and particularly in the interfacial layer. The different interfacial behavior of the two anions is reflected by the opposite concentration dependence (beyond 0.5 M) of surface tension of the corresponding magnesium salts. Measurements, supported by simulations, show that the surface tension of Mg(NO3)2(aq) increases with concentration as for other inorganic salts. However, in the case of Mg(OAc)2(aq) the surface tension isotherm exhibits a turnover around 0.5 M, after which it starts to decrease, indicating a positive net solute excess in the interfacial layer at higher concentrations.

Exploring Intramolecular Reactions in Complex Systems with Metadynamics: The Case of the Malonate Anions

We have determined the optimized structures, relative energies and intramolecular reactions for two anionic forms of malonic acid, anion malonate(-1) (HO(2)CCH(2)CO(2)(-)) and malonate(-2) ((-)O(2)CCH(2)CO(2)(-)). For this purpose we employed accurate quantum chemistry calculations using second-order Möller-Plesset perturbation theory and Density Functional Theory with an aug'-cc-p-VTZ basis set to determine the structures and energies, and a novel metadynamics method based on Car-Parrinello molecular dynamics for the thermal reactions. For both malonates, we found new isomers (keto and enol structures) characterized by CO(2) rotations and intramolecular proton transfers. These proton transfers characterize the keto-enol tautomerism that takes place both in the monoanion and dianion. In all cases, the keto tautomer is the more stable configuration. The metadynamics method allows the system to explore the potential energy surface in a few picoseconds, crossing activation barriers of 20-50 kcal/mol.