Solvation effects on glyphosate protonation and deprotonation states evaluated by mass spectrometry and explicit solvation simulations.

Glyphosate is a widely used herbicide, and its protonation and deprotonation sites are fundamental to understanding its properties. In this work, the sodiated, protonated, and deprotonated glyphosate were evaluated in the gas phase by infrared multiple photon dissociation spectroscopy to determine the exact nature of these coordination, protonation, and deprotonation states in the gas phase. In this context, Natural Bond Orbital analyses were carried out to unravel interactions that govern glyphosate (de)protonation states in the gas phase. The solvent effect on the protonation/deprotonation equilibria was also investigated by implicit (Solvation Model Based on Density and polarizable continuum models) and explicit solvation models (Monte Carlo and Molecular Dynamics simulations). These results show that glyphosate is protonated in the phosphonate group in the gas phase because of the strong hydrogen bond between the carboxylic oxygen (O7) and the protonated phosphonate group (O8-H19), while the most stable species in water is protonated at the amino group because of the preferential interaction of the NH2 + group and the solvent water molecules. Similarly, deprotonated glyphosate [Glyp-H]- was shown to be deprotonated at the phosphonate group in the gas phase but not in solution, also because of the preferential solvation of the NH2 + group present in the other deprotomers. Therefore, these results show that the stabilization of the protonated amino group by the solvent molecules is the governing factor of the (de)protonation equilibrium of glyphosate in water.

p-Aminobenzoic acid protonation dynamics in an evaporating droplet by ab initio molecular dynamics.

Protonation equilibria are known to vary from the bulk to microdroplet conditions, which could induce many chemical and physical phenomena. Protonated p-aminobenzoic acid (PABA + H+) can be considered a model for probing the protonation dynamics in an evaporating droplet, as its protonation equilibrium is highly dependent on the formation conditions from solution via atmospheric pressure ionization sources. Experiments using diverse experimental techniques have shown that protic solvents allow formation of the O-protomer (PABA protonated in the carboxylic acid group) stable in the gas phase, while aprotic solvents yield the N-protomer (protonated in the amino group) that is the most stable protomer in solution. In this work, we explore the protonation equilibrium of PABA solvated by different numbers of water molecules (n = 0 to 32) using ab initio molecular dynamics. For n = 8-32, the protonation is either at the NH2 group or in the solvent network. The solvent network interacts with the carboxylic acid group, but there is no complete proton transfer to form the O-protomer. For smaller clusters, however, solvent-mediated proton transfers to the carboxylic acid were observed, both via the Grotthuss mechanism and the vehicle or shuttle mechanism (for n = 1 and 2). Thermodynamic considerations allowed a description of the origins of the kinetic trapping effect, which explains the observation of the solution structure in the gas phase. This effect likely occurs in the final evaporation steps, which are outside the droplet size range covered by previous classical molecular dynamics simulations of charged droplets. These results may be considered relevant in determining the nature of the species observed in the ubiquitous ESI based mass spectrometry analysis, and in general for droplet chemistry, explaining how protonation equilibria are drastically changed from bulk to microdroplet conditions.

Development of a photoinduced fragmentation ion trap for infrared multiple photon dissociation spectroscopy.

RATIONALE: Methods for isomer discrimination by mass spectroscopy are of increasing interest. Here we describe the development of a three-dimensional ion trap for infrared multiple photon dissociation (IRMPD) spectroscopy that enables the acquisition of the infrared spectrum of selected ions in the gas phase. This system is suitable for the study of a myriad of chemical systems, including isomer mixtures. METHODS: A modified three-dimensional ion trap was coupled to a CO2 laser and an optical parametric oscillator/optical parametric amplifier (OPO/OPA) system operating in the range 2300 to 4000 cm-1 . Density functional theory vibrational frequency calculations were carried out to support spectral assignments. RESULTS: Detailed descriptions of the interface between the laser and the mass spectrometer, the hardware to control the laser systems, the automated system for IRMPD spectrum acquisition and data management are presented. The optimization of the crystal position of the OPO/OPA system to maximize the spectroscopic response under low-power laser radiation is also discussed. CONCLUSIONS: OPO/OPA and CO2 laser-assisted dissociation of gas-phase ions was successfully achieved. The system was validated by acquiring the IRMPD spectra of model species and comparing with literature data. Two isomeric alkaloids of high economic importance were characterized to demonstrate the potential of this technique, which is now available as an open IRMPD spectroscopy facility in Brazil.

Revisiting the Mechanism of Hydrolysis of Betanin.

Betanin (betanidin 5-O-ß-D-glucoside) is a water-soluble plant pigment used as a color additive in food, drugs and cosmetic products. Despite its sensitivity to light and heat, betanin maintains appreciable tinctorial strength in low acidic and neutral conditions, where the color of other plant pigments, such as anthocyanins, quickly fades. However, betanin is an iminium natural product that experiences acid- and base-catalyzed hydrolysis to form the fairly stable betalamic acid and cyclo-DOPA-5-O-ß-D-glucoside. Here, we show that the decomposition of betanin in aqueous phosphate solution pH 2-11 is subject to general base catalysis by hydrogen phosphate ion and intramolecular general acid and base catalysis, providing new insights on the mechanism of betanin hydrolysis. UV/Vis absorption spectrophotometry, 1 H NMR spectroscopy and mass spectrometry were used to investigate product formation. Furthermore, theoretical calculations support the hypothesis that the nitrogen atom of the tetrahydropyridine ring of betanin is doubly protonated, as observed for structurally simpler amino dicarboxylic acids. Our results contribute to the study of betanin and other pigments belonging to the class of betalains and to deepen the knowledge on the chemical properties of imino acids as well as on iminium-catalyzed modifications of carbonyl compounds in water.

Raman spectroscopic study of temperature and pressure effects on the ionic liquid propylammonium nitrate.

Raman spectroscopy has been used to decipher structural rearrangements in the protic ionic liquid propylammonium nitrate, [C3H7NH3][NO3], as a function of temperature (180-420 K) at atmospheric pressure and as a function of pressure (0.1 MPa-2.0 GPa) at room temperature. Spectral modifications of the Raman bands belonging to the anion and cation normal modes indicate structural changes occurring in both the polar and nonpolar nanoscale domains of [C3H7NH3][NO3]. The crystalline phase of [C3H7NH3][NO3] at low temperature has cations in the anti conformation and undertakes a transition with increasing temperature to a phase with cations mostly in the gauche conformation. The distorted network of hydrogen bonds gives a distribution of local environments around the anions that remains in the normal liquid phase at high temperature. The sample under high pressure might become microscopically heterogeneous, allowing for micro-Raman imaging of different ordered phases of [C3H7NH3][NO3] in a diamond anvil cell.

Pressure and temperature effects on intermolecular vibrational dynamics of ionic liquids.

Low frequency Raman spectra of ionic liquids have been obtained as a function of pressure up to ca. 4.0 GPa at room temperature and as a function of temperature along the supercooled liquid and glassy state at atmospheric pressure. Intermolecular vibrations are observed at ~20, ~70, and ~100 cm(-1) at room temperature in ionic liquids based on 1-alkyl-3-methylimidazolium cations. The component at ~100 cm(-1) is assigned to librational motion of the imidazolium ring because it is absent in non-aromatic ionic liquids. There is a correspondence between the position of intermolecular vibrational modes in the normal liquid state and the spectral features that the Raman spectra exhibit after partial crystallization of samples at low temperatures or high pressures. The pressure-induced frequency shift of the librational mode is larger than the other two components that exhibit similar frequency shifts. The lowest frequency vibration observed in a glassy state corresponds to the boson peak observed in light and neutron scattering spectra of glass-formers. The frequency of the boson peak is not dependent on the length scale of polar∕non-polar heterogeneity of ionic liquids, it depends instead on the strength of anion-cation interaction. As long as the boson peak is assigned to a mixing between localized modes and transverse acoustic excitations of high wavevectors, it is proposed that the other component observed in Raman spectra of ionic liquids has a partial character of longitudinal acoustic excitations.