Liquid-phase Chemistry Research Articles

Microphysics of aqueous droplets in clouds and fogs as applied to PM-fine modeling

Clouds and fogs transform atmospheric gases through gas- and liquid-phase chemistry, and cause the production of PM-fine aerosols. The aqueous-phase concentrations of pollutants in the clouds and fogs depend on gas-phase concentrations, their solubility, and mass transfer rates. Therefore, it is important to study cloud microphysics and dynamics to understand in what portions of an aerosol size distribution oxidation occurs during a supersaturation event, when and how it occurs, and as a result understand why chemical composition depends on droplet size. The growth rate of individual cloud condensation nuclei to cloud droplets is governed by the mass transfer equation which is traditionally expressed in terms of first moment change, i.e., rate of change of cloud droplet diameters. Since droplet aqueous chemistry is directly related to the liquid water mass, the third moment more fully elucidates their growth behavior. By viewing cloud activation in these terms, it is observed that some sections of the aerosol size distribution trade water thus limiting the time that a section may experience the aqueous chemistry. Three characteristic times explain the trading of water between different sections during the cloud's lifetime, and why transport to particles is fast or slow in different distributions.

Matrix assisted laser desorption/ionisation (MALDI)-TOF mass spectrometry of supramolecular metalloporphyrin assemblies: a survey †

Analysis of a representative series of metalloporphyrins and of their supramolecular assemblies using laser desorption/ionisation time of flight mass spectrometry (LDI-TOF MS) revealed that non-covalent metal–ligand interactions in the complexes remain effective in the gas phase. Detectable assemblies range from simple ruthenium( II) porphyrin complexes with N- or P-donor ligands, or tin( IV) and zirconium( IV) porphyrin complexes with O-donor ligands, up to trimeric arrays. Crucial to successful recording of intact complexes is avoiding protic matrices which interfere with Lewis acidic analytes and which may induce ligand exchange reactions to form complexes with the matrix itself. Also described is the in situ generation of a series of ruthenium( II) porphyrin dimers via laser induced vaporisation of the corresponding monomers and the suppression of dimerisation by nitrogen and phosphorus ligands. Careful adjustment of instrument parameters such as incident laser energy requires special attention. Relative affinities in complexation reactions are found to be consistent with liquid phase chemistry, therefore LDI-MS can provide a fast and simple, yet effective screening method for reactivity exploration. However, it also induces side-reactions such as iodine abstraction/transfer reactions. Furthermore, structurally different porphyrins show variations up to 90% in relative ion formation. Extreme caution is therefore required in the interpretation, and particularly in the quantitation, of such mass spectra.

Development of an easy-to-use mass spectrometric technique to monitor solid-phase reactions on polystyrene supports.

By using mass spectrometry as an analytical tool to characterise substituted, cross-linked polystyrene resins, it is possible to directly monitor the progress of the solid-phase reactions performed on these resins without prior cleavage of the resin-bound molecules. Therefore, this is a true on-resin analytical method. The mass-to-charge ratios observed in the mass spectra are readily assigned to fragments of the polymer that include the chemically bound substituents. This is the first time that the formation and breaking of bonds have been directly observed on the polymeric support. Furthermore, the relative intensities of the signals in the mass spectra provide a measure of the completeness of the reaction. Because these measurements are rapidly performed without further chemical transformations or cleavage procedures, and because only minimal amounts of material are needed, this technique could become the solid-phase equivalent of thin-layer chromatography used in classical liquid-phase chemistry.

ChemInform Abstract: Combinatorial Chemistry and Contemporary Pharmacology

Both solid- and liquid-phase combinatorial chemistry have emerged as powerful tools for identifying pharmacologically active compounds and optimizing the biological activity of a lead compound. Complementary high-throughput in vitro assays are essential for compound evaluation. Cell-based assays that use optical endpoints permit investigation of a wide variety of functional properties of these compounds including specific intracellular biochemical pathways, protein-protein interactions, and the subcellular localization of targets. Integration of combinatorial chemistry with contemporary pharmacology now represents an important factor in drug discovery and development.

CAPRAM2.3: A chemical aqueous phase radical mechanism for tropospheric chemistry

A Chemical Aqueous Phase Radical Mechanism (CAPRAM) for modelling tropospheric multiphase chemistry is described. CAPRAM contains (1) a detailed treatment of the oxidation of organic compounds with one and two carbon atoms, (2) an explicit description of S(IV)-oxidation by radicals and iron(III), as well as by peroxides and ozone, (3) the reactions of OH, NO 3 ,C l 2 , Br 2 ,a nd CO 3 radicals, as well as reactions of the transition metal ions (TMI) iron, manganese and copper. A modelling study using a simple box model was performed for three different tropospheric conditions (marine, rural and urban) using CAPRAM coupled to the RADM2-mechanism (Stockwell et al., 1990) for liquid and gas phase chemistry, respectively. In the main calculations the droplets are assumed as monodispersed with a radius of 1m and a liquid water content of 0.3 g m 3 .I n the coupled mechanism the phase transfer of 34 substances is treated by the resistance model of Schwartz (1989). Results are presented for the concentration levels of the radicals in both phases under variation of cloud duration and droplet radius. The effects of the multiphase processes are shown in the loss fluxes of the radicals OH, NO 3 and HO2 into the cloud droplets. From calculations under urban conditions considering gas phase chemistry only the OH maximum concentration level is found to be 5:5 10 6 cm 3 . In the presence of the aqueous phase (r D 1 m, LWC D 0: 3g m 3 ) the phase transfer constitutes the most important sink (58%) reducing the OH level to 1:0 10 6 cm 3 . The significance of the phase transfer during night time is more important for the NO3 radical (90%). Its concentration level in the gas phase (1:9 10 9 cm 3 ) is reduced to 1:4 10 6 cm 3 with liquid water present. In the case of the HO2 radical the phase transfer from the gas phase is nearly the only sink (99.8%). The concentration levels calculated in the absence and presence of the liquid phase again differ by three orders of magnitude, 6 10 8 cm 3 and 4:9 10 5 cm 3 , respectively. Effects of smaller duration of cloud occurrence and of droplet size variation are assessed. Furthermore, in the present study a detailed description of a radical oxidation chain for sulfur is presented. The most important reaction chain is the oxidation of (hydrogen) sulphite by OH and the subsequent conversion of SO 3 to SO 5 followed by the interaction with TMI (notably Fe 2C )a nd chloride to produce sulphate. After 36 h of simulation ((H2O2U0 D 1 ppb; (SO2U0 D 10 ppb) the direct oxidation pathway from sulfur(IV) by H2O2 and ozone contributes only to 8% (2:9 10 10 M s 1 ) of the total loss flux of S(IV) (3 :7 10 9 Ms 1 ).

The Role of Multiphase Chemistry in the Oxidation of Dimethylsulphide (DMS). A Latitude Dependent Analysis

A kinetic model for the OH-initiated homogeneous gas phase oxidation of dimethylsulfide (DMS) in the atmosphere (Saltelli and Hjorth, 1995), has been extended here to include the liquid phase chemistry. The updated model has then been employed to predict the temperature dependency of the MSA/nss-SO42- ratio. Model predictions have been compared with observational data reported in Bates et al. (1992). Sensitivity and uncertainty analysis has been performed in a Monte Carlo fashion to identify which are the important uncertainties on the input parameters and which are the possible combinations of parameter values that could explain the field observations. Results of the analysis have indicated that the temperature dependencies of the interactions between gas phase and liquid phase chemistry may to a large extent explain the observed T-dependence of the MSA/nss- SO42- ratio. The potential role of multi-phase atmospheric chemistry, not only in the case of SO2 but also of other oxidation products of DMS and, particularly, of DMS itself, has been highlighted.

Liquid-phase chemistry: recent advances in soluble polymer-supported catalysts, reagents and synthesis

Chemistry on soluble polymer-matrices, termed liquid-phase organic synthesis, is emerging as a viable alternative or adjunct to the classical solid-phase approach across the broad spectrum of polymer-supported organic chemistry. This review details the significant advances in liquid-phase synthetic methodologies, reagents, catalysts and supports that have appeared from 1997 to the present.

Role of Ester Intermediates in Isobutane Alkylation and Its Consequence for the Choice of Catalyst System

Alkylation of isobutane by olefins catalyzed by Bronsted acids involves initial reaction of the olefin with the acid to form an ester intermediate. The esters formed by the reaction of olefins with triflic acid are very short-lived under typical alkylation reaction conditions but have been made by reaction of the olefin with frozen triflic acid and characterized by NMR spectroscopy. The ester intermediates are formed in an olefin-rich environment and reacts predominantly in an environment where the acid activity is high and the olefin concentration is low. This protects against oligomerization reactions, and the formation and stability of these ester intermediates in the absence of free acid, therefore, play an important role in the selectivity to the desired high-octane components typical of alkylate gasoline. The formation of relatively stable ester type intermediates represents a route to passivation of solid acid catalysts and thus favors a liquid acid catalyst for isobutane alkylation. A new fixed-bed alkylation technology using an SLP type catalyst has been developed for isobutane alkylation. It is a compromise between the wish for improved control of the acid catalyst and the chemistry of isobutane alkylation as it provides an effective control of an otherwise liquid acid catalyst without compromising the liquid phase chemistry. Results from bench-scale tests show product qualities comparable to the qualities obtained using the established technologies.

Regulation of Platelet Plasma Membrane Ca2+-ATPase by cAMP-dependent and Tyrosine Phosphorylation

As a consequence of its central role in the regulation of calcium metabolism in the platelet, the plasma membrane Ca2+-ATPase (PMCA) was assessed for cAMP-dependent and tyrosine phosphorylation. Addition of forskolin or prostaglandin E1, agents known to elevate platelet cAMP and calcium efflux, to platelets pre-labeled with [32P]PO4 resulted in the direct phosphorylation of platelet PMCA. Similarly, addition of the catalytic subunit of protein kinase A to platelet plasma membranes resulted in a 1.4-fold stimulation of activity. Thus, the previously reported inhibition of platelet activation by elevated intracellular cAMP may be accomplished in part by stimulation of PMCA, likely resulting in a decrease in intracellular calcium. Treatment with thrombin evoked tyrosine phosphorylation of platelet PMCA, while PMCA from resting platelets exhibited little tyrosine phosphorylation. Phosphorylation of platelet plasma membranes by pp60(src) resulted in 75% inhibition of PMCA activity within 15 min. Similarly, membranes isolated from thrombin-treated platelets exhibited 40% lower PMCA activity than those from resting platelets. Phosphorylation of erythrocyte ghosts and purified PMCA by pp60(src) also resulted in up to 75% inhibition of Ca2+-ATPase activity, and inhibition was correlated with tyrosine phosphorylation. Sequencing of a peptide obtained after 32P labeling of purified erythrocyte PMCA in vitro showed that tyrosine 1176 of PMCA4b is phosphorylated by pp60(src). These results indicate that tyrosine phosphorylation of platelet PMCA may serve as positive feedback to inhibit PMCA and increase intracellular calcium during platelet activation.

Feasibility Study of Raman Spectroscopy as a Tool to Investigate the Liquid-Phase Chemistry of Aliphatic Organic Peroxides

The described investigations are focused on peroxides occurring as products in atmospheric chemical processes, namely, hydrogen peroxide, methylhydroperoxide, hydroxymethylhydroperoxide, bis-(hydroxymethyl)peroxide, 1-hydroxyethylhydroperoxide, bis-(hydroxyethyl)peroxide, and hydroxymethylmethylperoxide. The compounds are identified and determined through the position and intensity of their characteristic O–O stretching bands in the range between 767 and 878 cm−1. Time-resolved Raman spectroscopy of peroxide solutions permits the in situ investigation of pathways and kinetics of reactions between peroxides and aldehydes.

Homogeneous and heterogeneous radiation induced NO and SO 2 removal from power plants flue gases—modeling study

The generalized mathematical model has been developed for radiation induced removal of NO and SO 2 from flue gases of power plants. This model includes energy absorption of electron beam with active species generation, reactions in gas phase, aerosol formation and growth, and liquid-phase chemistry. To investigate the role of various process parameters (initial NO and SO 2 concentrations, temperature, humidity, absorbed dose) a number of numerical calculations has been carried out. Computer modeling results are in good agreement with reported experimental data.

Liquid-phase combinatorial synthesis: in search of small-molecule enzyme mimics.

The applications, advantages and recent advances in liquid-phase combinatorial chemistry using poly-(ethyleneglycol) as a soluble polymer support are reviewed. Our recent efforts towards the synthesis of peptide-based catalysts on polyethyleneglycol are reported. The screening of libraries of peptides for catalysis is discussed.

A lognormal model for the size-resolved treatment of particle dynamics and liquid-phase chemistry

A lognormal model for the size-resolved treatment of particle dynamics and liquid-phase chemistry

Solvent Mode Participation in the Nonradiative Relaxation of the Hydrated Electron

Nonadiabatic molecular dynamics simulations are used to analyze the role of different solvent molecular degrees of freedom in the nonradiative relaxation of the first excited state of the hydrated electron. The relaxation occurs through a spatially diffuse multimode coupling between the adiabatic electronic states, indicating that the process cannot be described by a single-mode promotion model frequently used in the “large molecule” limit of gas phase theories. Solvent librations and vibrations, and the H 2O asymmetric stretch in particular, are found to be the most effective promotors of the electronic transition. Dissipation of the released energy to the solvent proceeds on two time scales: a fast 10-20 fs heating of the first solvation shell, where most of the energy is accepted by the librational degrees of freedom, and a several hundred femtosecond global reconstruction of the solvent as the first shell transfers its excess energy to the rest of the molecules. The implications of our use of a semiclassical approximation as the criterion for good promoting and energy dissipating modes are discussed. The majority of chemical processes involve radiationless energy transfer in one or more of their elementary steps, be it an intermolecular conversion, intramolecular relaxation, or an ordinary heating necessary to activate a reaction. The role of nonradiative processes is particularly pronounced in liquid phase chemistry, where the solvent serves as a thermal bath providing or withdrawing energy from the reacting species. During the past decade new experimental and theoretical techniques have been developed allowing for detailed dynamical studies on ultrafast processes in polar solutions. The hydrated electron, being a relatively simple as well as ubiquitous species in solution photo- and electrochemistry, represents a unique system for such studies. Because the hydrated electron is merely an extra electron in pure water, its evolution is governed solely by nuclear motions of water molecules. The instantaneous configuration of the solvent determines the Born-Oppenheimer (B-O) electronic energy spectrum. Vibrations and librations couple these electronic states, leading to nonadiabatic radiationless transitions. The electronic energy released during the transitions is accepted by specific solvent modes, and as the solvation structure approaches equilibrium, the energy is dissipated into the rest of the solvent. The present work was carried out to elucidate in detail what particular solvent motions and solvent regions have the greatest impact on these aspects of hydrated electron relaxation dynamics. This analysis represents the first such

Design and synthesis of novel inhibitors of prohormone convertases

Prohormone convertase-1 (PC1) and furin are subtilisin-like endopeptidases involved in the biosynthesis of peptide hormones. Five decapeptides representing the junction between the pro-region and the catalytic region of PC1 were prepared. The core sequence corresponded to D-Tyr-Arg-Ser-Lys-Arg- Xaa-Val-Gln-Lys-Asp where D-Tyr replaces the native Glu residue and Xaa, representing the P1' position, corresponds to L-Ser, L-Leu or the unnatural amino acids, D-Ser, beta-Ala, gamma-Abu, beta-Cha or gamma-Hyp. Another analog incorporating an Orn residue in place of the Arg at the P1 site was also prepared. These peptides, synthesized by solid-phase Fmoc chemistry, were fully characterized by FAB-MS, 1H-NMR and amino acid composition. Except for Orn, gamma-Hyp, L/D-Ser and L-Leu containing analogs, the others were found to be moderate to potent competitive inhibitors of hPC1 activity in the following order: gamma-Abu > beta-Cha > beta-Ala, with Ki values ranging from 1 to 8.6 microM. Both L-Ser and L-Leu analogs were correctly cleaved at the acyl carbon COOH-terminal to the Lys-Arg pair by human PC1, whereas beta-Cha, gamma-Abu, beta-Ala and D-Ser analogs proved to be very poor substrates. The Orn and gamma-Hyp derivatives were not cleaved by the enzyme at all. The three analogs containing beta-Cha, gamma-Abu and beta-Ala also proved to be potent inhibitors of the human furin activity in the following order: beta-Ala > beta-Cha > gamma-Abu, with Ki ranging from 0.8 to 2.2 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloud chemistry effects on tropospheric photooxidants in polluted atmosphere — Model results

The mathematical model presented in this paper describes in detail the gas-phase chemistry (22 reactions), gas-phase/liquid-phase equilibrium (18 equilibria) and liquid-phase chemistry (57 reactions and equilibria) in a stratiform cloud system. The model is used to analyze the influence of the liquid phase on the photooxidant formation and destruction for different gaseous SO2 concentrations with and without consideration of organic aqueous phase chemistry. It has been shown that for [SO2]>1 ppb the cloud is quantitatively a sink for H2O2, OH, HO2 and O3. The ozon destruction via O3+O2-, which is most important in remote areas, is in polluted areas only significant at summer days. The role of organic components in cloud water consists in the transformation OH → HO2 where HO2 is further transformed into H2O2.

Liquid-phase metal vapor chemistry: rotary reactors and electron-beam evaporation sources

A new rotatable metal atom reactor that allows electron-beam vaporizations of refractory materials into liquid solutions at reduced temperatures is described. This innovation demonstrably broadens the scope of synthesis from metal atoms and molecular high-temperature species. Details of the instrument and its operation are provided, together with results of studies of charges particle emissions, a description of techniques for quantifying the rate of metal atom deposition, and a description of a variable-temperature transfer tube for removing thermally labile compounds. The paper concludes with some test applications of the device in the liquid-phase electron-beam synthesis of some bis(η 6 -arene) complexes of titanium, vanadium, and molybdenum

A study of atmospheric acid formation in different environments

The mathematical model presented in this paper describes in detail the gas-phase chemistry (80 reactions), gas-phase/liquid-phase equilibrium (12 equilibria) and liquid-phase chemistry (27 reactions and equilibria) of acid formation. The model is used to simulate acid formation under various conditions: 1. (1) nonprecipitating clouds in the Adirondacks, 2. (2) raining clouds in the Ohio River valley, 3. (3) night-time fog in the Los Angeles basin and 4. (4) night-time nonprecipitating clouds in the Los Angeles basin. Model calculations are compared with observed values of sulfate and nitrate formation. The model is used to analyze the relative importance of sulfate and nitrate formation pathways under these various conditions.