The effect of ammonium sulfate on the solubility of amino acids in water at (298.15 and 323.15) K

Using the analytical gravimetric method the solubility of glycine, dl-alanine, l-isoleucine, l-threonine, and l-serine in aqueous systems of (NH₄)₂SO₄, at (298.15 and 323.15) K, were measured for salt concentrations ranging up to 2.0 molal.In the electrolyte molality range studied the experimental observations showed that ammonium sulfate is a salting-in agent for most of the amino acids studied. Furthermore, the change of the relative solubility with electrolyte concentration shows a maximum, which makes the representation of the data by a simple empirical correlation such as the Setschenow equation difficult. For the development and evaluation of a robust thermodynamic framework that makes it possible to more profoundly understand aqueous amino acid solutions with ammonium sulfate additional experimental information is needed.     

SIFT studies of the reactions of H3O+, NO+ and O+2 with a series of volatile carboxylic acids and esters

We report the results of a selected ion flow tube (SIFT) study of the reactions of H₃O⁺, NO⁺ and O⁺₂ with some nine carboxylic acids and eight esters. We assume that all the exothermic proton transfer reactions of H₃O⁺ with all the acid and esters molecules occur at the collisional rate, i.e. the rate coefficients, k, are equal to k_c; then it is seen that k values for most of the NO⁺ and O⁺₂ reactions also are equal to or close to k_c. The major ionic products of the H₃O⁺ reactions with both the acids and esters are the protonated parent molecules, MH⁺, but minor channels are also evident, these being the result of H₂O elimination from the excited (MH⁺)1 in some of the acid reactions and an alcohol molecule elimination (CH₃OH or C₂H₅OH) in some of the ester reactions. The NO⁺ reactions with the acids and esters result in both ion-molecule association producing NO⁺M in parallel with hydroxide ion (OH⁻) transfer with some of the acids, and parallel methoxide ion (CH₃O⁻) and ethoxide ion (C₂H₅O⁻) transfer as appropriate with some of the esters. The O⁺₂ reactions proceed by dissociative charge transfer with the production of two or more ionic fragments of the parent molecules, the different isomeric forms of both the acid and the ester molecules resulting in different product ions.     

Pharmaceutical salts of emoxypine with dicarboxylic acids

New salt forms of the antioxidant drug emoxypine (EMX, 2‐ethyl‐6‐methylpyridin‐3‐ol) with pharmaceutically acceptable maleic (Mlt), malonic (Mln) and adipic (Adp) acids were obtained {emoxypinium maleate, C₈H₁₂NO⁺·C₄H₃O₄⁻, [EMX+Mlt], emoxypinium malonate, C₈H₁₂NO⁺·C₃H₃O₄⁻, [EMX+Mln], and emoxypinium adipate, C₈H₁₂NO⁺·C₆H₉O₄⁻, [EMX+Adp]} and their crystal structures determined. The molecular packing in the three EMX salts was studied by means of solid‐state density functional theory (DFT), followed by QTAIMC (quantum theory of atoms in molecules and crystals) analysis. It was found that the major contribution to the packing energy comes from pyridine–carboxylate and hydroxy–carboxylate heterosynthons forming infinite one‐dimensional ribbons, with [EMX+Adp] additionally stabilized by hydrogen‐bonded C(9) chains of Adp⁻ ions. The melting processes of the [EMX+Mlt] (1:1), [EMX+Mln] (1:1) and [EMX+Adp] (1:1) salts were studied and the fusion enthalpy was found to increase with the increase of the calculated lattice energy. The dissolution process of the EMX salts in buffer (pH 7.4) was also studied. It was found that the formation of binary crystals of EMX with dicarboxylic acids increases the EMX solubility by more than 30 times compared to its pure form.     

Adsorption of cadmium(II) onto goethite and kaolinite in the presence of benzene carboxylic acids

The effect of benzene carboxylic acids on the adsorption of Cd(II) (5×10⁻⁵ M) by goethite and kaolinite has been studied in 0.005 M NaNO₃ at 25°C. The concentrations of phthalic (benzene-1,2-dicarboxylic acid), hemimellitic (1,2,3), trimellitic (1,2,4), trimesic (1,3,5), pyromellitic (1,2,4,5) and mellitic (1,2,3,4,5,6) acids varied from 2.5×10⁻⁵ to 1×10⁻³ M. Mellitic acid complexes Cd(II) strongly above about pH 3, but the other acids only at higher pH, phthalic acid forming the weakest complexes. Phthalic, trimesic and mellitic acids adsorbed strongly to goethite at pH 3, but adsorption decreased at higher pH; however, mellitic acid was still about 50% adsorbed at pH 9, by which the other two were almost entirely in solution. At 10⁻³ M all the acids enhanced the adsorption of Cd(II) to goethite, the higher members of the series being the most effective. The higher members of the series suppressed Cd(II) adsorption onto kaolinite, but phthalic and trimesic acids caused slight enhancement. The effects of mellitic acid on Cd(II) adsorption depended strongly on its concentration. The maximum enhancement of Cd(II) adsorption onto goethite was at 10⁻⁴ M. The greatest suppression of Cd(II) adsorption onto kaolinite was at 10⁻³ M, and at 2.5×10⁻⁵ M mellitic acid enhanced Cd(II) adsorption onto kaolinite at intermediate pH. The results are interpreted in terms of complexation between metal and ligand (acid), metal and substrate, ligand and substrate, and the formation of ternary surface complexes in which the ligand acts as a bridge between the metal and the surface.     

Effects of the feed solution concentration on the separation degree in Donnan dialysis for binary systems of amino acids

The separations of amino acids by Donnan dialysis using an ion-exchange membrane were studied. Donnan dialytic experiments were carried out using an anion-exchange membrane, glutamic acid–phenylalanine or glutamic acid–alanine mixed solutions as the feeds, and sodium hydroxide solutions as the stripping ones. The initial concentrations of the two kinds of amino acids in the feed solutions were equal and in the range of 0.5–50 mol m⁻³. The amino acid fluxes were measured for each feed solution. Above the feed concentration of 10 mol m⁻³, the glutamic acid flux was over 100 times greater than that of the other amino acid, and it was found that the Donnan dialysis was applicable to the separation of the amino acids. On the other hand, below 10 mol m⁻³, the amino acid fluxes varied in a complicated manner with the concentration, and below 1 mol m⁻³ there was little difference between the fluxes of the two amino acids.Furthermore, after soaking the membrane in solutions having the same concentrations as the feed in the Donnan dialysis, uptake of the amino acids into the membrane was also measured. By comparing the experimental results of both the flux and uptake of the amino acids, the reason why the flux varied in a complicated manner with the concentration was discussed.     

Interactions of some amino acids and glycine peptides with aqueous sodium dodecyl sulfate and cetyltrimethylammonium bromide at T=298.15 K: a volumetric approach

The apparent molar volumes V_φ of glycine, alanine, valine, leucine, and lysine have been determined in aqueous solutions of 0.05, 0.5, 1.0 mol · kg⁻¹ sodium dodecyl sulfate (SDS) and 1.0 mol · kg⁻¹ cetyltrimethylammonium bromide (CTAB) by density measurements at T=298.15 K. The apparent molar volumes have also been determined for diglycine and triglycine in 1 mol · kg⁻¹ SDS and CTAB solutions. These data have been used to calculate the infinite dilution apparent molar volumes V₂⁰ for the amino acids and peptides in aqueous SDS and CTAB and the standard partial molar volumes of transfer (Δ_trV_2,m⁰) of the amino acids and peptides to these aqueous surfactant solutions. The linear correlation of V₂⁰ for a homologous series of amino acids has been utilized to calculate the contribution of the charged end groups (NH₃⁺, COO⁻), CH₂ group and other alkyl chains of the amino acids to V₂⁰. The results on the partial molar volumes of transfer from water to aqueous SDS and CTAB have been interpreted in terms of ion–ion, ion–polar and hydrophobic–hydrophobic group interactions. The volume of transfer data suggests that ion–ion or ion–hydrophilic group interactions of the amino acids and peptides are stronger with SDS compared to those with CTAB. Comparison of the hydration numbers of amino acids calculated in the present studies with those in other solvents from literature shows that these numbers are almost the same at 1 mol · kg⁻¹ level of the cosolvent/cosolute. Increasing molality of the cosolvent/cosolute beyond 1 mol · kg⁻¹ lowers the hydration number of the amino acids due to increased interactions with the solvent and reduced electrostriction.     

Effect of glycine,dl-alanine and dl-2-aminobutyric acid on the temperature of maximum density of water

The effect of glycine, dl-alanine and dl-2-aminobutyric acid on the temperature of maximum density of water was determined from density measurements using a magnetic float densimeter.Densities of aqueous solutions were measured within the temperature range from T = (275.65 to 278.65) K at intervals of T = 0.50 K over the concentration range between (0.0300 and 0.1000) mol · kg⁻¹. A linear relationship between density and concentration was obtained for all the systems in the temperature range considered.The temperature of maximum density was determined from the experimental results. The effect of the three amino acids is to decrease the temperature of maximum density of water and the decrease is proportional to molality according to Despretz equation. The effect of the amino acids on the temperature of maximum density decreases as the number of methylene groups of the alkyl chain becomes larger. The results are discussed in terms of (solute + water) interactions and the effect of amino acids on water structure.     

Assignment problems of amino acids,di- and tripeptides and proteins in the near infrared region

Nine amino acids, four dipeptides, one tripeptide and several types of proteins were studied by reflectance and absorption FT-IR spectroscopy. Mid-IR and, where necessary, Raman frequencies were applied to assign the combination and overtone bands occurring in the near-IR region. On one occasion, an isotope effect (deuterization) was also employed for the recognition of the combinations involving N⁺H₃ vibrations. The overtone and combination bands of N⁺H₃, CH₃, CH₂, NH₂ and amide groups were, in part, assigned. On the basis of this assignment and with the use of second derivative and/or Fourier deconvolved spectra, attempts were made to recognize the characteristic features of glutamine and asparagine components of some proteins. The IR spectra were taken on an FT-IR spectrometer with a configuration allowing spectra to be measured in the 10,000 to 400 cm⁻¹ range.     

On the prediction of equilibrium phase behavior of amino acids in aqueous and aqueous-electrolyte solutions using SAFT equation of state

We present an approach based on the statistical associating fluids theory (SAFT) to predict the solubility of amino acids in aqueous and aqueous-electrolyte solutions. This approach can describe the association interactions and their effects on the solubility of amino acids. Using the experimental data of activity coefficients of amino acids in water, the parameters of SAFT model for amino acids are obtained. The solubility of several amino acids in the temperature range of 273.15–373.15 K is predicted. Results obtained from the model are in a good accordance with the experimental data. Also, we examine the effect of pH on the solubility of dl-methionine. Addition of an extra amino acid to the binary solution of amino acid + water makes the system more complex. To check the accuracy of model, we study the ternary solution of dl-serine + dl-alanine + water and dl-valine + dl-alanine + water. Predicted results depict that the proposed model has the ability to describe the ternary solution of amino acids, accurately. Finally, the solubility of amino acids in aqueous-electrolyte solutions is investigated. The long-range interactions caused by the presence of ions affects the solubility of amino acids, leading them to be salted in or out. To treat this kind of interaction, the restrictive primitive mean spherical approximation (RP-MSA) is coupled with the SAFT equation of state. The proposed model can accurately predict the solubility of amino acids in aqueous-electrolyte solutions.     

On Demand One-Pot Mild Preparation of Layered Double Hydroxides and Their Hybrid Forms: Advances through the Epoxide Route

Epoxide ring opening driven alkalinization process was explored with the aim of preparing layered double hydroxide (LDH) phases on demand, at room temperature. Employing iodide as nucleophilic agent, the precipitation reaction can be driven under much lower halide concentrations. This scenario favors the selective intercalation of concomitant bulky oxo anions as nitrate or perchlorate in the LDH products, allowing for the one-pot synthesis of an LDH able to delaminate in formamide. Even large dicarboxylic acids, ⁻O₂C-(CH₂)_n-CO₂⁻, with n up to 8, can be quantitively intercalated within the growing LDH phase, providing a versatile one-pot route for hybrid LDHs as well. Under the mild conditions employed, governed by a continuous pH rise from a starting acid condition, a M^II to M^*III ratio of 2 prevails, independently from the overall cationic composition. However, after moderate hydrothermal aging LDH phases bearing a cationic ratio higher than 2 could result. The solubility of a given chloride-containing M^II₂M^*III LDH can be approximated as a linear combination of the solubility of the pure hydroxylated phases of the constitutive cations, M(OH)₂ and M^*(OH)₃.     

Effect of initiators,lewis acids,deactivators, additives and medium on controlled/“living” carbocationic systems

Effect of nucleofugacity of leaving group, X, nucleophilicity/basicity of the formed counteranion, X⁻ or MtX_n+1⁻, and groups stabilizing positive charge in the carbocation, R⁺, generated from initiator, RX, as well as metal and ligands in the Lewis acids, MtX_n, strength of protonic acids, HA, added salts, NR₄⁺X⁻, nucleophiles, Nu, some other additives and also reaction conditions on controlled/“living” carbocationic polymerization is discussed. The role of all of the components is explained and their rational design for various monomers which can be polymerized cationically to well defined polymers and copolymers is provided.     

Carbon‐Nanotube‐Supported Bio‐Inspired Nickel Catalyst and Its Integration in Hybrid Hydrogen/Air Fuel Cells

A biomimetic nickel bis‐diphosphine complex incorporating the amino acid arginine in the outer coordination sphere was immobilized on modified carbon nanotubes (CNTs) through electrostatic interactions. The functionalized redox nanomaterial exhibits reversible electrocatalytic activity for the H₂/2 H⁺ interconversion from pH 0 to 9, with catalytic preference for H₂ oxidation at all pH values. The high activity of the complex over a wide pH range allows us to integrate this bio‐inspired nanomaterial either in an enzymatic fuel cell together with a multicopper oxidase at the cathode, or in a proton exchange membrane fuel cell (PEMFC) using Pt/C at the cathode. The Ni‐based PEMFC reaches 14 mW cm⁻², only six‐times‐less as compared to full‐Pt conventional PEMFC. The Pt‐free enzyme‐based fuel cell delivers ≈2 mW cm⁻², a new efficiency record for a hydrogen biofuel cell with base metal catalysts.     

Effects of clustering structure on volumetric properties of amino acids in (DMSO + water) mixtures

For a better understanding on the functions of DMSO in biological systems at a relatively lower concentration, apparent molar volumes of three typical amino acids, glycine, l-alanine and l-serine in (DMSO + water) mixtures were determined and the transfer volumes from water to the mixtures were evaluated. Together with static light scattering measurement, the results were utilised to reveal the microscopic solvent structure of (DMSO + water) mixtures and its influence on the interaction between DMSO and amino acids from a clustering point of view. The results demonstrate that the interaction between amino acids and DMSO is greatly related to the clustering structure of the mixed solvent and that amino acids interacted with already established solvent clusters. The linear dependence of transfer volume of amino acids on DMSO concentration up to 2.0 mol ⋅ dm⁻³ could be attributed to the increasing interaction with (DMSO)₁(H₂O)_n clusters. The formation of (DMSO)_m(H₂O)_n cluster via hydrophobic aggregating at higher DMSO concentration led to a decrease in hydrophobic effect of DMSO and its hydrophobic–hydrophilic and hydrophobic–hydrophobic interaction with amino acids. The structure change of solvent and the interaction between amino acid residues and DMSO was reflected by the solvation of proteins. It was found that dependence of hydrodynamic radius of bovine serum albumin and lysozyme on DMSO concentration was the same and similar to that of static light scattered by the mixed solvent, regardless of the difference in conformational change between the two proteins.     

Solubilities of p-coumaric and caffeic acid in ionic liquids and organic solvents

The solubilities of two cinnamic acid derivatives, namely p-coumaric acid and caffeic acid, in six 1-alkyl-3-methyl imidazolium based ionic liquids composed of the PF₆⁻, BF₄⁻, TFO⁻ and TF₂N⁻ anions, and in two organic solvents, t-pentanol and ethyl acetate, have been measured at the temperature range of about (303 to 317) K. The p-coumaric acid was found to be more soluble than caffeic acid in all studied solvents. Higher solubilities of both acids were observed in the ionic liquids composed of the BF₄⁻ and TFO⁻ anions. The increase of the alkyl chain length on the cation invokes a decrease in solubility in the case of hydrophilic ionic liquids composed of BF₄⁻ anion, while in the case of hydrophobic ones composed of PF₆⁻ anion an increase in the solubility is observed. Between the two organic solvents t-pentanol is better solvent than ethyl acetate for both acids. Moreover, using the van’t Hoff equations the apparent Gibbs energy, enthalpy, and entropy of solution were calculated. Finally, successful correlation of the experimental data was achieved with the UNIQUAC and the NRTL activity coefficient models, while poor predictions of the solubility of the two acids in the organic solvents were obtained with two UNIFAC models.     

Studies on the oxidation of α-amino acids by N-Bromo oxidants: Kinetics of the reaction of bromide ion with N-Bromoacetamide

The rate of oxidation of amino acids (AA) by N-Bromoacetamide (NBA) was studied in aqueous buffered medium at 35°C. The rate of disappearance of [NBA] is catalyzed by the Br⁻ produced from the reduction of NBA. Analysis of the autocatalyzed reaction gives the kinetic data for the oxidation of bromide ion by NBA. The results suggest that the protonated NBA reacts with Br⁻ to form Br₂ which rapidly oxidizes amino acids. The rate constant for the reaction between protonated NBA and Br⁻ at 35°C is estimated. © 1996 John Wiley & Sons, Inc.     

Transport of amino acids through liquid membranes III. The alkaline ion role

This paper discusses the alkaline ion (Na⁺) role in the uphill transport of amino acids through a bulk liquid membrane. The aqueous phases (source phase - S and receiving phase - R) are made up of equimolar concentrations of amino acid (4.38 mM p-aminobenzoic acid (PABA)) and alkaline ion (75 mM Na⁺). A chloroform solution containing 5 mM dibenzo-18-crown-6 (DB18C6) represents the bulk liquid membrane (M). The data obtained show that at the S-M interphase, the amino acid is coupled with the carrier via the H₃N⁺ group rather than being transported to the R-M interphase, where Na⁺ substitutes the amino acid. If Na⁺ is absent, the amino acid is transported to the opposite direction. These results support the hypothesis that the presence of Na⁺ ion in the aqueous phases assures the ‘biological’ direction of aminobenzoic acids transport through membranes.     

Explanation of Ionic Sequences in Various Phenomena. III. Salting-in and -out of Amino Acids

Abstract

Previous developed theories were applied in explaining the mechanism for the salting-in and -out of various amino acids. Glycine is salted-in according to the cationic sequences Li⁺ > Na⁺ > K⁺ > Rb⁺ and Ca²⁺ > Ba²⁺ > Sr²⁺. The ability of a cation to increase the solubility of an amino acid therefore corresponds to the destruction of the ion-ion bond between the - CO-₂and the -NH_{+ 2}group of the amino acid by forming an insoluble ion-ion bond between the added cation and the - CO?² group. This insolubilizing effect produces a positive charge on the amino acid. If, however, the anion of the added salt forms a relatively insoluble ion-ion bond with the -NH₊₂ group of the amino acid, then the effect is minimized because now both charges on the amino acid are reduced. Consequently, the more insoluble the cation amino acid salt and the more soluble the anion amino acid salt (or vice versa), the greater will be the salting-in effect. Titration of either charged group on the amino acid zwitterion has the same effect, since now the ion-ion bond of the amino acid is again destroyed. Aliphatic and carboxylic acid groups also effect the salting-in sequence, since these groups are salted-out by addition of salt when D^± < D_H2o. These mechanisms explain how leucine is first salted-out, then salted-in (at 4 M) and finally salted-out again (at 9 M) in LiCl solutions. Urea salts-in hydrophobic amino acids by increasing the dielectric constant and salts-out polar amino acids by increasing the interaction between the two charge groups on the amino acid. Glycine reverses the salting-in effect of NaCl on asparagine by competing for the Na⁺ ion.     

Electrochemical oxidation of HCOONa on Pt in acidic solutions

The electrooxidation of HCOONa was carried out over a wide range of pH on Pt. HCOO⁻ and its associated form of HCOOH do not show any difference in electrochemical behaviour. A voltammetric study demonstrates the formation of two kinds of poisoning species in the hydrogen (X₁) and double-layer (X₂) regions. Their dependences on the potential and pH were examined. Constant polarization measurements give the rate expression, i = k(α_H⁺)^−0.43 exp(0.4Fφ_n/RT), independent of the concentrations of HCOO⁻ and HCOOH. The rate-determining step is concluded to be HCOO⁻ (a) → COO⁻ (a)+H⁺ + e⁻ or HCOOH(a) → COOH(a)+H⁺ + e⁻. The negative reaction order with respect to H⁺ was explained through the retarding action of X₂. The nature of X₁ and X₂ is discussed.     

Arsenate hydroxyapatite: A physico-chemical and thermodynamic investigation

Arsenate hydroxyapatite, Ca₁₀(AsO₄)₆(OH)₂ (AsHA), was synthesized by precipitation at 100°C, and characterized by X-ray studies, IR spectra, electron microscopy, and thermography (TGA, DTA and DTG). The solubility of AsHA was investigated between temperatures of 308 and 323 K, and in the pH range 4.0–8.0. It exhibited a retrograde solubility. The solubility behaviour can be explained on the basis of the chemical potential and free energy in solution. Standard thermodynamic parameters, ΔH^θ, ΔS^θ and ΔC_p^θ, are reported. The solubility products of AsHA were found to be 4.0 x 10⁻⁹¹, 2.7 x 10⁻⁹¹, 2.1 x 10⁻⁹¹ and 1.2 x 10⁻⁹¹ at temperatures of 3.08, 313, 318 and 323 K, respectively.