Thermochemical investigation of interaction of L-serin with glycerol,ethylene glycol,and 1,2-propylene glycol in aqueous solutions

By the method of dissolution calorimetry integral enthalpies of dissolution Δ_sol H ^m of L-serine are measured in the mixtures of water with glycerol, ethylene glycol, and 1,2-propylene glycol at the concentration of the organic solvent up to 0.42 mole fraction. The standard values of enthalpies of dissolution (Δ_sol H ⁰) and transfer (Δ_tr H ⁰) of amino acids from water to mixed solvents are calculated. The calculated values of the enthalpy coefficients of pair interactions of L-serine with the molecules of co-solvents are positive. The data obtained are interpreted in terms of prevalence of different types of interactions in solutions and the influence of nature of co-solvents on the thermochemical characteristics of the dissolved amino acids.     

Enthalpy characteristics of dissolution of <Emphasis Type="SmallCaps">l</Emphasis>-cysteine and <Emphasis Type="SmallCaps">l</Emphasis>-asparagine in aqueous solutions of acetonitrile and dimethyl sulfoxide at 298.15 K

The integral enthalpies of dissolution Δ_sol H ^m of l-cysteine and l-asparagine in mixtures of water with acetonitrile and dimethyl sulfoxide at the concentration of organic solvent up to 0.32 molar fractions were measured by means of dissolution calorimetry. The standard enthalpies of dissolution (Δ_sol H°) and transfer (Δ_trans H°) of the amino acids from water to a mixed solvent were calculated. The enthalpy coefficients of pair interactions for L-cysteine and L-asparagine with cosolvent molecules are positive, except for the L-asparagine-water-acetonitrile system. The concepts on the prevailing effect of specific interactions in solutions and the influence of the nature of the cosolvents and lateral substituents of the amino acids on the thermochemical characteristics of dissolution were used to explain the data obtained.     

Standard enthalpies of dissolution of L-alanine in the water solutions of glycerol, ethylene glycol, and 1,2-propylene glycol at 298.15 K

The integral enthalpies of dissolution Δ_sol H ^m of L-alanine in mixtures of water with glycerol, ethylene glycol, and 1,2-propylene glycol under the concentration of organic solvents up to 0.32 mole fraction were measured by means of calorimetry. The standard values of the dissolution enthalpies (Δ_sol H ^o) and transfer enthalpies (Δ_tr H ^o) of amino acids from water to the mixed solvent were calculated. It was shown that the calculated enthalpic coefficients of pair interactions of L-alanine with cosolvent molecules have positive values. The data obtained are interpreted from the viewpoint of prevalence of different types of interactions in the solutions and influence of the cosolvents nature on the thermo-chemical characteristics of amino acid dissolution.     

Enthalpy characteristics of L-asparagine interaction with glycerol, ethylene glycol, and 1,2-propylene glycol in aqueous solutions

Integral dissolution enthalpies ??_sol H ^m of L-asparagine in the mixtures of water with glycerol, ethylene glycol, and 1,2-propylene glycol at the concentration of organic solvents up to 0.24 mole fraction were measured by the calorimetry method. The standard enthalpies of dissolution (??_sol H ⁰) and transport (??_tr H ⁰) of amino acids from water to the mixed solvents were derived. The calculated enthalpy coefficients of the molecule pair interactions of L-asparagine-cosolvent are positive, except for the amino acid-glycerol-water system. The data obtained are interpreted in terms of prevalence of different types of interactions in solution and effect of the cosolvent nature on the thermochemical characteristics of the L-asparagine dissolution.     

Enthalpy pair coefficients of interaction for DL-valine in aqueous solutions of polyatomic alcohols at 298 K

Integral enthalpies of dissolution Δ_sol H ^m of DL-valine are measured via calorimetry of dissolution in aqueous solutions of glycerol, ethylene glycol, and 1,2-propylene glycol. Standard values of the enthalpies of dissolution (Δ_sol H ^○) and transfer (Δ_tr H ^○) of amino acid from water to mixed solvent are calculated from the resulting experimental data. The enthalpy coefficients for pair interactions h_xy of amino acid with polyatomic alcohol molecules are calculated using the McMillan-Meyer theory and have positive values. The obtained results are discussed in light of the theory of the predomination of various types of interactions in mixed solutions and the effect of structural features of interacting biomolecules on the thermochemical parameters of the dissolution of amino acids.     

The Enthalpies of Solution and Solvation of L-Serine in Aqueous Alcoholic Solutions at 298.15 K

The integral enthalpies of solution (Δ_sol H ^m ) of L-serine in water-alcohol (ethanol, n-propanol, isopropanol) mixtures were measured over the range of alcohol concentrations up to 0.32 mole fractions. The standard enthalpy of solution (Δ_sol H°), enthalpy of transfer of L-serine from water into a mixed solvent (Δ_tr H°), and enthalpy of solvation (Δ_solv H°) were calculated. The dependences of Δ_sol H°, Δ_solv H°, and Δ_tr H° on the composition of mixtures had extrema. The calculated enthalpy coefficients of the pair interactions of L-serine with alcohol molecules were positive and increased along the series ethanol, n-propanol, isopropanol. The data obtained were interpreted in terms of different types of interactions in solutions and the influence of the nature of amino acid residues on the thermochemical solution characteristics. Original Russian Text ? I.N. Mezhevoi, V.G. Badelin, 2008, published in Zhurnal Fizicheskoi Khimii, 2008, Vol. 82, No. 4, pp. 789–791.     

Enthalpic characteristics of solution of amino acids and aliphatic dipeptides in aqueous solutions of KCl

The integral enthalpies of dissolution Δ_sol H ^m of L-α-serine and L-α-asparagine in mixtures of water with KCl were measured in electrolyte concentrations of up to 4 mol/kg at 298.15 K. The standard enthalpies of dissolution (Δ_sol H ^o) and transfer (Δ_tr H ^o) of amino acids from water to aqueous solutions of KCl were calculated. The enthalpic pair interaction coefficients h _xy of biomolecules with KCl were estimated within the McMillan-Mayer theory. The changing nature of the interaction between the components of the solution (depending on the structure of the dissolved biosubstance side substituents) is shown on the basis of data we obtained earlier for amino acids and dipeptides series. Estimates of the contributions from the electrostatic and other interactions of dipolar ions of amino acids and dipeptides with ions of electrolyte KCl in the enthalpic pair interaction coefficients h _xy are obtained using the Kirkwood approach.     

Thermochemical characteristics of interactions of DL-alanine in aqueous solutions of glycerol, ethylene glycol, and 1,2-propylene glycol

The method of dissolution calorimetry was used to measure the integral enthalpy of dissolution ??_sol H ^m of DL-alanine in mixtures of water with glycerol, ethylene glycol, and 1,2-propylene glycol at a concentration of organic solvent up to 0.32 mole fraction. The standard dissolution enthalpy (??_sol H ⁰) and transport enthalpy of amino acids from water to mixed solvent (??_tr H ⁰) were calculated. The calculated enthalpy coefficients of pair interactions of the DL-alanine molecules with the polyol molecules are positive and less than these values for L-alanine. The effect of interactions of different types in solution and the structural features of biomolecules and co-solvents on the enthalpy of dissolution characteristics of amino acids were considered.     

Dissolution properties of oxymatrine in citric acid solution

In this article, the enthalpy of dissolution for oxymatrine in 0.15 M citric acid solution is measured using a RD496-2000 Calvet Microcalorimeter at 36.5 °C under atmospheric pressure. The differential enthalpy (Δ _dif H _m) and molar enthalpy (Δ _sol H _m) were determined for oxymatrine dissolution in 0.15 M citric acid solution. On the basis of these experimental data and calculated results, the kinetic equation, half-life, Δ _sol H _m, Δ _sol G _m, and Δ _sol S _m of the dissolution process were also obtained.     

Thermochemical analysis of intermolecular interactions between <Emphasis Type="Italic">N</Emphasis>-acetylglycine and polyols in aqueous solutions

The integral enthalpies of dissolution Δ_sol H _m for N-acetylglycine in aqueous solutions of glycerol, ethylene glycol and 1,2-propylene glycol are measured via solution calorimetry. The standard enthalpies of dissolution (Δ_sol Н ⁰) and transfer (Δ_tr Н ⁰) for N-acetylglycine from water to aqueous solutions of polyhydric alcohols are calculated from experimental data. Positive values of enthalpy coefficients of pair interactions h _xy for amino acids and polyol molecules are calculated using the McMillan–Mayer theory. The results are discussed using an approach for evaluating different types of interactions in ternary systems and the effect the structural features of interacting biomolecules have on the thermochemical characteristics of N-acetylglycine dissolution.     

Energetics of the molecular interactions of L-alanine and L-serine with xylitol, D-sorbitol, and D-mannitol in aqueous solutions at 298.15 K

Integral enthalpies of dissolution Δ_sol H ^m of L-alanine and L-serine are measured via the calorimetry of dissolution in aqueous solutions of xylitol, D-sorbitol, and D-mannitol. Standard enthalpies of dissolution (Δ_sol H ^○) and the transfer (Δ_tr H ^○) of amino acids from water to binary solvent are calculated from the experimental data. Using the McMillan-Mayer theory, enthalpy coefficients of pairwise interactions h _xy of amino acids with molecules of polyols are calculated that are negative. The obtained results are discussed within the theory of the prevalence of different types of interactions in mixed solutions and the effect of the structural features of interacting biomolecules on the thermochemical parameters of dissolution of amino acids.     

Effect of organic co-solvents on the solvation enthalpies of amino acids and dipeptides in mixed aqueous solutions

Transition enthalpies (Δ_tr H ^o) of substances from water to binary solutions were calculated at 298.15 K on the basis of standard dissolution enthalpies (Δ_sol H ^o) for six amino acids and five dipeptides in the mixtures of water with organic solvents of various chemical natures. The enthalpy pair coefficients of interaction h _xy for biomolecules with organic component of mixture were estimated within the formalism of the McMillan-Mayer theory. The change in the character of the interaction of the components of solution was demonstrated in dependence on the physicochemical properties of the solvent and nature of the side radical of the dissolved bioorganic substance. Quantitative estimation of the type of interaction of the substance with the solvent was performed on the basis of correlation ratios relating the enthalpy characteristics of bioorganic substances to properties of organic cosolvents. It was shown that in the solutions under study, the effects of both the specific (mainly electron donor) and non-specific solvation of amino acids and peptides are observed.     

Dependence of the solution and transfer enthalpies of DL-α-alanyl-DL-α-asparagine on the composition of water-amide binary solvents at 298.15 K

Solution enthalpies of DL-α-alanyl-DL-α-asparagine (AlaAsn) in water-formamide, water-N-methylformamide, water-N,N-dimethylformamide, and water-N,N-dimethylacetamide mixtures were measured in the range of amide mole fractions x ₂ = 0–0.3. The standard enthalpies of solution (Δ_sol H°) and transfer (Δ_tr H°) of AlaAsn from water to the binary solvent and enthalpy coefficients of pair-wise interactions (h _xy ) of AlaAsn with amide molecules were calculated. The influence of the composition of the water-organic mixture on the enthalpy characteristics of AlaAsn is discussed. It is shown that the enthalpy characteristics of solution and transfer of AlaAsn are related to the structure of amides.     

The thermochemical characteristics of solution of L-cysteine and L-asparagine in aqueous 1,4-dioxane and acetone

The integral heats of solution Δ_sol H ^m of L-cysteine and L-asparagine in mixtures of water with 1,4-dioxane and acetone were measured by solution calorimetry at organic solvent concentrations of up to 0.30 mole fractions. The standard enthalpies of solution (Δ_sol H ^o) and transfer (Δ_tr H ^o) of amino acids from water to mixed solvents were calculated. The calculated enthalpy coefficients of pair interactions of L-cysteine and L-asparagine with cosolvent molecules were positive. The results were interpreted from the point of view of the predominance of interactions of various types in solutions and the influence of the nature of cosolvents and amino acid side substituents on the thermochemical characteristics of solution.     

The Dissolution Properties of 2-(1,1-Dinitromethylene)-1,3-diazepentane in N-methyl Pyrrolidone

The dissolution properties of 2-(1,1-dinitromethylene)-1,3-diazepentane in N-methyl pyrrolidone(NMP) were studied with a RD496-2000 Calvet microcalorimeter at three different temperatures. The measured molar enthalpies (Δ_sol H) for 2-(1,1-dinitromethylene)-1,3-diazepentane in NMP at T=(298.15,306.15,311.15) K are (5.02, 5.59, 6.67) kJ⋅mol⁻¹, respectively. The differential molar enthalpies (Δ_dif H), the specific enthalpies (Δ_sol h), and the standard heat effect (Q ^Θ) for 2-(1,1-dinitromethylene)-1,3-diazepentane in NMP were obtained at the same time. The kinetic parameters of activation energy E and pre-exponential factor A are 2.26×10⁴ J⋅mol⁻¹ and 10^2.06 s⁻¹, which indicate that NMP is a good solvent for the title compound.     

Thermodynamic characteristics of protolytic equilibria of L-serine in aqueous solutions

The heat effects of the reaction of aqueous solution of L-serine with aqueous solutions of HNO₃ and KOH were determined by calorimetry at temperatures of 288.15, 298.15, and 308.15 K, and ionic strength values of 0.2, 0.5, and 1.0 (background electrolyte, KNO₃). Standard thermodynamic characteristics (Δ_r H ^o, Δ_r G ^o, Δ_r S ^o, ΔC _p ^o) of the acid-base reactions in aqueous solutions of L-serine were calculated. The effect of the concentration of background electrolyte and temperature on the heats of dissociation of amino acid was considered. The combustion energy of L-serine by bomb calorimetry in the medium of oxygen was determined. The standard combustion and formation enthalpies of crystalline L-serine were calculated. The heats of dissolution of crystalline L-serine in water and solutions of potassium hydroxide at 298.15 K were measured by direct calorimetry. The standard enthalpies of formation of L-serine and products of its dissociation in aqueous solution were calculated.     

Thermal properties of tetraalkylammonium bromides in several solvents

The enthalpies of solution of tetraethyl- and tetra-n-hexylammonium bromides have been measured in mixtures of formamide with ethylene glycol at 298.15 and 313.15 K in the whole mole fraction range by the calorimetric method. The standard enthalpies of solution in binary mixtures have been calculated with Redlich–Rosenfeld–Meyer type equation. The enthalpy and heat capacity parameters of pair interaction of organic electrolytes with EG in FA and with FA in EG have been computed and discussed. The enthalpy interaction parameters of single ions with EG in FA medium have been evaluated and compared with those for ion–water and ion–MeOH interaction in FA. The standard heat capacities of solution have been evaluated. The excess enthalpies of solution, Δ_sol H ^E, of Et₄NBr, Bu₄NBr, and Hex₄NBr have been determined. The Δ_sol H ^E values are positive for Et₄NBr and negative for Bu₄NBr and Hex₄NBr and become more negative from Bu₄NBr to Hex₄NBr.     

Low-temperature heat capacities and standard molar enthalpy of formation of the complex

Low-temperature heat capacities of a solid complex Zn(Val)SO₄·H₂O(s) were measured by a precision automated adiabatic calorimeter over the temperature range between 78 and 373 K. The initial dehydration temperature of the coordination compound was determined to be, T _D=327.05 K, by analysis of the heat-capacity curve. The experimental values of molar heat capacities were fitted to a polynomial equation of heat capacities (C _p,m) with the reduced temperatures (x), [x=f (T)], by least square method. The polynomial fitted values of the molar heat capacities and fundamental thermodynamic functions of the complex relative to the standard reference temperature 298.15 K were given with the interval of 5 K. Enthalpies of dissolution of the [ZnSO₄·7H₂O(s)+Val(s)] (Δ_sol H _m,l ⁰) and the Zn(Val)SO₄·H₂O(s) (Δ_sol H _m,2 ⁰) in 100.00 mL of 2 mol dm^–3 HCl(aq) at T=298.15 K were determined to be, Δ_sol H _m,l ⁰=(94.588±0.025) kJ mol^–1 and Δ_sol H _m,2 ⁰=–(46.118±0.055) kJ mol^–1, by means of a homemade isoperibol solution–reaction calorimeter. The standard molar enthalpy of formation of the compound was determined as: Δ_f H _m ⁰ (Zn(Val)SO₄·H₂O(s), 298.15 K)=–(1850.97±1.92) kJ mol^–1, from the enthalpies of dissolution and other auxiliary thermodynamic data through a Hess thermochemical cycle. Furthermore, the reliability of the Hess thermochemical cycle was verified by comparing UV/Vis spectra and the refractive indexes of solution A (from dissolution of the [ZnSO₄·7H₂O(s)+Val(s)] mixture in 2 mol dm^–3 hydrochloric acid) and solution A’ (from dissolution of the complex Zn(Val)SO₄·H₂O(s) in 2 mol dm^–3 hydrochloric acid).     

Thermochemical, volumetric and spectroscopic properties of lysozyme–poly(ethylene) glycol system

Investigations of lysozyme–polyethylene glycol system were made by differential scanning calorimetry, fluorescence and density techniques. The values of unfolding enthalpies, ΔH_N^U, unfolding temperatures, T_m, excess molar heat capacities, ΔC_p, and apparent molar volumes, V_Φ , were determined as functions of PEG concentration. The three PEGs of average molecular mass (MW) 6000, 10000, 20000 were used as macromolecular crowding agents. The concentration of polymers was changed in the range 0–30% mass per volume (w/v). The values of ΔH_N^U remained constant with no dependence on PEG concentration, while PEG addition to buffered lysozyme solutions caused linear decrease of T_m. The values of ΔC_p and V_Φ of lysozyme dramatically changed in the range of 8–10% of PEG concentration. The fluorescence spectroscopy was used in order to investigate the polymer influence on possible solvent–lysozyme interactions. The electrical properties of polymer–water and polymer–buffer systems, the dielectric constants of solutions were determined with use of impedance spectroscopy.     

Determination of the Enthalpies of Vaporization of Cyclic Organic Peroxides by Correlation of Changes in Gas Chromatographic Retention Times

Liquid and solid cyclic peroxides derived from aliphatic ketones are explosive materials so their enthalpies of vaporization and other thermodynamic or condensed-phase properties cannot be measured directly. In this work the enthalpies of vaporization of peroxides at 298.15 K were estimated simply from gas chromatographic retention times measured at different temperatures. The technique correlates changes in the retention times of compounds whose enthalpies of vaporization are known (called the reference series), with those of the compounds of interest. If t _R′ is the adjusted retention time (retention time of each compound minus the retention time of unretained diethyl ether, used as solvent) a plot of ln t _R′ against 1/T for each compound (reference compounds and cyclic peroxides) results in a straight line (r ² > 0.99 for all compounds). The enthalpy of transfer from solution to the vapor state (Δ_sol^g H _m) can be obtained by multiplying the slope by the gas constant (R). A second plot correlates the enthalpies of transfer from solution to the vapor state (Δ_sol^g H _m), as measured by gas–liquid chromatography (GLC), with enthalpies of vaporization of reference materials (Δ_vap H _m at 298.15 K) available in the literature. C₉–C₁₅ fatty acid methyl esters and hydrocarbons were used as reference compounds. The enthalpies of vaporization of the cyclic organic peroxides were calculated from the equation of the line obtained in this second correlation, the slope of which was Δ_vap H _m (at 298.15 K)/Δ^g _sol H _m. The experiments were performed under isothermal conditions with a DB-5 capillary column, flame-ionization detection (FID), and nitrogen as carrier gas. The column temperature was varied over a range of at least 30–70 K between 403 and 473 K, with chromatograms being acquired at 10 K intervals. Enthalpies of vaporization of cyclic organic peroxides are not available in the literature, and the values given in this paper, obtained by gas chromatography, are the first to be reported.