Thermodynamic investigations on derivatives of pyrimidine nucleic acid bases: Joint use of calorimetric, volumetric and structural data for the description of properties of pyrimidine nucleic acid bases and their derivatives

The experimental enthalpies of solution Δ_solH_m^∞, van’t Hoff enthalpies of sublimation Δ_s^gH_m⁰ of solid compounds, partial molar volumes V₂⁰, and partial molar heat capacities C_p,2⁰ of aqueous solutions of pyrimidine nucleic acid bases and their derivatives, determined previously and collected here, are discussed in terms of calculated structural parameters. Relations have been established between the calorimetric and volumetric properties. Correlations have been developed to relate both the enthalpies of solvation and the partial molar heat capacities to the polar and apolar parts of the accessible molecular surface areas.     

Thermochemistry of some cytisine derivatives

The enthalpies of solution of cytisine derivatives (phosphoric acid cytiside dimethyl ester, cytisinovinyloxyethylaminothiourea) in 96% ethanol at various dilutions were determined by isothermal calorimetry. Equations describing the dependences ΔH _s ⁰ = f(m ^1/2) (m is the molal concentration) were obtained. The standard enthalpies of solution, combustion, melting, and formation (at 298.15 K) of the compounds were calculated.     

Saturation molalities and standard molar enthalpies of solution of cytidine(cr), hypoxanthine(cr), thymidine(cr), thymine(cr), uridine(cr), and xanthine(cr) in H2O(l)

Saturation molalities m(sat) in H₂O(l) have been measured for the substances cytidine(cr), hypoxanthine(cr), thymidine(cr), thymine(cr), uridine(cr), and xanthine(cr) by using h.p.l.c. The states of hydration were established by performing Karl-Fischer analyses on samples of these substances, which had been allowed to equilibrate with their respective aqueous saturated solutions for several days at T≈298 K and then dried with air at T≈296 K for ≈24 h. The crystalline forms of the substances were identified by comparison of the results of X-ray diffraction measurements with results from the literature. Also, molar enthalpies of solution Δ_solH_m(cal) for these substances were measured by using an isoperibol solution calorimeter. A self-association (stacking) model was used to estimate values of the activity coefficients γ and relative apparent molar enthalpies L_φ for these substances. These γ and L_φ values were used to adjust the measured values of m(sat) and Δ_solH_m(cal) to the standard state and thus obtain values of the standard molar Gibbs free energy Δ_solG^∘_m and enthalpy changes Δ_solH_m^∘ for the dissolution reactions of these substances. The values of the pKs and of the standard molar enthalpies of the ionization reactions were also used to account for speciation of the substances in the calculations of Δ_solG_m^∘ and Δ_solH_m^∘. Values of standard molar enthalpies of formation Δ_fH_m^∘, standard molar Gibbs free energies of formation Δ_fG_m^∘, and standard partial molar entropies S_2,m^∘ for the aqueous species of hypoxanthine and xanthine were calculated. A detailed summary and comparison of thermodynamic results from the literature for these substances is presented.     

Measuring the enthalpies of interaction between glycine,L-cysteine,glycylglycine, and sodium dodecyl sulfate in aqueous solutions

Calorimetric measurements of enthalpies of solution Δ_sol H ^m for glycine, L-cysteine, and glycylglycine in aqueous solutions of sodium dodecyl sulfate (SDS) with concentrations of up to 0.05 mol kg^–1 are made. Standard enthalpy of solution Δ_sol H ⁰ and enthalpy of transfer Δ_tr H ⁰ of the dipeptide from water into mixed solvent are calculated. The calculated enthalpy coefficients of paired interactions of amino acids and dipeptide with SDS prove to be positive. Hydrophobic interactions between the biomolecules and SDS are found to have a major impact on the enthalpies of interaction in the three-component systems under study, within the indicated range of concentrations.     

Ein isoperiboles lösungskalorimeter zur messung von lösungsenthalpien der doppelchloride des magnesiums

For measuring solution enthalpies of the strong hygroscopic double chlorides, an isoperibol solution calorimeter was built. Samples of 2–5 g could be solved to a molar dilution 1 : 3000. The temperature difference between reaction vessel and thermostat was measured by a thermopile; the temperature of the thermostat was constant to 2 · 10^?4°C. From the molar enthalpies of solution (ΔH^L), enthalpies for the reactions nACl + MCl₂ = A_sMCl(_{s + 2}) were calculated: ΔHP^R = ? ΔH^L (double chloride) + ΔH^L(n Cl) + ΔH^L(MCL₂) These values are relatively small: about ? 50 kJ for the Cs-compounds, nearly zero for the K- and Na-compounds.     

Thermochemical investigations of some electrolytes in water-acetamide mixtures at 298.15 K

The enthalpies of solution of NaCl, KCl, KBr, KNO₃ and CsI have been measured in mixtures of acetamide with water at 298.15 K in the miscibility range. The standard enthalpies of solutions (ΔH⁰) of the investigated electrolytes have been evaluated The corresponding enthalpies of solvation (ΔH_s⁰) and enthalples of transfer (ΔH_t⁰) from water to water-acetamide mixtures have been calculated The dependence of the standard enthalpies of solution and enthalpies of transfer of the investigated electrolytes on the solvent composition is discussed     

Relationships among strain energies of mono- and poly-cyclic cyclohexanoid molecules and strain of their component rings

The standard molar enthalpies of vaporizationΔ_l^gH_m^o of N -(hydroxyethyl)-piperazine, bis-piperidino-methane, 1,3,5-tri-methyl-hexahydro-s-triazine, and also the enthalpies of sublimation Δ_cr^gH_m^oofN,N^′ -di(hydroxyethyl)-piperazine, and hexamethylenetetramine were obtained from the temperature dependence of the vapor pressure measured in a flow system. The standard molar enthalpies of formationΔ_fH_m^o (l) at the temperatureT =  298.15 K were measured by means combustion calorimetry for bis-piperidino-methane and 1,3,5-tri-methyl-hexahydro-s-triazine. Strain enthalpies (ring-correction terms) of these cyclohexanoid molecules were derived from the experimental enthalpies of formation of the gaseous compounds. These experimental results, together with data available from the literature, provided a quantitative understanding of the interrelations of structure and energetics of a mono- and poly-cyclic cyclohexanoid molecules and strain of their component rings.     

Thermodynamic properties of lanthanum and lanthanide halides: IV. Enthalpies of atomization of LnCl,LnCl<Superscript>+</Superscript>, LnF,LnF<Superscript>+</Superscript>, and LnF<Subscript>2</Subscript>

The results of measurement of equilibrium constants of 30 reactions involving lanthanum and lanthanide fluorides (LnF, LnF₂, and LnF₃) and 14 reactions involving lanthanum and lanthanide monochlorides (Ln = La-Lu) have been summarized. These constants have been used for calculating the enthalpies of reactions by the second and third laws, from which the enthalpies of atomization Δ_at H ₀ ⁰ of LnCl, LnF, and LnF₂ have been determined. Comparison of the calculation results shows that the thermodynamic functions of LnCl and LnF (Ln = Ce-Yb) in which the electronic excitation contribution has been calculated from the excitation energies of Ln⁺ ions allow one to adequately determined the Δ_at H ₀ ⁰ values from experimental data. Using the trends in the change in Δ_at H ₀ ⁰ as a function of the atomic number of a lanthanide, the enthalpies of atomization of compounds for which experimental data are lacking have been estimated. The Δ_at H ₀ ⁰ values for LnCl⁺ ions have been calculated. The reliability of the Δ_at H ₀ ⁰ values for LnF⁺ ions have been assessed.     

Coordination Reaction of Alanine with Neodymium(III) and Erbium(III) Nitrate. Thermochemical study

The solid-state coordination reaction: Nd(NO₃)₃·6H₂O(s)+4Ala(s) → Nd(Ala)₄(NO₃)₃·H₂O(s)+5H₂O(_l) and Er(NO₃)₃·6H₂O(s)+4Ala(s) → Er(Ala)₄(NO₃)₃·H₂O(s)+5H₂O(l) have been studied by classical solution calorimetry. The molar dissolution enthalpies of the reactants and the products in 2 mol L^–1 HCl solvent of these two solid-solid coordination reactions have been measured using a calorimeter. From the results and other auxiliary quantities, the standard molar formation enthalpies of [Nd(Ala)₄(NO₃)₃·H₂O, s, 298.2 K] and[Er(Ala)₄(NO₃)₃·H₂O, s,298.2 K] at 298.2 K have been determined to be Δ_f H _m ⁰ [Nd(Ala)₄(NO₃)₃·H₂O,s, 298.2 K]=–3867.2 kJ mol^–1, and Δ_f H _m ⁰ [Er(Ala)₄(NO₃)₃·H₂O, s, 298.2 K]=–3821.5 kJ mol^–1. This revised version was published online in August 2006 with corrections to the Cover Date.     

Enthalpy characteristics of solution of L-cysteine and L-asparagine in water-alcohol mixtures at 298.15 K

The integral enthalpies of solution Δ_sol H ^m of L-cysteine and L-asparagine in mixtures of water with ethanol, n-propanol, and isopropanol at a mole fraction of alcohol of up to 0.32 were determined by calorimetry of solution. The standard enthalpies of solution (Δ_sol H ⁰) of L-serine and of its transfer (Δ_tr H ⁰) from water to a mixed solvent were calculated. The dependences of Δ_sol H ⁰ and Δ_tr H ⁰ on the composition of water-alcohol mixtures pass through a maximum. The calculated enthalpy coefficients of pair interaction of amino acids with alcohol molecules are positive and increase in the order ethanol, n-propanol, isopropanol. The data obtained were interpreted from the viewpoint of various types of interaction in solution and effect of the amino acid residue on the thermochemical characteristics of solution.     

Solute–solvent interactions of oligo(m-benzamide)s in N,N-dimethylacetamide

Studies on solute–solvent interactions of oligo(m-benzamide)s in N,N-dimethylacetamide (DMA) have been carried out. The enthalpies of solution have been measured for oligo(m-benzamide)s and oligo(m-phenylene)s in DMA and benzene. Contributions of enthalpies of cavitation and dispersion interaction to the enthalpy of transfer from benzene to DMA, ΔH_tr (Ben → DMA), have been examined for oligo(m-phenylene)s. A considerable contribution of excess enthalpy, ΔH_E (Ben → DMA), to ΔH_tr (Ben → DMA) has been found, which increases with the number of benzene rings of the solute. By assuming that ΔH_E (Ben → DMA) of diphenyl (DP) is equal to that of benzanilide (BA) in DMA, the amide hydrogen bond enthalpy of BA in DMA, ΔH estimated by “the pure base method” corrected for the enthalpies of cavitation and dispersion interaction. The ΔH value has been given by the following expression including the unknown solubility parameter of BA, δ^BA: The evaluation of δ^BA has resulted in the conclusion that ?ΔH is smaller than 10.9 kJ mol^?1. Moreover, ΔH_tr (TMU → DMA) for oligo(m-benzamide)s has been examined. It has been shown that the amide hydrogen bonding ability of DMA is lower than that of TMU. The linearity of the plot of ΔH_tr (TMU → DMA) against the number of amide bonds in the molecule has been explained by the increase in hydrogen bond enthalpies with the number of amide bonds in the molecule.     

Anomalies in the enthalpy of freezing in alpha-phase indium—lead,due to brillouin-zone effects

The enthalpy of freezing has been measured with DTA using electrical calibration for 33 compositions of indium—lead in the range 6.9–7.9 at.% lead, at which range the electron—lattice interaction causes anomalies in several physical and structural properties. Enthalpies of mixing, ΔH_m^s, have been calculated. The results reveal three maximum and minimum pairs corresponding to partial shift of the Fermi surface to higher Brillouin zones in three steps at 7.1, 7.2 and 7.3–7.4 at.% lead. These details in the overlapping have not been found earlier. The observed change in ΔH_m^s in the steps is about 1 meV atoms^?1.     

Saturation molalities and standard molar enthalpies of solution of adenosine(cr), guanosine · 2H2O(cr), inosine(cr), and xanthosine · 2H2O(cr) in H2O(l)

Saturation molalities m(sat) in H₂O(l) have been measured for the substances adenosine(cr), guanosine · 2H₂O(cr), inosine(cr), and xanthosine · 2H₂O(cr) over the temperature range T=287.91 K to T=308.18 K by using h.p.l.c. The indicated states of hydration of these substances were established by performing Karl–Fischer analyses of samples of these substances which had been equilibrated over H₂O(l) and of samples obtained by passing air over the wet crystals (air dried samples). The crystalline phases of these substances were identified by comparison of the results of X-ray diffraction measurements with results from the literature. Molar enthalpies of solution Δ_solH_m for adenosine(cr) and inosine(cr) were measured by using an isoperibol solution calorimeter. A “stacking” or “self-association” model was used to estimate values of the activity coefficients γ and relative apparent molar enthalpies L_φ for these substances. These γ and L_φ values were used to adjust the measured values of m(sat) and Δ_solH_m to the standard state and obtain values of the standard molar Gibbs free energy and enthalpy changes Δ_solG^∘_m and Δ_solH^∘_m, respectively, for the dissolution reactions of these substances. Values of Δ_solH^∘_m calculated from the temperature dependence of values of Δ_solG^∘_m were in good agreement with the values of Δ_solH^∘_m obtained by using calorimetry.     

Thermochemical investigations of the systems KCl−KBr−H2O,K2SO4−(NH4)2SO4−H2O and KNO3−NH4NO3−H2O at 298.15 K

For the equilibrium solid phases occurring in the systems: KCl?KBr?H₂O, K₂SO₄?(NH₄)₂SO₄?H₂O and KNO₃?NH₄NO₃?H₂O, the concentration dependencies of differential solution enthalpies, Δ_sol H ₂ for several crystallization paths, were measured. The limiting differential solution enthalpies, Δ_sol H ₂ ⁰ , were determined by extrapolation of the above dependencies to the ionic strength,I _m ⁰ , corresponding to the appropriate binary solutions. For KCl?KBr?H₂O system only, the clear dependence between Δ_sol H ₂ ⁰ andI _m ⁰ values was found and discussed.     

Thermodynamic study of 2-methyl-tetrahydrofuran with isomeric chlorobutanes

Excess molar volumes, V^E, isentropic compressibility deviations, Δκ_S, and excess molar enthalpies, H^E, for the binary mixtures 2-methyl-tetrahydrofuran with 1-chlorobutane, 2-chlorobutane, 2-methyl-1-chloropropane and 2-methyl-2-chloropropane have been determined at temperatures 298.15 and 313.15 K, excess molar enthalpies were only measured at 298.15 K. We have applied the Prigogine-Flory-Patterson (PFP) theory to these mixtures at 298.15 K.     

Enthalpies of transfer of amino acids from water to aqueous solutions of sodium nitrate and sodium perchlorate at T = 298.15 K

Enthalpies of solution of glycine, l-alanine and l-serine in water and aqueous solutions of NaNO₃ and NaClO₄ have been determined at T = 298.15 K with a calorimeter. Enthalpies of transfer (Δ_trH) from water to aqueous solutions of salts were derived and interpreted in terms of electrostatic interaction and structural interaction. Δ_trH decreases with increasing salt concentration in the composition range studied. The transfer enthalpies of amino acids from water to NaNO₃ solution and low concentration NaClO₄ solution vary in the sequence l-serine < glycine < l-alanine while glycine < l-serine < l-alanine in NaClO₄ solution above 2 mol kg⁻¹. The difference may be due to ion association at high concentration, weakening the interaction with l-serine more than with glycine.     

Simultaneous determination of enthalpies of dilution and dissociation of protocatechuic acid in aqueous salt solutions by calorimetric measurements

Protocatechuic acid, a major metabolite of antioxidant polyphenols found in green tea, has been shown to prevent carcinogenesis or antitumor growth in vitro and in vivo studies. The dilution enthalpies Δ_dilH_m, dissociation enthalpies Δ_disH_m and thermodynamic dissociation constants K_a for protocatechuic acid in aqueous NaCl or KCl solutions were simultaneously determined by mixing-flow microcalorimetry at T = 298.15 K. In order to verify the reliability of the fitted dissociation parameters, the values of dissociation enthalpies and thermodynamic dissociation constants were also determined by isothermal titration calorimetry and electrical conductivity methods. The K_a values obtained through the proposed method agree well with those reported in literatures and obtained by other techniques. Enthalpic interaction coefficients (h₂, h₃ and h₄) were computed according to the McMillan–Mayer model. The trends of h₂ and Δ_disH_m for protocatechuic acid with increasing salt molality in both the salt solutions were obtained. The different influence of KCl and NaCl on the values of h₂ and Δ_disH_m were discussed in terms of (solute + solute) and (solute + solvent) interactions. The results showed that it is possible to perform in a single calorimetric experiment the simultaneous determination of dilution enthalpies, dissociation enthalpies and constants in a given solvent.     

Determination of combustion energies and the standard enthalpies of formation for the complexes of RE(Et<Subscript>2</Subscript>dtc)<Subscript>3</Subscript>(phen)

Solid complexes, RE(Et₂dtc)₃(phen) (RE=La, Pr, Nd, Sm—Lu), were synthesized with sodium diethyldithiocarbamate (NaEt₂dtc3H₂O), 1,10-phenanthroline (o-phen?H₂O) and hydrated lanthanide chlorides in absolute ethanol. The constant-volume combustion energies of complexes, Δ _C U, were determined by a precise rotating-bomb calorimeter at 298.15 K. The standard enthalpies of combustion, Δ_CH_m ^θ, and standard enthalpies of formation, Δ_fH_m ^θ, were calculated for these complexes, respectively. The experiment results showed the “tripartite effect” of rare earth.     

Estimation of physico-chemical properties of ionic liquid [C7mim][BF4]

Ionic liquid (IL) [C₇mim][BF₄] (1-heptyl-3-methyl-imidazolium tetrafluoroborate) was prepared and characterized. The density and surface tension of the IL were determined in the temperature range of 293.15–343.15 K. In terms of Glasser's theory, the standard molar entropy and lattice energy of the IL were estimated. Using Kabo's method, the molar enthalpy of vaporization of the IL, Δ_l^gH_m⁰ (298 K), was estimated. According to the interstice model, the thermal expansion coefficient of IL, α, was calculated and in comparison with experimental value, they are within one order of magnitude.     

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.