W. Zielenkiewicz

The effect of magnesium ions on the precipitation of calcium phosphates.

Abstract The effect of magnesium ions on the precipitation of calcium phosphates has been studied at 30°C by measuring pH, calcium ion activity and the calorimetric heat power as a function of time. The results can be interpreted by magnesium mainly inhibiting the formation of the recently reported new amorphous phase called ACP2 and the growth of OCP. Magnesium does not enter the solid phases to any great extent, nor are the apparent solubility constants significantly affected by the presence of magnesium. The enthalpy of transformation of ACP1 to ACP2 is found to be an endothermic reaction. The enthalpy of the mixing process and the formation of ACP1 or ACP2, calculated per mol both Ca and Mg bound, is comparable to the enthalpy of formation of the CaHPO 4 ion pair. This could indicate that an important contribution to the change in enthalpy is related to the dehydration of calcium and magnesium ions.

Thermochemistry of aqueous solutions of alkylated nucleic acid bases. III. Enthalpies of hydration of uracil, thymine and their derivatives.

Enthalpies of sublimation DeltaH(0)(subl) crystalline uracil, thymine and their methylated derivatives as well as of N,N-diethylthymine were determinated by the quartz-resonator method and mass spectrometry. Enthalpies of solution at infinite dilution DeltaH(0)(sol) in water of aBcylated compounds were obtained calorimetrically. Hence the calculated enthalpies of hydration: DeltaH(0)(hydrsubal) = DeltaH(0)(sol) - DeltaH(0)(subl), were corrected for energies of cavity formation in pure liquid water to yield enthalpies of interaction DeltaH(0)(sint) of the solutes with their hydration shells. For uracil DeltaH(0)(int) = -59.8 kJ mole(-1) was obtained in this way. This value decreased linearly on N-methyl substitution with a mean increment of about 6.5 kJ mole CH2(-1). After C(5) or C(6) ring substitution it increased by about 3 kJ. These results are discussed in connection with heat of dilution data and theoretical schemes of hydration.

Thermochemistry of aqueous solutions of alkylated nucleic acid bases. I. Apparent molar heat capacities of uracil, thymine and their derivatives.

Apparent molar heat capacities phiC(p(1,3)) of uracil, thymine and a series of their alkylated derivatives: m(1)Ura,m(1,3)(2) Ura, m(1,3)(2)Thy, mi(1,3,6)(3)Ura, m(1,3)(2),e(5)Ura and e(1,3)(2)Thy in dilute aqueous solutions were measured in the temperature range of 293.15-388.15 K, using a differential adiabatic scanning microcalorimeter. They were found to lie (i) much higher than the estimated heat capacities C(p)(s) of solid compounds, (ii) comparable with the respective partial molar heat capacities at infinite dilution, C(o)(p2), and (iii) linearly related to the number nH of hydrogen atoms covalenuy bound to the solute molecules. The increment thus obtained DeltaC(o)(p2)=42.8 J mole(-1) K(-1)n(-1)(H) per each hydrogen atom at 298.15 K proved (i) to coincide closely with those found previously for homologous series of aliphatic amides and hydrocarbons, and (ii) to decrease with a rise of temperature. These findings imply the involvement of hydrophobic hydration of the solutes.

Phosphorylated Calixarenes as Receptors of L-Amino Acids and Dipeptides: Calorimetric Determination of Gibbs Energy, Enthalpy and Entropy of Complexation

Thermodynamics of 5,17bis(dihydroxyphosphorylhydroxymethyl)-25,27-dipropoxy-calix[4]arene in pure Racemic form and 1:1 mixture of the Meso and Racemic forms (host molecule) interacting in methanol solution with free amino acids (guest molecule), and additionally with five dipeptides, has been studied using isothermal titration calorimetry. A moderate variation in the changes of enthalpy, entropy and Gibbs free energy, depending on the nature of the guest molecule as well as the stereomeric form of the host molecule, was observed. The stability constants in the range of 3000–17000 M− 1 (25000–45000 M− 1 for dipeptides) and enthalpy changes in the range of − 10– − 2 kJmol− 1 ( − 10.5– − 5.9 for dipeptides) were evaluated experimentally by ITC. The decreased variation in the estimated Gibbs free energy ( − 25– − 20 kJmol− 1 for amino acids, and − 26.5– − 25.3 for dipeptides, respectively) was attributed to the effect of enthalpy-entropy compensation. The complexation phenomenon was found driven by electrostatic interactions between protonated N-terminal amino group of the guest and calixarene phosphoryl groups. The complex stability correlates with the hydrophobicity of amino acid residues, indicating significant partition of the solvatophobic interactions.

Excess enthalpies and apparent molar volumes of aqueous solutions of crown ethers and cryptand(222) at 25°C

The enthalpies of dilution and densities of aqueous solutions of 12-crown-4, 15-crown-5, 18-crown-6, 1,10-diaza-18-crown-6 and cryptand (222) were measured at 25°C. The excess enthalpies and enthalpic coefficients of solute-solute interactions were calculated by the McMillan-Mayer theory formalism. Values for the apparent molar volumes at infinite dilution were determined by extrapolation. The contributions of the-CH2CH2O-group to values of h2 and to the limiting partial molar volume were calculated for the series of crown ethers studied. It is concluded that the hydrophobic hydration and the hydrophobic solute-solute interaction are predominant in the solutions investigated.

Physico-chemical properties of peptides and their solutions

Abstract Complex investigations of a series of dipeptides and their solutions in H 2 O were conducted by the methods of differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), densimetry, and IR and Raman spectroscopy. The results of investigations by DSC and TGA show that the differences in the spatial structure of solid dipeptides of a homologous series sharply influences the character of phase transformations. Linear correlations between the values of the heat capacity of crystalline amino acids and peptides and the number of atoms, number and length of interatomic bonds. The dipeptides studied are classified into three groups: those with predominantly hydrophilic, those with predominantly hydrophobic and those with hydrophobic-hydrophilic (at different concentrations) characters of hydration in aqueous solutions. The ratio between hydrophilic and hydrophobic contributions to the hydration of dipeptides, and role of separate fragments of the molecules is considered.

Thermochemistry of aqueous solutions of alkylated nucleic acid bases: IV. Enthalpies of hydration of 5-alkyluracils

Enthalpies of sublimation, DeltaH degrees (subl) and of solution in water, DeltaH degrees (sol) were determined for a series of crystalline 1,3-dimethyl-uracil derivatives substituted at the C5-ring carbon atom with alkyl groups (-C(n)H(2n+1), n = 2-4) and some of their C(5.6)-cyclooligomethylene analogues (-(CH2)(n)-, n = 3-5). From these data. enthalpies of hydration DeltaH degrees (hydr)= DeltaH degrees (sol) - DeltaH degrees (subl) were calculated and corrected for energies of cavity formation in pure liquid water in order to obtain enthalpies of interaction, DeltaH degrees (int) of the solutes with their hydration shells. The latter are discussed together with the recalculated DeltaH degrees (int) for variously methylated uracils, obtained previously according to a simplified correction procedure, in terms of perturbations in the energy and scheme of hydration of the diketopyrimidine ring brought about by alkyl substitution. It was found that each -CH2-group added with an alkyl substitution contributes favorably about -20 kJ mol(-1) toDeltaH degrees (int). This contribution is partially cancelled by the unfavorable contribution to DeltaH degrees (int) connected with removal of some water molecules bound in the first and subsequent hydration layers by an alkyl substituent. This is particularly evident on substitution at the polar side of the diketopyrimidine ring on which water molecules are expected to be bound specifically.

Calorimetry for thermokinetic determinations

The possibilities of using direct calorimetric measurements for the determination of kinetic parameters of processes are discussed in terms of the choice of an appropriate method of reproduction of the course of changes in the heat power generated in time, i.e. thermokinetics. Such methods are presented and it is concluded that many of these methods permit reproduction of the thermokinetics in the same way as if the experimental thermal curve were obtained in a calorimetric system characterized by a time constant 100–200 times as large as that for the thermal inertia of this calorimetric system.ZusammenfassungDie Möglichkeiten zur Benutzung direkter kalorimetrischer Messungen zur Bestimmung kinetischer Parameter von Prozessen werden hinsichtlich der Wahl geeigneter Methoden zur Reproduzierung des Verlaufs der Änderungen der Wärmekraft mit der Zeit, d.h. zur Reproduzierung der Thermokinetik diskutiert. Solche Methoden werden angeführt und es wird die Schlussfolgerung gezogen, dass viele dieser Methoden die Reproduzierung der Thermokinetik auf gleiche Weise wie in den Fällen erlauben, in denen die thermische Kurve mit einem kalorimetrischen System erhalten wird, dessen Zeitkonstante um 100–200mal grösser als die des diskutierten Systems ist.РезюмеВозможности использ ования прямых калориметрических и змерений обсуждены в плане выбора соотве тствующего метода воспроизводимости х ода изменения выделя ющегося тепла во времени т. е. те рмокинетики. Предста влены эти методы и сделан вы вод, что многие из этих методо в позволяют воспроиз вести термокинетику тем же самым путем, как если бы экспериме нтальные термически е кривые были получены в калор иметрической системе, характеризу ющейся постоянной вр емени в 100–200 раз выше по сравнению с термической инерци онностью такой калориметрической с истемы.

Thermodynamic and structural study of tolfenamic acid polymorphs.

The article deals with the study of two polymorphic modifications in the space groups P2(1)/c (white form) and P2(1)/n (yellow form) of the tolfenamic acid. It also describes how the white form vapor pressure temperature dependence was determined by using the transpiration method and how thermodynamic parameters of the sublimation process were calculated. We have estimated the difference between the crystal lattice energies of the two polymorphic forms by solution calorimetry and found that the crystal lattice energy of the yellow form is 6.7+/-1.2 kJ mol(-1) higher than that of the white form, whereas Gibbs free energies of the forms obtained from the vapor pressure temperature dependence are practically the same. The modifications under consideration are monotropically related. From the practical point of view, the white form is more preferable due to its lower crystal lattice energy and better performing procedure. We have also studied the solubility, solvation and transfer processes of the tolfenamic acid white form in buffers (with various values of pH and ionic strengths), n-hexane and n-octanol. The thermodynamic parameters of the investigated processes have been discussed and compared with those determined for others fenamates. In the study we estimated specific and non-specific contributions of the solvation enthalpic term of the fenamate molecules with the solvents as well. The driving forces of the transfer processes from the buffers with pH 7.4 and different ionic strengths to n-octanol were analyzed. It was found that the relationship between the enthalpic and entropic terms depends essentially on the ionic strength. For the considered fenamates the transfer processes of the neutral molecules and the ionic forms are enthalpy-determined, whereas for the niflumic acid this process is entropy-determined.

Partial Molar Volumes of Aqueous Solutions of Some Halo and Amino Derivatives of Uracil

Apparent molar volumes were experimentally determined at 25, 35, and 45°Cfor aqueous solutions of uracil, 5-fluorouracil, 5-chlorouracil, 6-chlorouracil,5-bromouracil, 5-iodouracil, 5-aminouracil, 6-aminouracil,6-amino-1,3-dimethyluracil, and 5-nitrouracil. The solubility of each compound in water was measuredat 25°C. Partial molar volumes and structural parameters of the moleculesinvestigated were determined. The relations between these data and solubility andpolarity were found and discussed. The partial molar volumes for halo derivativesand 6-amino-1,3-dimethyluracil increase, with increasing temperature whereasthose for 5-aminouracil, 6-aminouracil, and 5-nitrouracil remain constant withinthe limit of experimental error.