A. 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.

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.

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.

Molar heat capacities and volumes of transfer of cytosine, thymine, caffeine and 1,3-diethylthymine to aqueous solutions of glycyl-glycine and L-α-alanyl-L-α-alanine at 25°C

Densities and specific heat capacities of ternary aqueous systems containing dipeptides (glycyl-glycine or L-α-alanyl-L-α-alanine) and nucleic acid bases (cytosine or thymine) or their alkyl derivatives (1,3-diethylthymine or caffeine) were determined at 25°C by flow calorimetry and flow densimetry. The partial molar volumes and heat capacities of transfer at infinite dilution of the different nucleic acid bases from water to water+dipeptide solutions were obtained therefrom. Except for the case of the transfer of cytosine to aqueous glycyl-glycine solutions where a small positive dependence of the transfer quantities was observed with the dipeptide concentration, the values of the heat capacities of transfer were in general low, positive or negative, depending on the compensation of hydrophobic-hydrophilic interactions between the dipeptide and the base. The volumes of transfer of most of the bases are very small, within the limit of the experimental error.

Thermochemistry of aqueous solutions of alkylated nucleic acid bases: II. Apparent molar relative enthalpies of dilution of solutions containing uracil and thymine derivatives

Apparent molar relative enthalpies of dilution phi(L)of aqueous solutions of a series of alkylated diketopyrimidines: m(1)Ura, m(1,3)(2)Ura, m(1,3)(2)Thy, m(1,3,6)(3)Ura and e(1,3)(2)Thy were measured as a function of concentration of the solutes at three temperatures 298.15, 308.15 and 318.15 K. Dilution proved to be an endothermic process over the whole range of molalities m and temperatures studied for all compounds except the e(1,3)(2)Thy solution, the dilution of which, with the exception of the lowest concentrations (m > 0.2-0.3) was an exothermic process. Partial molar relative enthalpies of dilution -L(2)(m) derived from phi(L)(m) functions were analysed as if they were composed of two additive contributions: an endothermic one -L(2), (m(1)) and an exothermic one 1.2, (m(as)0), owing to the presence in the solutions of a free monomer m(1) or associated species m(as), respectively. Partial molar heat capacities of the solutes, evaluated by differentiation of -L(2)(m) functions in respect to temperature, decreased with the rise of concentration in the order of the tendency of the solutes to stacking association. Changes in heat content and in heat capacity of solutions upon their dilution are interpreted in terms of involvement of hydrophobic hydration and association of the solutes.

Apparent molar heat capacities and volumes of some alkylated derivatives of uracil and adenine in aqueous solution at 25°C

Densities and apparent molar heat capacities of some alkylated derivatives of uracil and adenine: 1-methyluracil, 1,3-dimethyluracil, 1,3-diethylthymine, 5,6-trimethylene-1,3-dimethyluracil, 5,6-tetramethylene-1,3-dimethyluracil, 5,6-pentamethylene-1,3-dimethyluracil, 2,9-dimethyladenine, 2-ethyl-9-methyladenine, 2-propyl-9-methyladenine, 8-ethyl-9-methyladenine, 6,8,9-trimethyladenine and 8-ethyl-6,9-dimethyladenine were determined using flow calorimetry and flow densimetry at 25°C. It was found that the partial molar volumes and heat capacities correlate linearly with the number of substituted methylene groups-CH2-as well as to the number of hydrogen atoms, nH, belonging to the skeleton of the molecule. In the case of alkylated uracils a difference was observed in the values at infinite dilution V2o and Cp2o, depending on the substitution of alkyl and cyclooligomethylene groups.

Calorimetric investigations of calcium phosphate precipitation

The kinetics of the early stages of crystallization of calcium phosphate under different initial solution compositions have been studied by calorimetric and light scattering methods. It is found that the total heat of formation of calcium phosphate is negative and depends on the molar ratio Ca/P and the initial pH of the mixture. At initial pH 7.40 and pH 8.62, there are relatively strong repeated endothermic heat effects. The character of this heat conversion is not clear. The light scattering studies do not rule out the possibility of the existence of Ca4H(PO4)3·½H2O (OCP) crystals in this region at these initial solution pHs rather than Ca5(PO4)3OH (HAP) and CaHPO4·2H2O (DCPD) expected to crystallize at initial higher and lower values of pH, respectively.

Partial Molar Volumes of Alkylated Uracils--Insight into the Solvation Shell? Part II

The solute–solvent interactions in aqueous solutions of alkylated uracils are discussed. The partial molar volume data are interpreted using a new model of interaction of the solute molecule with the solvent. The model is based on the assumption that the density of solvent in the hydration shell depends on the structure and polarity of the solute molecule. The relation among molecular volume, partial molar volume, and volume of the solvation shell is expressed by α parameter, α which is defined as the relative density of the solvation shell. It is found that in compounds with the same number of CH2- groups, the α values depend on substitution on the C or N atoms of the uracil skeleton. The α values also depend to some extent, on screening of the oxygen atoms by methylation of the neighboring atoms of the uracil ring. A correlation is presented between the relative density of solvation shell and polarity (defined as the ratio of the surface of polar groups and atoms exposed to the solvent to the total accessible molecular surface of the molecule) for the compounds studied. It was demonstrated that of the various solute–solvent interactions the dominant role is played by polar interactions.

Apparent molar heat capacities and volumes, van der Waals volumes and accessible surface areas of alkylated derivatives of cytosine and uracil in aqueous solutions at 25°C.

Densities and specific heat capacities of aqueous solutions: 1,3,5,6-tetramethyluracil, 1,6-dimethyl-3-ethyluracil, 1,6-dimethyl-3-propyluracil, 1,6-dimethyl-3-butyluracil, 1,N4-trimethylcytosine, 1,N4-dimethyl-5-ethylcytosine, 1,N4 dimethyl-5-propylcytosine, 1,N4-dimethyl-5-butylcytosine were determined using flow calorimetry and flow densimetry at 25°C. Apparent molar volumes and heat capacities, van der Waals volumes and accessible surface areas were determined. It was stated that for alkylcytosines and alkyluracils partial molar volumes and heat capacities correlate linearly with the number of substituted methylene groups-CH2-as well as with the van der Waals volumes and accessible surface areas of the compounds studied; for cyclooligouracils the cyclization effect was discussed.