Thermochemistry of aqueous solutions of alkylated nucleic acid bases V. Enthalpies of hydration of N-methylated adenines

Enthalpies of solution in water, H _sol ^o , and of sublimation, H _subl ^o , were determined experimentally for a number of crystalline N-methyl adenines: m⁶Ade, m ₂ ^6,6 Ade, m⁹Ade, m ₂ ^6,9 Ade, and m ₃ ^6,6,9 Ade. Derived standard enthalpies of hydration H _hydr ^o , were corrected for the calculated cavity terms H _cav ^o to yield enthalpies of interaction H _int ^o of the solutes with their hydration shells. The increments of H _int ^o per unit area of the water-accessible molecular surface S _B , H _int ^o (CH₃)/S _B (CH₃), for the particular methyl groups: is considered to be the net effect of the gain in the energy resulting from van der Waals' interactions and of the loss in the energy due to polar interactions upon methyl substitution. It proved to vary somewhat numerically in agreement with the theoretically predicted hydration schemes of adenine. Comparison of H _int ^o /S _B value for adenine with those previously determined for uracil and thymine indicates that the aminopurine moiety is less hydrated than the diketopyrimidine ring.     

Strain Energies of α-Alkylsubstituted Styrenes, 1,1-Di-phenyl-ethene,Tri-, and Tetra-phenyl-ethene

The standard (p° = 0.1 MPa) molar enthalpies of formation _fH°_m (1 or cr) at the temperature T = 298.15 K were determined by using combustion calorimetry for -ethyl-styrene (A), -iso-propyl-styrene (B), -tert-butyl-styrene (C), 1,1-di-phenyl-ethene (D), tri-phenyl-ethene (E), and tetra-phenyl-ethene (F). The standard molar enthalpies of vaporization _l ^g H°_m or sublimation _cr ^g H°_m of compounds A to F were obtained from the temperature variation of the vapor pressure measured in a flow system. Molar enthalpies of fusion _cr ^l H°_m of solid compounds were measured by d.s.c. Resulting values of _fH°_m (g) were obtained at the temperature T = 298.15 K and used to derive strain enthalpies of phenylalkenes. The interactions of the substituents are discussed in terms of deviations of _fH°_m (g)from the group additivity rules. These values provide a further improvement on the group-contribution methodology for estimation of the thermodynamic properties of organic compounds.     

Thermochemistry of aqueous solutions of alkylated nucleic acid bases. IX. Enthalpies of hydration of 9-methyl-8-alkyladenines and 6,9-dimethyl-8-alkyladenines

Enthalpies of solution in water, H _sol ^o , and enthalpies of sublimation, H _sub ^o , were determined experimentally for a number of crystalline derivatives of adenine: 6,8,9-trimethyladenine; 6,9-dimethyl-8-ethyladenine; 6,9-dimethyl-8-propyladenine; 6,9-dimethyl-8-butyladenine; 8,9-dimethyl-adenine and 9-methyl-8-ethyladenine. Standard enthalpies of hydration, H _hydr ^o , derived from these data were calculated. The latter were discussed together with the values for variously alkylated adenines, determined previously. The data obtained show that the dependence of enthalpy of hydration on the number of methylene groups added upon substitution with 8-n-alkyl groups of 9-methyladenine and 6,9-dimethyladenine is nonlinear.     

Model calculations of changes of thermodynamic variables for the transfer of nonpolar solutes from water to water-d2

A recently introduced modified hydration shell hydrogen bond model for rationalizing the thermodynamic consequences of hydrophobic hydration is adapted for use with heavy water. The required adjustment of parameters employs the assumption that breaking hydrogen bonds in water-d₂ involves a greater enthalpy change and a larger entropy increase than bond breaking in ordinary water. It also makes some use of information derived from studies of gas solubilities in the two solvents, although a review of the data leads to serious questions about the reliability of results obtained in this way. The model permits calculations of hydrogen bonding contributions to the changes, G _t ^o , H _t ^o , S _t ^o , and C _p,t ^o , for transfer of nonpolar solutes from water to water-d₂ and implies that such data should show regular trends. Although some of the numerical results depend strongly on the values chosen for the parameters, the pattern defined by these trends is nearly independent of parameters. Predicted values of C _p,t ^o are large and positive for all nonpolar solutes, while S _t ^o is expected to be negative near 0°C, becoming progressively less negative on warming and eventually positive. Both of these quantities should be proportional to the molecular surface area of the solute. Analogous predictions regarding G _t ^o and H _t ^o can also be made, but only if it is permissible to neglect possible contributions to these quantities from van der Waals interactions.     

Thermodynamic quantities for the protonation of amino acid amino groups from 323.15 to 398.15 K

Flow claorimetry has been used to study the interaction of protons with glycine, DL--alanine, -alanine, DL-2-aminobutyric acid, 4-aminobutyric acid, and 6-aminocaproic acid in aqueous solutions at temperatures from 323.15 to 398.15 K. By combining the measured heats for amino acid solutions titrated with NaOH solutions with the heat of ionization for water, the log K, H^o, S^o, and C_p ^o values for the protonation of the amino groups of these amino acids have been obtained at each temperature studied. Equations are given expressing these values as functions of temperature. The H^o and S^o values increase while log K values decrease as temperacture increases. The trends for log K, H^o, S^o, and C_p ^o are discussed in terms of changes in long-range and short-range solvent effects. The trend in H^o, S^o, and C_p ^o values with temperature and with charge separation in the zwitterions is interpreted in terms of solvent-solute interactions and the electrostatic interaction between the two oppositely charged groups within the molecule.     

Enthalpy of solution of carbohydrates using a modified differential scanning calorimeter

A differential scanning calorimeter (DSC) was modified for the determination of enthalpies of solution. The measurements were performed on aqueous solutions of the deoxy- and fluoro-deoxy derivatives of D-glucopyranose (Glu) where the OH group on the C1, C2, C3, and C6 is replaced by H (1HGlu, 2HGlu, 3HGlu, and 6HGlu) and by F (1FGlu, 2FGlu, 3FGlu, and 6FGlu), 4-deoxy 4-fluoro--D-glucopyranoside (4FGlu), 1-methoxy--D-glucopyranoside (MeOGlu), 1-phenoxy--D-glucopyranoside (PheOGlu), D-mannopyranose (Man), and 3-methoxy--D-glucopyranoside (3MeOGlu), at 15.1, 25.0, 35.0, and 45.1°C. The enthalpies of solution _sH⁰(T) ranged from 1.00±0.25 kJ-mol^–1 for 6HGlu at 15.1°C to 20.4±1.4 for PhOGlu at 45.1°C and were in good agreement with literature values for Man, Glu, MeOGlu, and 3MeOGlu at 25.0 and 35.0°C and for MeOMan and 2HGlu at 35.0°C. _sH⁰(T) for the derivatives were then extrapolated up to the melting temperature T_m and compared with their enthalpies of fusion, _fH also determined from DSC measurements. If the agreement between _sH⁰(T_m) and _fH was within the 95% confidence level, then it was concluded that intermolecular interactions between the carbohydrate molecules in the liquid phase were the same as between the carbohydrate and water molecules in the solution phase. This agreement was observed for aqueous solutions of Man, Glu, MeOGlu, 3HGlu, 3FGlu, and 6FGlu.     

Thermodynamics of the weak interaction between samarium cation and xylitol in water: A chemical model

The thermodynamic characterization of the weakly complexed model system Sm³⁺-xylitol has been carried out. The standard Gibbs energy enthalpy, entropy, volume and heat capacity of complexation of Sm³⁺ by xylitol have been determined in water at 25°. The stability constant and the enthalpy change have been simultaneously determined by using a calorimetric method. The thermodynamic properties characterizing solely the specific interaction between the cation and the complexing sequence of hydroxyl groups of the ligand have been isolated. The stability constant and the volume of complexation have also been estimated from a similar treatment of the apparent molar volumes. It was found that the reaction between Sm³⁺ and the complexing site of xylitol in water is characterized by: K = 8.1, _rG^o = –5.2 kJ-mol^–1, _rH^o = –13.7 kJ-mol^–1, T_rS^o = –8.5 kJ-mol^–1, _rV^o = 8.8 cm³-mol^–1 and _rC _p ^o = 51 J-K^–1-mol^–1.     

Thermodynamics of protonation of AMP,ADP, and ATP from 50 to 125°C

The interaction of adenosine 5-monophosphate (AMP), adenosine 5-diphosphate (ADP), and adenosine 5-triphosphate (ATP) ions with protons in aqueous solution has been studied calorimetrically from 50 to 125°C and 1.52 MPa. At each temperature, the reaction of acidic AMP with tetramethylammonium hydroxide was combined with the heat of ionization for water to obtain the enthalpy of protonation of AMP, while the reactions of HCl with deprotonated tetramethylammonium salts of ADP and ATP were used to obtain the enthalpies of protonation of ADP and ATP. Equilibrium constant K, enthalpy change H^o, entropy change S^o, and heat capacity change C _p ^o values were calculated for the stepwise protonation reactions as a function of temperature. The reactions involving the first protonation of AMP, ADP, and ATP and the third protonation of ADP and ATP were endothermic over the temperature range studied, while that involving the second protonation is exothermic for AMP and ADP, but is exothermic below 100°C and endothermic at 125°C in the case of ATP. Consequently, log K values for the first and third protonation reactions (phosphate groups) increase while those for the second protonation reaction (N₁-adenine) decrease in the cases of AMP and ADP and go through a minimum in the case of ATP as temperature increases. The H^o values for all protonation reactions increase with temperature. The magnitude and the trend for the H^o, S^o, and C _p ^o values with temperature are discussed in terms of solvent-solute interactions. The magnitude of the C _p ^o values for the second protonation is consistent with little interaction between the phosphate ion and the protonated N₁ site of the adenine moiety in AMP, but indicates moderate interaction between these groups in ADP, and strong interaction in ATP.     

Thermodynamics of lonization of aqueous lodic acid,an “almost-strong” electrolyte

We have made calorimetric measurements of enthalpies of dilution of aqueous iodic acid and have used these results for evaluation of the standard enthalpy of ionization of HIO₃(aq.). We have also made calorimetric measurements of enthalpies of addition of perchloric acid solution to aqueous solutions of KIO₃, KNO₃, NaIO₃, and NaNO₃ and have used these results to obtain further values for the standard enthalpy of ionization of HIO₃(aq.). On the basis of all these results, we have selected H^o=–660±125 cal-mole^–1 as the best available standard enthalpy of ionization of HIO₃(aq.) at 298.15°K, compared to the previously accepted –2400 cal-mole^–1. Using the best available K=0.157 for ionization, we also obtain G^o=1097 cal-mole^–1 and S^o=–5.9 cal-^oK^–1-mole^–1 for ionization of HIO₃(aq) at 298.15°K.On study leave from Department of Inorganic and Analytical Chemistry, LaTrobe University, Bundoora, Victoria, 3083, Australia, to University of Lethbridge.On study leave from Department of Chemistry, University of Wollongong, Wollongong, N.S.W. 2500, Australia, to University of Lethbridge.     

Enthalpies of formation of amidyl free radicals

The enthalpies of formation (#x0394;H°_f) of twenty-one amidyl radical (R) belonging to the formamidyl homological series were calculated using the published values of R—H bond dissociation energies. Among them, the H°_f values of nine radicals were first calculated and those of eight radicals were refined. Most of the H°_f values of corresponding starting molecules RH (H°_f(RH)) were obtained using the macroincrementing schemes. Based on the group additivity scheme, the structure—enthalpy of formation relationships for the radicals considered were examined, the H°_f(R) values were analyzed, and their reliability was confirmed. Parameters for calculating the H°_f values of radicals belonging to this homologous series were suggested.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1574–1577, August, 2004.     

Radiochemical measurement of solubility product and thermodynamic functions for erbium trifluoride-water system

The solubility and solubility product of erbium trifluoride in aqueous solution were measured using three different techniques. The more acceptable radiometric values for the solubility and solubility product are 1.98×10^–5M and 3.5×10^–18, respectively. The values for the thermodynamic functions, H ₂₉₈ ^o , G ₂₉₈ ^o and S ₂₉₈ ^o for the dissolution of ErF₃ and the stability constant for ErF²⁺ have also been measured.     

Influence of temperature on the micellization of sodium dodecylsulfate in water from speed of sound measurements

Speed of sound measurements for aqueous solutions of sodium dodecylsulfate (NaDS) have been carried out from 20 to 45°C at intervals of 5°C. The critical micellar concentrations (cmc) were determined and their change with temperature is discussed. A second change in the speed of sound has been found between 15 to 20mM for temperatures above 20°C. Values for G _m ^o , H _m ^o and S _m ^o for the micellization process have been determined and compared with data previously obtained from other experimental properties. These results confirm that the micellization process of NaDS in water is mainly entropically driven.     

Thermodynamics of transfer of non-ionic solutes between immiscible liquid phases. I. Transfer of normal alcohols fromn-octane to water at 25°C

A recently developed calorimetric method has been employed to estimate the thermodynamic functions for transfer of 1-propanol, 1-butanol, 1-pentanol, 1-hexanol and 1-heptanol from n-octane to water at 25°C. A linear correlation for G _t ^o as a function of the number of carbon atoms of the alchohol molecule has been found but for H _t ^o and S _t ^o the dependence gave well defined minima.     

Kinetics of the solvolysis oftrans-dichlorotetra-(4-t-butylpyridine)-cobalt(III) ions in water +t-butyl alcohol mixtures

Rates of solvolysis of the complex cation [Co(4tBupy)₄Cl₂]⁺ have been determined in mixtures of water with the hydrophobic solvent, t-butyl alcohol. The solvent composition at which the extremum is found in the variation of the enthalpy H^* and the entropy S^* of activation correlates well with the extremum in the variation of the relative partial molar volume of t-butyl alcohol in the mixture and the straight line found for the variation of H^* with S^* is coincident with the same plot for water + 2-propanol mixtures. A free energy cycle is applied to the process initial state (C ⁿ⁺) going to the transition state [M⁽ⁿ⁺¹⁾⁺...Cl^–] in water and in the mixture using free energies of transfer of the individual ionic species, G _t ^o (i), from water into the mixture. Values for G _t ^o (i) are derived from the solvent sorting method and from the TATB/TPTB method: using data from either method, changes in solvent structure on going from water into the mixture are found to stabilize the cation in the transition state, M⁽ⁿ⁺¹⁾⁺, more than in the initial state, C ⁿ⁺. This is compared with the application of the free energy cycle to the solvolysis of complexes [Co(Rpy)₄Cl₂]⁺ and [Coen₂LCl]⁺ in mixtures of water with methanol, 2-propanol or t-butyl alcohol: the above conclusion regarding the relative stabilization of the cations holds for all these complexes in their solvolyses in water+alcohol mixtures using values of G _t ^o (Cl^–) from either source.     

Enthalpies of solution of thymine and uracil in water and dimethylsulfoxide

Enthalpies of solution of thymine and uracil in water and in dimethylsulfoxide (DMSO) were measured calorimetrically in the temperature range 25–40°C. H _s ^o at 25°C for thymine and uracil in water were found to be 23.1±0.5 and 29.5±0.3 kJ-mol^–1, respectively. In DMSO, H _s ^o were 7.9±0.1 and 10.2±0.1 kJ-mol^–1, respectively. In aqueous solution C _p ^o for the two nucleic acid bases were relatively large and positive with C _p ^o of thymine being larger. Both transfer quantities H _t ^o and C _p,t ^o for the proceses H₂ODMSO for the two nucleic acid bases were negative. It is proposed that, the differences in the values obtained for the two bases is due principally to increased order in the water adjacent to the methyl group in thymine.     

Enthalpy of dissociation of water at 325°C and LogK, ΔH, ΔS,and ΔC p values for the formation of NaOH(aq) from 250 to 325°C

The aqueous reactions H⁺+OH^–=H₂O at 325°C and Na⁺+OH^–= NaOH(aq) at 250–325°C, were studied using a flow calorimeter. Heats of mixing of aqueous NaOH and HCl solutions were measured at 325°C. The enthalpy of water formation (H=95.9 kJ-mol^–1, valid at 12.4 MPa and infinite dilution) was obtained at this temperature from the heat of mixing data but differs significantly from that calculated from the Marshall-Franck equation. This calorimetric H at 325°C was used in combination with literaturelog K and H values at lower temperatures to derive equations representinglog K, H, S, and C_p for the formation of water from 250 to 325°C. Heats of dilution of aqueous NaOH solutions were measured at 250, 275, 300, and 325°C. Log K, H, and S for the formation of NaOH(aq) were determined at these temperatures from the fits of the calculated and measured heats while C_p values were calculated from the variation of H with temperature. No previous experimental results have been reported for the formation of NaOH(aq). The isocoulombic reaction principle is tested using thelog K values obtained in this study. The plot oflog K vs. 1/T for the isocoulombic reaction NaOH(aq) +H⁺=H₂O+Na⁺ is approximately linear.Presented at the Second International Symposium on Chemistry in High Temperature Water, Provo, UT, August 1991.Taken in part from the Ph.D. Dissertation of Xuemin Chen, Brigham Young University, 1991.     

The enthalpies of dilution of phosphate solutions at 30°C

The enthalpies of dilution of aqueous solutions of HCl, H₃PO₄, NaOH, NaH₂PO₄, Na₂HPO₄ and Na₃PO₄ in the molality range 0.1 to 1.0 mole-kg^–1 have been determined at 30°C. The relative apparent molal enthalpies _L of HCl, NaOH, NaH₂PO₄ and Na₂HPO₄ have been determined with the aid of an extended form of the Debye-Hückel limiting law. The relative apparent molal enthalpies for Na₃PO₄ solutions have been corrected for hydrolysis. A value of H _H ^o =9525±150 cal-mole^–1 was determined for the heat of hydrolysis of PO ₄ ^–3 . This value gives H ₃ ^o =3815±150 cal-mole^–1 for the ionization of H₂PO ₄ ^– , which is in good agreement with the value of H ₃ ^o =3500±500 cal-mole^–1 determined directly by Pitzer at 25°C. The relative apparent molal enthalpies for H₃PO₄ solutions have been corrected for ionization. A value of H ₁ ^o =–1900±150 cal-mole^–1 was obtained for the heat of ionization of H₃PO₄ to H⁺+H₂PO ₄ ^– . This value is in good agreement with the value of H ₁ ^o =–2031 cal-mole^–1 at 30°C determined by Harned and Owen from the temperature coefficient of the equilibrium constant and H ₁ ^o =–1950±80 cal-mole^–1 at 25°C determined from calorimetry by Pitzer.     

Enthalpy and entropy changes for the intercalation of small molecules to DNA. I. Substituted naphthalene monoimides and naphthalene diimides

Calorimetric studies have been performed on the intercalation of a series of nitro and amino substituted naphthalene monoimide cations to calf thymus DNA. For comparison, we also included in the study the unsubstituted naphthalene diimide dication. All of the substituted naphthalene monoimides formed dimers at the concentrations used in the calorimetric titrations, and dimerization constants for these compounds were derived from spectroscopic studies and used in calculating the H _B ^o parameters from the calorimetric data. The dimerization constants increase in the order 3-NO₂MI=4-NO₂MI>3-NH₂MI>4-NH₂MI. For the unsubstituted naphthalene monoimide and 3-NO₂MI and 4-NO₂MI, the H _B ^o parameters are within experimental error equal to that found for the naphthalene diimide, i.e., –4.3 kcal-mol^–1. Thus, changes in entropy cause the K _B for the diimide to be 40 times larger than that for the monoimide. This observation is consistent with the current electrostatic theory for counterion binding to DNA: a dication should cause the release of more counterions than a monocation and produce a more positive S _B ^o . For the amino substituted naphthalene monoimides, the K _B values are similar to the other monoimides, but H _B ^o =–6.7 kcal-mol^–1. We propose that a hydrogen bond is responsible for the unusual enthalpy and entropy effects seen for 3-NH₂MI and 4-NH₂MI.     

Absorption spectra of 2-styryl-4-phenyl-thiazole ethiodides in organic solvents of varying polarities and determination of the formation constant of the charge transfer complex formed

The electronic absorption spectra of some 2-styryl-4-phenyl-thiazole ethiodides are studied in organic solvents of different polarities. The shorter wavelength band appearing in the visible region is assigned to an intramolecular charge transfer (CT)-transition originating from the phenyl moiety to the positively charged hetero ring, while the longer wavelength one is due to an intermolecular CT-transition from the iodide ion to the 2-styryl-4-phenyl-thiazolinium cation. These assignments are based on the nature of the aldehydic residue and effects of solvent, concentration, and temperature on both the position and absorptivity of the CT complex-band. It is concluded that the CT complex formed will be highly solvated inDMF, DMSO, ethanol and methanol relative to in CHCl₃, dioxane and acetone. The formation constant of the CT complex in solutions of different polarities is determined at different temperatures. Furthermore, the thermodynamic parameters H ^o, G ^o and S ^o for complex formation are calculated and discussed.

---

Absorptionsspektren von 2-Styryl-4-phenyl-thiazol-ethiodiden in verschiedenen Lösungsmitteln und Bestimmung der Bildungskonstanten der Charge-Transfer-Komplexe

Zusammenfassung Die Elektronenanregungsspektren einiger substituierter 2-Styryl-4-phenyl-thiazol-ethiodide wurden in einigen Lösungsmitteln unterschiedlicher Polarität untersucht. Die Absorption bei kürzerer Wellenlänge wird einem intramolekularen Charge-Transfer (CT)-Übergang zugeordnet, die langwellige Bande einem intermolekularen CT-Übergang (Jodid—organ. Kation). Die Diskussion erfolgt basierend auf Substitutions-, Lösungsmittel-, Konzentrations-, und Temperatur-Effekten. Die Komplexbildungskonstanten und die thermodynamischen Parameter H ^o, G ^o und S ^o werden angegeben.

     

Synthesis of 19-hydroxysteroids I. New synthesis of 19-hydroxytestosterone

For obtaining 19-hydroxytestosterone from dehydroepiandrosterone a new scheme of synthesis has been developed the key stages of which are the reduction of the 17-keto group to a 17-alcohol, the functionalization of the 19-methyl group via the bromohydrin with the formation of a 6,19-epoxide, the selective hydrolysis of the free -acetoxy group, the conversion of the 3-hydroxy-5-bromo derivative into the ⁴-3-ketone, and the reductive cleavage of the 6,19-epoxide ring.Institute of Bioorganic Chemistry, Belorussian Academy of Sciences, Minsk. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 672–678, November–December, 1992.