Thermochemical study of the temperature effect on the l-α-alanine-l-α-alanine interactions in aqueous solutions of urea and ethylene glycol

The dilution enthalpies of l-α-alanine (Ala) solutions in aqueous solutions of urea and ethylene glycol were measured at 298.15 and 313.15 K. The enthalpy (h _xx ) and heat capacity (c _xx ) coefficients of pair interaction were used for characterization of the Ala-Ala interaction in solutions. The h _xx values are presented by the sum of contributions from the interactions of the nonpolar side chains (h _R-R) and polar groups (h _FG-FG) of the amino acid. The h _xx value of Ala in water increases with the temperature increase due to an increase in the contribution of h _R-R. The increase in h _xx of Ala in an aqueous-carbamide solvent with an increase in the urea concentration is determined by an increase in the contribution of h _FG-FG. The temperature rise and urea additives exert various denaturing effects. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1364–1368, July, 2008.     

大环疏水效应: 冠醚在DMF+H2O混合物中的焓对作用

利用等温滴定微量热法测定了298.15 K时12-冠-4、15-冠-5、18-冠-6和4,13-二氮杂-18-冠-6四种冠醚在纯水及不同质量分数(w=0-0.3)的N,N-二甲基甲酰胺(DMF)+H₂O混合物中的稀释焓, 根据McMillan-Mayer理论计算得到相应的焓对作用系数(h_xx). 实验发现, h_xx均为较大的正值, 表明在冠醚-冠醚自相互作用中疏水性成分占绝对优势, 主要表现为两种机制: (1) 当疏水-疏水作用发生时, 共球交盖使得水结构形成减少, 对h_xx有正的贡献; (2) 当疏水-亲水作用发生时, 共球交盖使得水结构破坏增加, 对h_xx有正的贡献. 此外, 四种冠醚h_xx的大小关系为: h_xx(18-冠-6)>h_xx(4,13-二氮杂-18-冠-6)≈h_xx(15-冠-5)>h_xx(12-冠-4), 表明冠醚环越大, 疏水-疏水作用越强, 存在显著的大环疏水效应.     

H 2 O-D2O Solvent Isotope Effect on Excess Molar Volumes of 3-Methylpyridine Solutions

Densities of 3-methylpyridine (3-MP) + water and 3-methylpyridine + heavy water were measured in the 3-MP mole fraction range 0.002–0.04 from 298 to 318 K. The excess molar volumes of 3-MP + D₂O mixtures were found to be more negative than those of 3-MP + H₂O mixtures. The partial molar volume of 3-MP at infinite dilution is smaller in D₂O than in H₂O which suggests that 3-MP causes a structure-breaking effect in water which is more pronounced in D₂O. It was found that the volume change with concentration in dilute solutions of 3-MP in water and heavy water can be adequately described by the pair-wise interaction of the solute molecules. The molal volume second-virial coefficient, V _xx , is positive indicating that the water molecules are less structured in the cospheres of the solute pairs than in the bulk solvent. The temperature dependence of V _xx displays a maximum at around 308 K in the case of D₂O solutions, whereas V _xx increases almost linearly with temperature in H₂O solutions.     

Enthalpies of Dilution and Enthalpies of Mixing of Amino Acids with Sucrose in Aqueous Solutions at 298.15 K

The enthalpies of mixing of aqueous solutions have been determined for sucrose with six different amino acids (glycine, l-alanine, l-serine, l-valine, l-proline and l-threonine) at 298.15 K, by using a LKB-2277 flow microcalorimetric system. These results, along with the enthalpies of dilution of these solutes for the initial solutions, were used to determine the enthalpic interaction coefficients (h _xy, h _xyy, h _xxy) of the McMillan–Mayer Theory. The pair-wise cross interaction coefficients of amino acids and sucrose are discussed from the viewpoint of solute–solute interactions.     

Chemical applications of topology and group theory. XXII : Lowest degree chirality polynomials for regular polyhedra [1]

The lowest degree chirality polynomials for the regular octahedron, cube, and regular icosahedron are discussed. All three of these regular polyhedra are chirally degenerate since they have more than one lowest degree chiral ligand parition by the Ruch-Schönhofer scheme. The two lowest degree chirality polynomials for the octahedron have degree 6 and can be formed from three degree 3 generating polynomialsf,g, andh through the relationshipsf(g +h) andf(g –h), wheref,g, andh measure the effects of the three separating reflection planes (h), the four threefold rotation axes, and the three fourfold rotation axes, respectively. The permutation groups of the vertices of the cube and icosahedron contain only even permutations, which leads to a natural pairing of their chiral ligand partitions according to equivalence of the corresponding Young diagrams upon reflection through their diagonals. The two lowest degree chirality polynomials for the cube have degree 4 and can be formed from two degree 4 generating polynomialsf andg through the relationships –2g andf –2g, wheref andg measure the effects of theS ₆ improper rotation andC ₄, proper rotation axes. respectively. The four lowest degree chiral ligand partitions for the icosahedron have degree 4 and lead naturally to a single degree 4 chirality polynomial with 120 terms of the general type (x –y)² (z –w)². This chirality polynomial for the icosahedron cannot be broken down into simpler generating polynomials, in contrast to the lowest degree chirality polynomials for the octahedron and cube. This appears to relate to the origin of the icosahedral group from the simple alternating groupA ₅. The full icosahedral chirality polynomial can be simplified to give a chirality polynomial for the chiral boron-monosubstituted ortho and meta carboranes of the general formula B₂C₁₀H₁₁X.     

Enthalpic interactions of some α-amino acids with glycol in aqueous solutions at 298.15 K

The enthalpies of mixing of six kinds of aqueous amino acid solutions (Glycine, l-alanine, l-valine, l-serine, l-threonine and l-proline) and aqueous glycol solution and their respective enthalpies of dilution have been determined at 298.15 K using flow microcalorimetry. The experimental data have been analyzed according to the McMillan–Mayer formalism to obtain the heterotactic enthalpic interaction coefficients (h_xy). h_xy coefficients have been discussed from the points of view of solute–solute interactions.     

Enthalpic interaction coefficients of NaI–alkanediol pairs in methanol and in water

The dissolution enthalpies of NaI in the mixtures of methanol with 1,2-alkanediols (1,2-propanediol, 1,2-butanediol, 1,2-pentanediol) and with ??,??-alkanediols (1,3-propanediol, 1,4-butanediol, 1,5-pentanediol), as well NaI in the mixtures of water with 1,3-propanediol and 1,2-pentanediol, were determined at 298.15?K. The energetic effect of interactions between the investigated alkanediols and NaI in methanol and in water was calculated using the enthalpic pair interaction coefficients (h _xy ) model. These results along with the other data concerning the NaI?Cnon-electrolyte pairs taken from our earlier reports and from the literature were analyzed with respect to the effect of the non-electrolyte properties on the variations of the h _xy values. The group contributions illustrating the interactions of NaI with selected functional groups in non-electrolyte (alkanediol and alkanol) molecules, namely: CH₂ and OH groups were calculated and discussed.     

α-氨基丁酸对映异构体在DMF+水混合溶剂中的焓对作用

利用等温滴定微量热法(ITC)分别测定了298.15K时L-α-氨基丁酸和D-α-氨基丁酸两种对映异构体在不同组成的二甲基甲酰胺(DMF)+水混合溶剂中的稀释焓.根据统计热力学的McMillan-Mayer理论计算各溶剂组成下的同系焓对作用系数(hxx).从溶质-溶质和溶质-溶剂相互作用的观点出发探讨了三元水溶液中疏水-疏水、疏水-亲水和亲水-亲水作用的竞争平衡.实验发现,在所研究的混合溶剂组成范围内(wDMF=0-0.3),α-氨基丁酸两种对映体的hxx都是较大的正值,且都随wDMF的增大而逐渐减小,而有趣的是L型对映体的hxx值普遍比D型的大(hLLhDD),说明ITC可以区分对映异构体的同手性焓对作用.结果表明:在α-氨基丁酸+水+DMF三元溶液体系中,疏水-疏水和疏水-亲水作用在分子对作用过程中占优势;在α-氨基丁酸的同种对映体发生分子对作用时,L-L分子对比D-D分子对在构型上更有利于α-碳上疏水侧链(CH3CH2-)的靠拢,疏水基团的水合壳层发生交盖,局部破坏而释放出部分结构化的水,过程自发、吸热且伴随显著熵增(ΔG0,ΔH0,ΔS=(ΔH-ΔG)/T0),因而在稀释时释放出更多热量,hxx具有较大的正值.     

Enthalpic interactions of N-glycylglycine with xylitol in aqueous sodium chloride and potassium chloride solutions at T = 298.15 K

The mixing enthalpies of N-glycylglycine with xylitol and their respective enthalpies of dilution in aqueous sodium chloride and potassium chloride solutions have been determined by using flow-mix isothermal microcalorimetry at the temperature of 298.15 K. These experimental results have been used to determine the heterotactic enthalpic interaction coefficients (h_xy, h_xxy, and h_xyy) according to the McMillan–Mayer theory. It has been found that the heterotactic enthalpic pairwise interaction coefficients h_xy between N-glycylglycine and xylitol in aqueous sodium chloride and potassium chloride solutions are negative and become less negative with an increase in the molality of sodium chloride or potassium chloride. The results are discussed in terms of solute–solute and solute–solvent interactions.     

Model calculations for Setchenow coefficients

The calculations of Setchenow coefficients reported earlier [H. L. Friedman, C. V. Krishnan, and C. Jolicoeur,Ann. N.Y. Acad. Sci. 204, 79 (1973)] have been extended to aqueous solutions of various nonelectrolytes mixed with alkali or alkylammonium halides. A few data for Setchenow coefficients in methanol have also been treated. The GurneyA _ij parameters for nonelectrolyte-ion interactions in models which fit the data are mostly negative, and more so the larger the solute molecules. A value ofA _ij near-100 cal-mole^–1 seems to characterize hydrophobic bonding. In several systems these is evidence that some nonsolvation contribution to theu _ij pair potential which is not explicitly accounted for in the models is important in the real systems. Quite possibly this contribution is due to dispersion forces or to the chargepolarizability interaction. On the whole, theA _ij parameters do not seem to depend upon the charges on solute particlesi andj; this is evidence that the model is fairly realistic.     

Enthalpies of Dilution of N,N′-hexamethylenebisacetamide in Pure Water and Aqueous Solutions of Alkali Halide at 298.15 K

Enthalpies of dilution of N,N′-hexamethylenebisacetamide in water and aqueous alkali halide solutions at the concentration of 0.150 mol⋅kg⁻¹ (approximately the concentration of physiological saline) have been determined by isothermal titration microcalorimetry at 298.15 K. The enthalpic interaction coefficients in the solutions have been calculated according to the excess enthalpy concept based on the calorimetric data. The values of enthalpic pair-wise interaction coefficients (h ₂) of the solute in aqueous solutions of different salts were discussed in terms of the different alkali salt ions and weak interactions of the diluted component with coexistent species as well as the change in solvent structure caused by ions.     

Heterogeneous interaction between zwitterions of amino acids and glycerol in aqueous solutions at 298.15 K

The enthalpies of mixing of six kinds of amino acid (glycine, L-alanine, L-valine, L-serine, L-threonine, and L-proline) with glycerol in aqueous solutions and the enthalpies of diluting of amino acid and glycerol aqueous solutions have been determined by flow microcalorimetry at 298.15 K. Employing McMillan–Mayer theory, the enthalpies of mixing and diluting have been used to calculate heterogeneous enthalpic pairwise interaction coefficients (h _xy ) between amino acids and glycerol in aqueous solutions. Combining h _xy values of amino acids with glycol in the previous study, the variations of the h _xy values between amino acids and glycerol have been interpreted from the point of view of solute–solute interactions.     

Adiabatic compressibility and structural features of non-electrolyte aqueous solutions

Structural characteristics of the hydration complexes of non-electrolytes such as the hydration numbers h, molar adiabatic compressibility of hydration complexes β _h V _h , the molar volume of water in the hydration sphere V _1h , the solute molar volume without hydration environments V _2h and others are determined using the data on the ultrasonic velocity, the density and heat capacity of aqueous solutions of urea, urotropine, acetonitrile, and a number of amides of N-acetyl amino acids. A theoretical model of solvation is also applied. A comparison of the environments of hydrated urotropine molecules with those of urea and acetonitrile molecules in an aqueous medium shows a considerable hydrophobic interaction of urotropine with a solvent.     

The Heterotactic Enthalpic Interaction Coefficients of <Emphasis Type="Italic">α</Emphasis>-Amino Acids with Maltose in Aqueous Solution at <Emphasis Type="Italic">T</Emphasis>=298.15 K

The mixing enthalpies of maltose with several typical α-amino acids (glycine, L-alanine, L-serine, L-valine, L-threonine and L-proline) and dilution enthalpies of each compound have been determined in aqueous solutions at T=298.15 K by a flow-mixing microcalorimeter. The heterotactic enthalpic pairwise interaction coefficients, h _xy , of each amino acid with maltose have been calculated by the McMillan–Mayer formalism, and are discussed in terms of intermolecular interactions of the hydrated solute species.     

Equilibrium dielectric properties of butanediols

Experimental investigations of static dielectric permittivity (ɛ _s ) for 1,3-, 1,4-, and 2,3-butanedilos determined in the temperature range from 283 to 423 K at 1 MHz and a density of (ρ) are presented. The values of the experimental correlation factor, g _exp, which characterizes the correlation of the orientation of molecules in a liquid due to short-range oriented intermolecular interactions, are calculated according to the Onsager-Kirkwood-Frohlich theory. Its values are used for the judgment of a character of a short-range orientation order in a liquid.     

Photocatalysis by titanium complexes of the reaction of ClO4 −, ions with alcohol molecules

In the photolysis of perchloric acid solutions of titanium(IV) with additions of ethanol, the formation of titanium(III) compounds was detected. Irradiated solutions ([HClO₄]=0.8 M) at 77K are characterized by an anisotropic ESR signal with parameters g ₁=1.994, g ₁=1.904, which correspond to pseudo-octahedral aqua-complexes of titanium(III). With the passage of time, the signal intensity of the titanium(III) compounds decreases, which is explained by their oxidation by ClO₄ ions. During prolonged photolysis of titanium(IV) compounds in 7 M HClO₄ with an addition of ethanol, a multicomponent signal of the paramagnetic chlorine oxide ClO₂ with parameters g _xx=2,004, a _xx(³⁷Cl)=6.0. mTl a _xx(³⁵Cl)=7,30 mTl, is recorded instead of the signal of titanium(III) compounds. Evidently ClO₂ is formed as a result of a rapid dark reaction between compounds of trivalent titanium (products of the photochemical step) and chlorate ions, which are accumulated in the solution as a result of successive photochemical and dark redox conversions of the titanium compounds, perchlorate ions, arid free radicals from the alcohol molecules. The formation of free radicals in the system was established by the ESR method after irradiation of frozen solutions.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 6, pp. 679–686, November–December, 1986.     

Excess enthalpies of aqueous solutions of polyols at 25°

The excess enthalpies of aqueous solutions of nine polyols were determined at 25°C and reported in the virial form. The most interesting and new feature of this family of solutes is that the sign of the enthalpic pair interaction coefficients h_xx is positive for the first members of the series and negative for the higher homologues. Other points are the large differences found among the values of h_xx for stereoisomers, whereas pairs of enantiomers show the same values within experimental errors. An application of the group additivity method is also discussed.     

Tetrahydroberberine,a pharmacologically active naturally occurring alkaloid

Tetrahydroberberine (systematic name: 9,10‐dimethoxy‐5,8,13,13a‐tetrahydro‐6H‐benzo[g][1,3]benzodioxolo[5,6‐a]quinolizine), C₂₀H₂₁NO₄, a widely distributed naturally occurring alkaloid, has been crystallized as a racemic mixture about an inversion center. A bent conformation of the molecule is observed, with an angle of 24.72 (5)° between the arene rings at the two ends of the reduced quinolizinium core. The intermolecular hydrogen bonds that play an apparent role in crystal packing are 1,3‐benzodioxole –CH₂...OCH₃ and –OCH₃...OCH₃ interactions between neighboring molecules.     

Enthalpy changes of salting processes of hen-egg white lysozyme in various electrolyte solutions

The enthalpy changes of salting process of hen-egg white lysozyme in buffer acetate solutions (pH=4.25) as a function of concentration of following electrolytes: LiCl, KCl, K₂SO₄, Li₂ SO₄ and (NH₄)₂SO₄ are determined. Obtained data according to McMillan and Mayer’s approach, has been analyzed in the terms of the enthalpic pairwise interaction coefficients: lysozyme – lysozyme h_xx, and lysozyme – salt h_xy. The ability of cations to precipitate lysozyme solution in relation to the concentration of cations can be seen from the series as follows: Li⁺> Na⁺>K⁺>NH₄+⁺