Relationship between the molecular structure of amino acids and dipeptides and thermal sublimation effects

This paper reports on our mass spectrometric study of sublimation of glycine and DL-alanyl-glycine (Ala-gly). The sublimation enthalpy of Ala-gly has been determined by generalization of the data obtained and the results of AM1 quantum-chemical calculations. A relationship has been found between the sublimation enthalpy (ΔH _subl), heat capacity (C _P), and the sum of bond lengths (Σn _i l _i ) in 17 α-amino acid and 9 dipeptide molecules. Correlations are suggested for evaluating ΔH _subl of amino acids and peptides.     

Solvation Interactions in Solutions of Lithium Hexafluoroarsenate in Propylene Carbonate

On the basis of the dissolution heat in propylene carbonate (PC) measured at 298.15 K, and the electroconductivity, ultrasound propagation velocity, and solution density and viscosity measured at 283.15–313.15 K, the LiAsF₆ solvation in PC is discussed. The LiAsF₆ solvation enthalpy and its ionic components are determined. It is shown that the donor–acceptor interaction between Li⁺ and the oxygen atom of the carbonyl group of PC makes the main contribution to the enthalpy. The effects of the concentration and temperature on the molar electroconductivity, viscosity, apparent molar volume, and electrolyte compressibility are analyzed, and thermodynamic parameters of the activation of viscous flow and electroconductivity are considered. Using the Onori approach, parameters of solvation complexes in solution, such as solvation numbers, volumes, compressibilities, and their dependences on the concentration are determined. A considerable solvent compressibility is noted in the inner sphere of the complex.     

Study on the Interaction of Nicotinic Acid with <Emphasis Type="SmallCaps">l</Emphasis>-Phenylalanine in Buffer Solution by Heat Capacity Measurements at Various Temperatures

The interaction between nicotinic acid (NA) and l-phenylalanine (Phe) was studied in aqueous phosphate buffer solutions (pH = 7.35) by differential scanning calorimetry. Heat capacities of nicotinic acid–buffer, l-phenylalanine–buffer, and nicotinic acid–l-phenylalanine–buffer mixtures were determined at (283.15, 288.15, 293.15, 298.15, 303.15, 308.15, 313.15, 318.15 and 323.15) K using the microdifferential scanning calorimeter SCAL-1 (Pushchino, Russia). The apparent molar heat capacities, ^? C _p , of nicotinic acid in buffer solution and in buffer 0.0216 mol·kg^?1 amino acid solutions were evaluated. The concentration of NA was varied from (0.0106–0.0701) mol·kg^?1. The interaction of NA with Phe is accompanied by complex formation. The partial molar heat capacities of transfer of nicotinic acid from buffer to buffer amino acid solutions are positive. The results are discussed in terms of various interactions operating in this system.     

Effect of Tryptophan and Asparagine Structure on the Enthalpic Characteristics of Their Dissolution in Aqueous Solutions of Sodium Dodecyl Sulfate

The integral enthalpies of dissolution of L-tryptophan and L-asparagine in aqueous solutions of sodium dodecyl sulfate (surfactant) at surfactant concentrations of up to 0.05 mol/kg of the solvent are determined and estimated calorimetrically. Standard values of the enthalpies of dissolution and transfer of amino acids from water to a mixed solvent are calculated. The calculated enthalpy coefficients of pair interactions between amino acids and surfactant molecules have positive values. Hydrophobic interactions between amino acids and surfactants have the dominant effect on the enthalpy characteristics of the interaction in a three-component solution.     

Ytterbium tris(hexafluoroacetylacetonate) Yb(C<Subscript>5</Subscript>O<Subscript>2</Subscript>HF<Subscript>6</Subscript>)<Subscript>3</Subscript>: The composition of overheated vapor and sublimation thermodynamics

A mass spectrometric study of the saturated vapor over ytterbium tris(hexafluoroacetylacetonate) Yb(hfa)₃ (hfa = CF₃-C(O)-CH-C(O)-CF₃) and of the vapor overheated up to the thermal decomposition temperature of the complex is presented. The vapor composition changes markedly with increasing temperature. At T ≈ 370 K, the mass spectrum of the vapor over Yb(hfa)₃ indicates the presence of ions containing one to three metal atoms. As the temperature is raised, the ion currents due to oligomer ions decrease. The oligomers are not detected at T > 440 K. The total decomposition temperature of Yb(hfa)₃ is 663(9) K. The second-law enthalpy of sublimation (ΔH _s^o (380 K)) is 134 ± 7 kJ/mol for the monomer and 138 ± 10 kJ/mol for the dimer. The enthalpy of dissociation of the dimer into monomer molecules is nearly equal to the enthalpy of sublimation of the monomer and dimer: ΔH _dis(380 K) = 130 ± 15 kJ/mol.     

Qualitative analysis and asymptotics of solutions of generalized KdV-class equations

We study the asymptotics and the structure of solutions of generalized KdV-class equations with an arbitrary nonlinearity index, which are extensively used in the physics of ionospheric and magnetospheric plasma, hydrodynamics, and aerodynamics. KdV-class equations are generalized by introducing higher-order dispersion correction and terms describing dissipation and instability caused by numerous factors. Using asymptotic analysis and analysis of solutions in the phase space, we distinguish among different classes of solutions of soliton and nonsoliton types. North-East Complex Research Institute of the Far East Branch of Russian Academy of Sciences; Pedagogical University, Magadan, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 3, pp. 328–344, March, 1997.     

Effect of hydrostatic pressure on certain thermodynamic properties of polymeric materials

By means of a thermodynamic approach it is demonstrated that the mechanical properties of homogenous polymeric materials depend only on specific volume. Expressions are derived for the entropy and volume coefficient of thermal expansion as functions of hydrostatic pressure and temperature. It is shown that for both crystalline and amorphous polymeric materials the bulk modulus depends on reduced temperature.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 5, pp. 825–829, September–October, 1970.     

Molecular Interactions of L-Histidine in an Aqueous Buffer Solution in the Temperature Range of 288–313 K

Russian Journal of Physical Chemistry A - The behavior of L-histidine (His) in an aqueous buffer solution (pH 7.4) is studied via densimetry and differential scanning calorimetry. The densities and...