Studies On Thermodynamics Of The Interaction Between Amino Acid And Rare Alkali Metal Ions (Rubidium And Cesium) In Water

Studies on the interaction of Rb+/Cs+ with amino acid in aqueous solution is of importance to explore the folding, denaturation, hydration and stretch, especially the stabilization of protein in body fluid.In this paper, the phase equilibrium of RbCl/CsCl-HCl-H2O ternary solution was studied at different temperatures, and based on the results, a method for the preparation of high purity RbCl/CsCl was proposed, as well as the technics procedure. In the mean time, an analysis system for the determination of the alkali metal impurities in products by flame atomic absorption spectrometry was set up so as to monitor the quality of products. .Then these high purity Rb/Cs compounds were used to investigate the interaction between Rb+/Cs+ and the a-amino acid in water in this work.(1) Calorimetric technique was employed to determine the molar mixing enthalpies(mixHm) of aqueous electrolyte (RbCl/CsCl) solution with aqueous amino acid (glycine, alanine and aminobutyric acid) solution at different concentration from 298.15K to 313.15K, and the molar dilution enthalpies of electrolyte and amino acid in pure water [dilHm(E) and dilHm(A)]. Then the molar transfer enthalpies (trHm) of electrolyte from pure water to aqueous amino acid solutions can be calculated based on these experimental data. The enthalpic interaction parameters (hEA) between electrolyte and amino acid in water are evaluated in term of McMillan-Mayer theory, as well as the expression of dependence of isobaric capacity parameters (CP,EA.) on temperature in RbCl/CsCl-alanine acid-water system.(2) A non-liquid-junction electrochemical cell composed by two ion selectivity electrodes was designed to measure the electromotive force of electrolyte-water duality system and electrolyte-amino acid-water ternary system. Both the activity coefficient of electrolyte in amino acid aqueous solutions and those of amino acid in electrolyte aqueous solutions, Gibbs free energy interaction parameters (gES) and salting constant (kS) were obtained, and the entropic interaction parameters (TsEA) were calculated using hEA and gEs.(3) The experimental results show that the enthalpic parameters (hEA) and salting constant (ks) of RbCl/CsCl with amino acid in water have close relationship with the numbers of carbon atom in amino acid. As the same electrolyte concerned, the values of hEA become less negative with the numbers of carbon atom increasing, and as the same amino acid concerned, the values of gES ,hEA and TsEA indicate some regulation with the cation radii becoming bigger. These conclusions can be interpreted by Savage-Wood group additivity principle, electrostatic and structural interaction model.(4) The enthalpic interaction parameters of RbCl/CsCl-alanine acid-water systemhave an extremum at 298.15K. It is interesting that this temperature point is close to basic body temperature.