Thermochemical characteristics of the formation of aqueous solutions of imino acids

Abstract

    The calorimetry methods are an important source of thermodynamic information in the physicochemistry of solutions of biologically active substances, including amino acids. The goal of the work was to prepare a thermodynamic description of the formation of an aqueous solution of heterocyclic imino acids, proline and hydroxyproline, that have different structures and sizes of the side radical, in a wide range of concentrations, which can be used for a qualitative analysis of changes occurring in their solutions.
     Thermochemical measurements of the formation of an aqueous solution of imino acids in the concentration range 1.0·10-3 – 40.0·10-3 mol/kg was performed on a MID-200 differential heat-conducting microcalorimeter at 293 K. The equilibrium moment in the solution formation was determined by the output of the thermokinetic curve to the zero line. The enthalpy of the formation of an aqueous solution of imino acids was calculated by the integration of the time dependence of thermal power.
     It is shown that the increase in the equilibrium time, the increase in the maximum heat flow, and the decrease in the rate of change of the heat flow during the dissolution of hydroxyproline is due to the  formation of intra and intermolecular bonds in the Hypro structure with the participation of the OH group. The difference in the structure of imino acids is reflected in the sign of the thermal effect and the form of the concentration dependence of the enthalpy of formation of aqueous solutions. The exoeffect of proline dissolution is due to the stabilisation of the water structure influenced by imino acid.