Effect of the Nature of Supporting Electrolyte on the Thermodynamic Parameters of the Stepwise Dissociation of Glycyl-D-Phenylalanine in Aqueous Solution

Abstract

The thermal effects of acidic and basic dissociation of glycyl-D-phenylalanine dipeptide at a temperature of 298.15 K and ionic strengths of solution of 0.5, 0.75, and 1.0 M against the background of different supporting electrolytes were calculated from the results of direct calorimetric measurements performed on a calorimeter with an isothermal shell and automatic recording of the temperature–time curve. The influence of the nature of supporting electrolytes NaCl, NaClO4, NaNO3, KNO3, and LiNO3 on the thermal effects of stepwise dissociation of the dipeptide is considered. The standard thermal effects of ionization of glycyl‑D‑phenylalanine in two steps were found by extrapolation to zero ionic strength. The standard changes in thermodynamic functions (enthalpy, entropy, and Gibbs energy) in the acidic and basic dissociation of g-lycyl-D-phenylalanine dipeptide were calculated.