Constants of acid-base equilibria of glycylglycine in water-dimethyl sulfoxide solutions at 298 K

Constants of acid-base equilibria of glycylglycine were obtained by potentiometry at a temperature of 298 K and an ionic strength of 0.1 M (NaClO4) in water-dimethyl sulfoxide solutions. It was determined that upon an increase in the content of the organic component in solution, the dissociation constant of the glycylglycinium ion declines stepwise. It was concluded that the pK of the dissociation of glycylglycine versus the composition of a mixed solvent is of a critical character with a minimum in the water-organic solvent composition range of 0.1 to 0.3 molar fractions of DMSO. It is suggested that upon an increase in the concentration of DMSO, the zwitter-ion form of glycylglycine transitions to the molecular form.