Some thermodynamic properties of dl-Tyrosine and dl-Tryptophan. Effect of the ionic medium, ionic strength and temperature on the solubility and acid–base properties

A study on the acid–base properties and solubility of two amino acids, namely dl-Tyrosine and dl-Tryptophan, was carried out in different experimental conditions (ionic medium, ionic strength and temperature). The protonation of dl-Tyrosine and dl-Tryptophan was investigated in both (CH3)4NCl and NaCl aqueous solutions by potentiometry (ISE-[H+] glass electrode) in different ionic strength ranges (up to ∼3.0 mol L−1 in (CH3)4NCl and ∼5 mol L−1 in NaCl). Measurements in NaCl were also performed by spectrophotometry at three different temperatures (T = 293.15, 298.15 and 308.15 K). The potentiometric and spectrophotometric data were analyzed altogether in order to obtain reliable protonation constants.Solubility investigations were performed only at T = 298.15 K, in NaCl and (CH3)4NCl aqueous solutions in the ionic strength range investigated for the protonation constants, with the aim to determine the Setschenow coefficient and the activity coefficients of the neutral species of the two amino acids. The dependence of protonation constants on ionic strength was modelled by means of a Debye–Hückel type equation and the SIT approach.The temperature coefficients, in NaCl aqueous solutions, were calculated by using the protonation constants at different temperatures. As a further validation of the data, for the dl-Tyrosine, the ΔH values were also determined by isoperibol calorimetric titrations, at T = 298.15 K, I = 0.15 and 0.5 mol L−1; these values were compared with the ΔH calculated by using the temperature coefficients.

► Acid–base properties and solubility of dl-Tyrosine and dl-Tryptophan.
► Protonation constants determined at different I and T.
► Dependence on ionic strength modeled by means of a Debye–Hückel and SIT approach.
► Setschenow coefficients and activity coefficient of the neutral species.
► Enthalpy and entropy change associated to each protonation step.