Modeling solubility and acid–base properties of some amino acids in aqueous NaCl and (CH3)4NCl aqueous solutions at different ionic strengths and temperatures

New potentiometric experiments have been performed in NaCl and in (CH3)4NCl media, to determine the protonation constants, the protonation enthalpy changes and the solubility of six natural α-amino acids, namely Glycine (Gly), Alanine (Ala), Valine (Val), Leucine (Leu), Serine (Ser) and Phenylalanine (Phe). The aim of the work is the rationalization of the protonation thermodynamics (log Kᵢᴴ, solubility and ΔHᵢ⁰) in NaCl, determining recommended, tentative or provisional values in selected experimental conditions and to report, for the first time, data in a weak interacting medium, as (CH₃)₄NCl. Literature data analysis was performed selecting the most reliable values, analyzed together with new data here reported. Significant trends and similarities were observed in the behavior of the six amino acids, and in some cases it was possible to determine common parameters for the ionic strength and temperature dependence. In general, the first protonation step, relative to the amino group, is significantly exothermic (average value is ΔH⁰₁ = −44.5 ± 0.4 kJ mol⁻¹ at infinite dilution and T = 298.15 K), and the second, relative to the carboxylate group, is fairly close to zero (ΔH⁰₂ = −2.5 ± 1.6, same conditions). In both cases, the main contribution to the proton binding reaction is mainly entropic in nature. For phenylalanine and leucine, solubility measurements at different concentrations of supporting electrolyte allowed to determine total and specific solubility values, then used to obtain the Setschenow and the activity coefficients of all the species involved in the protonation equilibria. The values of the first protonation constant in (CH₃)₄NCl are lower than the corresponding values in NaCl, due to the weak interaction between the deprotonated amino group and (CH₃)₄N+. In this light, differences between the protonation functions in NaCl and (CH₃)₄NCl were used for the quantification of the stability of the weak [(CH₃)₄N+–L−] complexes that resulted log K = −0.38 ± 0.07 as an average value for the six amino acids.