Modeling the Dependence on Medium and Ionic Strength of GlutathioneAcid-Base Behavior in LiClaq, NaClaq, KClaq, RbClaq, CsClaq, (CH3)4NClaq,and (C2H5)4NIaq

Acid−base properties of γ-l-glutamyl-l-cysteinyl-glycine (glutathione, GSH) in LiClaq, NaClaq, KClaq, RbClaq, CsClaq, (CH3)4NClaq, and (C2H5)4NIaq were investigated at T = 298 K, by potentiometry (ISE-H+, glass electrode), and at different ionic strengths [Ic ≤ 5.0 mol·L-1 for LiClaq and NaClaq, Ic ≤ 3.0 mol·L-1 for KClaq, RbClaq, and (CH3)4NClaq, Ic ≤ 4.0 mol·L-1 for CsClaq, and Ic ≤ 1.0 mol·L-1 for (C2H5)4NIaq]. The dependence on medium and ionic strength of protonation constants was modeled by an extended Debye−Hückel (EDH) type equation and by specific ion interaction theory (SIT) and Pitzer models. All three approaches give comparable and satisfactory results. Protonation constants in various ionic media at different ionic strengths, EDH parameters, SIT interaction coefficients, and Pitzer parameters were calculated, together with protonation constants at infinite dilution:  log T = pTKa4 = 10.135 ± 0.004, log T = pTKa3 = 9.077 ± 0.004, log T = pTKa2 = 3.713 ± 0.004, and log T = pTKa1 = 2.124 ± 0.004 (in the molar concentration scale, ± 3 standard deviation).