Thermodynamic Data for the Modeling of Lanthanoid(III) Sequestration by Reduced Glutathione in Aqueous Solution

This contribution reports the results of an investigation on the speciation of various lanthanoid(III) cations (Ln3+), namely La3+, Nd3+, Sm3+, Eu3+, Gd3+, Dy3+, Yb3+, in the presence of reduced glutathione (or simply glutathione, GSH), in NaCl(aq) at I = 0.15 mol dm-3 and T = 298.15 K. GSH forms in all cases the same four species, namely: Ln(GSH)H3, Ln(GSH)H2, Ln(GSH)H, and Ln(GSH)OH, whose stability depends on the type of lanthanoid, although it is also possible to consider an average stability for a generic lanthanoid, at least for the protonated species. Measurements at different temperatures and ionic strengths for the La3+/GSH and Gd3+/GSH systems have also been performed, in order to evaluate their effect on the speciation of both GSH and the Ln3+ cations. The dependence of the determined stability constants on I and T has been modeled by an Extended Debye-Hückel type and by the van’t Hoff equations. In this last case, the formation enthalpy changes of La3+/GSH species have also been determined. The stability constants obtained have been therefore used to quantify the sequestering ability of GSH toward the investigated cations, in different pH, ionic strength and temperature conditions, by the calculation of several pL0.5 parameters. Their analysis showed that, generally, the sequestering ability of GSH increases with increasing pH and decreases with increasing temperature, while the ionic strength has only a small effect. Finally, results obtained may be exploited for the evaluation of the speciation (and, therefore, of the sequestration) of any Ln3+ cations in the presence of GSH in different conditions of T and I, like those featuring many natural waters and biological fluids.