Thermodynamic and spectroscopic study of Al3+ interaction with glycine, l-cysteine and tranexamic acid in aqueous solution

- Study of Al3+-amino acids interaction in NaCl at different ionic strengths.
- ΔH values of Al3+-amino acids species determined by titration calorimetry.
- Stability and speciation models confirmed by spectrophotometric and 1H NMR titrations.
- Empirical equation for the individual contribution estimate of complex stability of carboxylate, amino and thiolate groups.
- Sequestering ability of different amino acids towards Al3+ at different pH values.

In this paper a thermodynamic and spectroscopic study on the interaction between Al3+ and glycine (Gly), l-cysteine (Cys), tranexamic acid (Tranex) is reported. Speciation models have been obtained by processing potentiometric titration data to determine stability constants of the species formed in aqueous solution at T = 298.15 K, 0.15 ≤ I/mol L− 1 ≤ 1 in NaCl. Thermodynamic formation parameters have been obtained from calorimetric titration data, at T = 298.15 K, I = 0.15 mol L− 1 using NaCl as ionic medium. Al3+-Cys system was also investigated by spectrophotometric and 1H NMR measurements. 1H NMR experiments were performed on Al3+-Tranex system as well. Different speciation models have been observed for the three systems. The results showed the formation of MLH, ML and M2L2(OH)2 species for Gly, ML, M2L and MLOH for Cys, MLH and MLOH for Tranex. The formed species are quite stable, i.e. for ML, logβ = 7.18, 11.91 for Gly and Cys, respectively, at I = 0.15 mol L− 1 and T = 298.15 K. For all the systems the dependence of formation constants on ionic strength over the range 0.1-1 mol L− 1 is reported. The sequestering ability of the ligands under study was also evaluated by pL0.5 empiric parameter. For Gly, Cys and Tranex, pL0.5 = 2.51, 3.74, 3.91 respectively, at pH = 5, I = 0.15 mol L− 1 and T = 298.15 K.

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