Understanding the Solution Behavior of Epinephrine in the Presence of Toxic Cations: A Thermodynamic Investigation in Different Experimental Conditions

Abstract

The interactions of epinephrine ((R)-(-)-3,4-dihydroxy-α-(methylaminomethyl)benzyl alcohol; Eph^-) with different toxic cations (methylmercury(II): CH₃Hg⁺; dimethyltin(IV): (CH₃)₂Sn²⁺; dioxouranium(VI): UO₂²⁺) were studied in NaCl_aq at different ionic strengths and at T = 298.15 K (T = 310.15 K for (CH₃)₂Sn²⁺). The enthalpy changes for the protonation of epinephrine and its complex formation with UO₂²⁺ were also determined using isoperibolic titration calorimetry: H_HL = -39 ± 1 kJ mol^-1, H_H2L = -67 ± 1 kJ mol^-1 (overall reaction), H_ML = -26 ± 4 kJ mol^-1, and H_M2L2(OH)2 = 39 ± 2 kJ mol^-1. The results were that UO₂²⁺ complexation by Eph^- was an entropy-driven process. The dependence on the ionic strength of protonation and the complex formation constants was modeled using the extended Debye-Hückel, specific ion interaction theory (SIT), and Pitzer approaches. The sequestering ability of adrenaline toward the investigated cations was evaluated using the calculation of pL_0.5 parameters. The sequestering ability trend resulted in the following: UO₂²⁺ >> (CH₃)₂Sn²⁺ > CH₃Hg⁺. For example, at I = 0.15 mol dm^-3 and pH = 7.4 (pH = 9.5 for CH₃Hg⁺), pL_0.5 = 7.68, 5.64, and 2.40 for UO₂²⁺, (CH₃)₂Sn²⁺, and CH₃Hg⁺, respectively. Here, the pH is with respect to ionic strength in terms of sequestration.

Keywords: dependence on ionic strength; enthalpy and entropy changes; epinephrine; sequestering ability; toxic cations.