Investigations of coordinating properties of oxydiacetate and thiodiacetate anions towards Zn2+ ions in solutions

The isothermal titration calorimetry (ITC) method was used to study the coordinating properties in solutions of oxydiacetate (ODA) and thiodiacetate (TDA) anions towards the Zn2+ ion. The calorimetric measurements (ITC) were run in a Mes buffer with a pH of 6.0 at 298.15 K. Furthermore, the potentiometric titration (PT) technique as well as ITC displacement titrations were applied to obtain conditional-independent thermodynamic parameters of the interaction of Zn2+ with ODA (ΔH = +3.49 [kcal/mol], ΔS = 30.30 [cal/mol K]) and TDA (ΔH = +2.93 [kcal/mol], ΔS = 25.99 [cal/mol K]). The MP2 method with the 6–311++G(d,p) basis set and the polarized continuum (PCM) solvation model were used to compare the relative stabilities of mer and fac conformations of the ODA and TDA ligands in the octahedral coordination sphere of the zinc ion. The relationship between the proposed coordination modes of the ligands and the thermodynamic parameters has been discussed.