Ca Complexation With Relevant Bioligands in Aqueous Solution: A Speciation Study With Implications for Biological Fluids.

A speciation study on the interaction between Ca and ligands of biological interest in aqueous solution is reported. The ligands under study are l-cysteine (), d-penicillamine (), reduced glutathione (), and oxidized glutathione (). From the elaboration of the potentiometric experimental data the most likely speciation patterns obtained are characterized by only protonated species with a 1:1 metal to ligand ratio. In detail, two species, CaLH and CaLH, for systems containing , , and , and five species, CaLH, CaLH, CaLH, CaLH, and CaLH, for system containing , were observed. The potentiometric titrations were performed at different temperatures (15 ≤ /°C ≤ 37, at = 0.15 mol L). The enthalpy and entropy change values were calculated for all systems, and the dependence of the formation constants of the complex species on the temperature was evaluated. H NMR spectroscopy, MALDI mass spectrometry, and tandem mass spectrometry (MS/MS) investigations on Ca-ligand solutions were also employed, confirming the interactions and underlining characteristic complexing behaviors of , , , and toward Ca. The results of the analysis of H NMR experimental data are in full agreement with potentiometric ones in terms of speciation models and stability constants of the species. MALDI mass spectrometry and tandem mass spectrometry (MS/MS) analyses confirm the formation of Ca-L complex species and elucidate the mechanism of interaction. On the basis of speciation models, simulations of species formation under conditions of some biological fluids were reported. The sequestering ability of , , , and toward Ca was evaluated under different conditions of pH and temperature and under physiological condition.