BINDING ABILITY OF SOME AMINOACIDS AND NUCLEOTIDES TOWARDS As(III): A COMPARISON

The increase of heavy metal concentration in the environment, especially for anthropogenic reasons, is source of serious concern regarding human health and global ecosystems. As it is well known, the toxicity, bioavailability and mobility of a metal are linked to the physicochemical form in which the element is present in an existing sample and, thus, a speciation analysis is required to better evaluate all these phenomena. As regards arsenic, in fact, it can be found in both organic and inorganic forms. Among these ones, the inorganic species are the most harmful and, in particular, arsenic in its trivalent oxidation state, since it shows the capability to interact with the sulfhydryl residues of the proteins, hindering their biological functions. Unfortunately, this metalloid can reach human organism through many pathways but it seems that the main source of arsenic intake is to be found in the consumption of contaminated water and food. In order to investigate the behavior of As(III) in presence of biological molecules, a thermodynamic study was performed in aqueous solution by using some aminoacids and nucleotides as ligands. The aim is also to enrich the existent literature, since to our knowledge, no data are reported. For each one, the speciation model was determined by means of potentiometry at I = 0.15 mol L-1 and t = 25 °C. In order to analyze the binding ability of all the ligands towards the metal, the pL0.5, an empirical parameter already proposed by the research group, was employed. It represents the total ligand concentration required to sequester the 50% of a metal present in traces and it takes into account all the variables that could affect the estimation of the complexing capability, such as the acid-base properties of the ligand, the hydrolysis of the metal and all the potential interactions with other components in solution. It is represented by plotting the mole fraction of the complex species vs. pL, where pL = -log CL (CL = total ligand concentration). Graphically, it is a sigmoid curve with asymptote 1 for pL → - ∞ and 0 for pL → + ∞. The study of the sequestering ability was performed at I = 0.15 mol L-1, t = 25 °C and at various pH values, in order to establish in which conditions the ligands better interact with As(III).