Study of As3+ interaction with thiols in aqueous solution

Arsenic is an ubiquitous element present in trace in all environmental compartments and in living organisms. About 60% of the arsenic is released by natural sources, as volcanic activity, but it is also due to human activities, since As compounds have numerous applications, for example in the glass and semiconductor industries, in agriculture, as pesticides and herbicide, and in many other fields. Accordingly, the environmental arsenic contamination represents one of the most important problem of environmental toxicology. Its toxicity, mobility and bioavailability depends on speciation, which represents the distribution of the element amongst defined chemical species in a system. Arsenic(III) compounds, for example, are more dangerous than those containing arsenic(V) owing to the high affinity for the proteins sulfhydryl groups. A possible bond with these proteins could alter their conformation and prevent the normal interaction with other functional proteins. Despite the importance of As3+ interactions with sulfhydryl groups, very few thermodynamic studies are reported in literature. In light of this, an investigation on As3+/S-donor ligand complexation was started in aqueous solution, with particular attention to physiological conditions. In this contribution, results obtained on As3+-TLA and As3+-TMA systems are reported. Studies were performed by different analytical techniques, such as potentiometry and spectrophotometry UV/Vis, in order to define the speciation model and all thermodynamic parameters concerning the species in solution, together with 1H NMR spectroscopy in order to gain information on coordination mode. The best speciation models obtained for the As-TLA and As-TMA systems are featured by five species: ML, MLH, ML2, ML2H and ML2H3 for As-TLA and ML, MLH2, MLH, M2L and MLOH for As-TMA system. For each ligand, the sequestering ability towards As(III) was determined using an empiric parameter known as pL0.5, that, numerically, represents the concentration of ligand required to sequester the 0.5 of metal fraction.