Dft Calculations and Raman Spectroscopy of As(III) Complexation with Thiol Ligands

The interest in the chemistry of arsenic complexes has increased in recent years. This metal is a natural pollutant, present in trace in the environment and living organisms. About 60% of arsenic is released by volcanic activity and largely derives from anthropogenic sources. It is well known, indeed, that arsenic compounds have several applications in many industrial fields, such as pesticides and herbicides in agriculture, as chemioterapic agent in medicine and as constituent of numerous consumer goods. For these reasons, humans are usually exposed in food, water, air and soil. Accordingly, the extensive arsenic contamination is considered one of the most important toxicological problem. The significance of the metal pollution, as well as its mobility and bioavailability, depends on arsenic speciation, which represents the distribution of the different chemical species in a given system. The predominant species in aqueous solutions are the inorganic As(III) and As(V) ones, derivatives of the arsenous and arsenic acids, respectively. It was found, already in the past, that arsenite(III) compounds are more poisonous than arsenate(V) ones and even more dangerous of its organic species. This toxicity is mainly due to its high affinity for the sulfhydryl groups present in active biomolecules, such as enzymes and proteins. Nevertheless, few data on the As(III) interaction with thiols are reported in the literature. In the light of these considerations, an investigation on the As3+ complexation with S-donor ligand in aqueous solution was performed, in particular with thiolactic (TLA) and thiomalic acids (TMA). This study is based on a speciation analysis on As3+-TLA and As3+-TMA systems and a Raman spectroscopic investigation combined with density functional calculations (DFT). Up to now, detailed Raman investigations of thiolactic and thiomalic ligands are missing in the literature. The Raman spectroscopy results combined with DFT calculations shed light on the arsenic chelation mechanisms with these S-donor ligands.