Arsenic toxicity and its association with human diseases such as blackfoot disease, diabetes, hypertension and some kinds of cancers have been widely considered in the literature. It is well known that arsenic can be present in the environment as arsenate (As(V)), arsenite (As(III)), monomethylarsonic acid, dimethylarsinic acid, arsenobetaine and arsenosugar, all of which have different degrees of mobility, bioavailability and toxicity. Nowadays, arsenic can be found in certain water supplies, seafood, glues, pigments and cigarette smoke. In places where geological structure introduces arsenic in water, which includes North of Chile, Bangladesh among others, drinking water contamination is a serious problem.
Some chelating agents are used for the remediation of As poisoning. Of which sodium 2,3-dimercaptopropanesulfate (DMPS and meso-2,3-dimercaptosuccinic acid (DMSA) are the most usual. These compounds include in their structure thiol groups that have a large capacity to complex several metals and metalloids.
In a ﬁrst attempt, several trials with a gold electrode were carried out due to its spread use for the determination of As(III) using different electroanalytical echniques. However, thiol groups of the chelating agents had more afﬁnity for the Au electrode surface than for complexation in solution. To overcome this problem, the systems were studied using a Hg electrode and DPP as the
voltammetric measuring technique. Although the signals related with the reduction of arsenic complexes do not appear in the voltammograms (they are hidden by the hydrogen discharge), the literature indicates that the anodic signals related to the interaction between the complexes and the Hg electrode are clearly visible (they appear at less negativ potential values than hydrogen discharge) and can be useful to understand the complexation of As(III). The data obtained by DPP were analyzed by Multivariate Curve Resolution using Gaussian Peak Adjustment GPA) method, and exceptionally by Alternating Least Squares (MCR-ALS) for the non-peak-shaped part of the data matrix, which corresponds to the free As(III) signal. This method has been successfully applied to voltammetric data in previous works. The voltammetric results were complemented and compared with new ITC data, and with other data described elsewhere. Electrospray Ionization Mass Spectrometry (ESI-MS) revealed the stoichiometries and relativestability of the complexes.
In order to study the complexation of arsenic with DMSA and DMPS, voltammetric titrations in PBS pH 7.4 were performed. Theobtained data were analyzed chemometrically by MCR-ALS and GPA.
The voltammograms obtained for titration of As(III) with DMSA. Initially, a small signal (not peak-shaped) related to free As(III) appears at −1.6 V. After DMSA additions, ﬁve different signals are observed. These signals are related to the anodic oxidation of the Hg electrode and they are typically observed in complexation studies with thiol-containing ligands. Of these, the signal at −0.6 V is of particular interest indicating the presence of free DMSA (component 2). The other signals are related to thecomplexes formed in the solution, being signal 3 the most relevant.
- As(III)–DMPS system
The voltammograms obtained from the addition of As(III) to DMPS solution. For the sake of simplicity, a nomenclature similar to the previous case has been used. Initially, the plot shows the peak of the free DMPS (component 2). When As(III) is added this peak decreases, and four anodic signals related to As(III) complexes appear (components 1, 3, 4, and 5). Finally, the free As(III) signal (not peak-shaped, component 6) appears.
For the ﬁrst time the complexation of As(III) has been successfully studied by a multiple approach combining ITC, ESI-MS and an electroanalytical technique supported by chemometric tools. This methodology yields a reliable and consistent picture of the binding of As(III) by the chelating therapy agents DMSA and DMPS.
Santiago Cavanillas, Analytica Chimica Acta 746 (2012) 47-52