# Binding of Arsenic by Common Functional Groups: An Experimental and Quantum-Mechanical Study
## 4 Conclusions  
Although the interactions between arsenic (As(III)) and thiol groups are quite established in the literature, potential interactions of As(III) with carboxylic and amino groups, to the best of our knowledge, have never extensively been investigated. With the aim of evaluating the strength underlying the single interactions and, above all, the effects of the simultaneous presence of different functional groups in competitive environmental situations, we here present a study on the interactions established by As(III) with carboxylic acids, amines, and amino acids. Detailed information on these interactions is useful for predictive purpose and, therefore, in all cases (such as removal processes from natural matrices or chelation therapy) in which the knowledge of the form under which arsenic species are present can help to simplify/optimize specific experimental procedures. Firstly, studies performed on carboxylic and amino ligands have evidenced the formation of simple AsLH\({}_{i}\) species with weak formation constants, which slightly increase with increasing the number of carboxylic and/or amino groups in the molecule. The proposed models, determined from experimental measurements performed varying the concentration of arsenic; the concentration of ligands; and, above all, their concentration ratio, provide the possibility to simulate arsenic speciation in any real situation. Moreover, quantum-mechanical calculations, carried out at the B3 LYP/6-311+G(d,p) Density Functional Theory (DFT) level and executed on a prototypical monocarboxylic species, have shown that the carboxylate group is able to displace one hydroxyl group in As(OH)\({}_{3}\) and to directly bind to As(III). By contrast, analogous quantum-mechanical calculations on the amino groups have not been useful to characterize the interactions because of the positively charged nature of the amino group (under the protonated NH\({}_{3}\)\({}^{+}\) form) in a wide pH range. This has led us to suppose, on the basis of the obtained experimental results, that an electrostatic interaction occurs between the As(OH)\({}_{2}\)O\({}^{-}\) (or H\({}_{2}\)AsO\({}_{3}\)\({}^{-}\)) and the RNH\({}_{3}\)\({}^{+}\) cation. Studies on amino acids containing the carboxylic, amino (glycine, aspartic acid, and lysine), and also the thiol group (cysteine) have confirmed that, under a competitive situation where -SH, -COOH, and -NH\({}_{2}\) groups are simultaneously present, the thiol group maintains its superior binding capabilities toward As(III). However, the empirical relationship (4) useful for a rough estimate of the stability of the species evidenced that, despite the thiol group mostly contributing to the stability of the species, the contribution of the other functional groups is two orders of magnitude lower, but not negligible for the calculation of the total stability of the species with polyfunctional ligands. Quantum-mechanical calculations performed on cysteine as a prototypical species containing the thiol, carboxylic, and amino groups indicate the concrete possibility of a simultaneous combined binding of As(III) by the thiol and the carboxylic groups. Finally, the superior capabilities of the sulfur atom of the thiol group in directly binding As(III) with respect to the atoms of the carboxylic and amino groups were quantitatively witnessed, within this DFT-based analysis, both under a simple direct interaction and under competitive more complex chelation modalities. On the other hand, the possibility of the simultaneous chelation of As(III) by the three functional groups here investigated was ruled out by the current calculations.