# Binding of Arsenic by Common Functional Groups: An Experimental and Quantum-Mechanical Study
## 3 Results  
### As(III)-Ligand Interactions  
The interactions between As(III) and each ligand were determined by potentiometric titrations. Data were elaborated taking into account the acid-base behavior of As(III) [26] and of the ligands at the same ionic strength and temperature conditions (in NaCl at I = 0.15 mol/L and T = 298.15 K). As is known from the literature [26], As(III) in water is present as As(OH)\({}_{3}\) but, for the sake of simplicity, it will be denoted as As throughout the manuscript. Literature data are summarized in Table S1 of the Supplementary Materials. Considering the extremely variable concentration of As(III) in natural systems, experimental measurements were performed, varying the concentration of arsenic; the concentration of ligands; and, above all, their concentration ratio, in order to evaluate the possible formation of species with different stoichiometries. In any case, for each class of ligands, the speciation models were featured by simple As-ligand species with different protonation degrees (AsLH\({}_{i}\) species, with 0 \(\leq\) i \(\leq\) 5). Results are resumed in Table 1, where the formation constant values log\(\beta\) referred to the overall formation equilibria are reported. To evaluate the stability of the species, in Table 2, data are reported as stepwise formation constants logK (As(OH)\({}_{3}\) + H\({}_{1}\)L). As can be observed, in the presence of carboxylic or amino groups, the stability of the species is always weak, with logK values falling in the range 0.83 \(\leq\) logK \(\leq\) 3.85. To better analyze the data, Table 2 also reports the total number of carboxylic (n\({}_{\rm COOH}\)), amino (n\({}_{\rm NH2}\)), and thiol (n\({}_{\rm SH}\)) groups in the molecule. In light of this, some general considerations can be made on the different ligand groups. Among the carboxylates, stability of the species generally increases with the n\({}_{\rm COOH}\) values and, therefore, from the di- (_mal_) to the hexa-carboxylic acids (_mlt_). Quantum-mechanical calculations simulated on a prototypical monocarboxylic species, and reported later in the text, show the displacement of the OH group in As(OH)\({}_{3}\) by the carboxylate, which directly binds to As(III). For _mlt_, the increase of the stability with the number of deprotonated carboxylic groups (hence from the AsLH\({}_{2}\) to AsL species) can probably be attributed to the involvement of many groups to the coordination (chelate effect). To highlight this, in Table 2, the number of deprotonated COO- groups (n\({}_{\rm COO}\)-) is also reported. If we consider the amines, a general increase of the stability can also be observed with the increase of the amino groups in the molecule (n\({}_{\rm NH2}\)), i.e., from mono- (_etan_) to penta-amine (_tetren_). In this case, it should be taken into account that, over a wide pH range, amino groups are protonated and, likely, an electrostatic interaction occurs between the As(OH)\({}_{2}\)O\({}^{-}\) (or H\({}_{2}\)AsO\({}_{3}\)\({}^{-}\)) and the RNH\({}_{3}\)\({}^{+}\) cation. The electrostatic nature is confirmed by the increase of stability (logK) for the same amine with the increase of the protonation degree (n\({}_{\rm NH3+}\)) and, therefore, of the positive charges on the molecule. Unfortunately, for these systems, the computational calculations, not being able to control the pH and therefore the protonation degree of the amine, did not allow us to reliably simulate the respective interaction modalities. The increase of stability with the number of carboxylic or amino groups was also observed for amino acids with the increase of logK values from _gly_, which contains one carboxylic and one amino group, to _asp_ and to _lys_, which contain a further carboxylic and amino group, respectively. Finally, to evaluate the effect of the thiol group present only in the _cys_ ligand, in Table 2, we also report some recent values referring to the interaction between As(III) and some ligands containing the thiol group, such as _tla_, _tma_, and _dmsa_ (Figure 1).