# Binding of Arsenic by Common Functional Groups: An Experimental and Quantum-Mechanical Study
## 3 Results
### Quantum-Mechanical Calculations  
As anticipated in the Methods section, two main chelation (de-hydration) reactions were considered and characterized. The first one was related to the complexation process of As(III) by involving the thiol functional group (reaction (1)). From quantum-mechanical calculations performed at the B3 LYP/6-311++G(d,p) level in the presence of the conductive polarizable continuum model (CPCM) mimicking the water electrostatics, it turns out that a system initially composed of CH\({}_{3}\)CH\({}_{2}\)SH and arsenous acid (As(OH)\({}_{3}\)) underwent an energetic advantage (\(\Delta\)E) of \(-4.60\) kcal/mol upon chelation. Similarly, but in a limited fashion, reaction (2), involving the complexation of As(III) by means of the carboxylic group, was also exergonic, with \(\Delta\)E = \(-2.15\) kcal/mol. Figure 3: Calculated logK values for As(III) -S, -O-, and -N donor ligand interactions vs. experimental ones. However, in such a case, the energetic gain was lower than that recorded for the thiol group case, indicating that this latter group chelated arsenic species more robustly and efficiently than the carboxylic group, a finding adherent with the experimental results. As explained in detail in the Methods section, the respective reaction involving the amino group cannot reliably and quantitatively be characterized by a similar analysis, mainly because of the positively charged nature of NH\({}_{3}\)+, over a wide pH range. To set the pH in quantum-mechanical simulations, computationally expensive ab initio simulations under explicit solvation similar to those some of ours reported for other systems [26; 29; 30; 31] will be considered in the future as an extension of the current work. In order to have a more complete picture shedding some light also to the chelation capabilities of the amino group with respect to the SH and COOH groups, a series of additional calculations at the B3 LYP/6-311++G(d,p) level with the CPMC water model was executed. In particular, the cysteine molecule was employed as a prototypical species possessing all the functional groups under investigation. This way, a one-to-one (1,1) interaction with each group at a time with As(III); a two-to-one (2,1) interaction considering each pair of groups simultaneously chelating As(III); and a three-to-one (3,1) interaction, where all the groups actively and simultaneously bind As(III), were considered. Although in this latter case (i.e., the (3,1) interaction), no stable configurations were identified, and the remainder cases allowed for the identification of a fundamental ranking of the binding capabilities of SH, NH\({}_{2}\), and COOH toward As(III) under ideal conditions. In fact, optimized molecular structures of the one-to-one (1,1) interaction of As(III) with the three functional groups are shown in Figure 4. The ground-state distance at which arsenic lies when it interacts directly with the thiol group was 2.27 A (Figure 4a), whereas such a value varied to 2.15 A and to 1.91 A in presence of the amino (Figure 4b) and of the carboxylic group (Figure 4c), respectively. Albeit these absolute values give a rough idea of the strength of the formed bonds per se, due to the different natural size of the atoms involved, in each case, these bond lengths have to be rescaled by the Van der Waals radii of the sulfur atom of the thiol group (i.e., 1.83 A), of the nitrogen atom of the amino group (i.e., 1.62 A), and of the oxygen atom of the carboxylic group (i.e., 1.37 A) [32]. The resulting dimensionless coefficient \(c\)=\(d\)/\(r_{vdW}\) provided, indeed, an intuitive and useful measure of the strength of the binding. Of course, at least in principle, the smaller the coefficient, the higher the bond strength.