# Binding of Arsenic by Common Functional Groups: An Experimental and Quantum-Mechanical Study
## 3 Results
### Quantum-Mechanical Calculations
This way, the coefficient associated with the S atom of the thiol group in the (1,1) interaction was equal to 1.24, whilst those of the N atom of the amino and the O atom of the carboxylic groups were equal to 1.34 and 1.39, respectively, as listed in Table 3. In accordance with the experimental findings, such an estimate quantitatively indicates that the thiol group is a better chelating agent than the remainder functional groups under the simplified conditions reproduced in the quantum-mechanical calculations. With the aim of evaluating potential competitive effects exhibited by the investigated groups in cysteine toward the binding of As(III) species, the previous analysis was also  
Figure 4: Molecular structures of the complexes formed by the direct one-to-one binding of As(III) with the thiol (**a**), amino (**b**), and carboxylic (**c**) groups optimized at the B3 LYP/6-311++G(d,p) under implicit solvation. Relevant optimal interatomic distances are shown in Å.  
conducted considering all permutations of the (2,1) interaction. The quantum-mechanically optimized molecular structures are shown in Figure 5 along with the relevant interatomic distances. The concerted chelation process of As(III) by the thiol and amino groups (Figure 5a) revealed that the arsenic atom lay at a shorter distance from the sulfur atom with respect to the nitrogen atom (i.e., 2.27 A vs. 2.73 A). Considering the fact that the size of the S atom is by its own nature larger than that of N, coefficients c equal to 1.24 and 1.69 can be associated to their respective functional groups once the ground-state distances are rescaled by the respective van der Waals radii. In other words, under a competitive situation, the binding capabilities of the thiol group remain unaltered, whilst those of the amino group are further weakened (i.e., c of the N atom was modified from 1.34 to 1.69) by the propensity of the S atom to attract the As(III) one. When the concerted chelation of As(III) by the thiol and the carboxylic groups was taken into account, no significant modifications with respect to the (1,1) interaction were recorded. In fact, once the ground-state distances displayed in Figure 5b were rescaled, coefficients equal to 1.23 and 1.38 were obtained for the SH and the COOH groups, respectively. This finding clearly and quantitatively manifests the superior binding capabilities of SH toward As(III) also with respect the COOH group. Incidentally, the fact that the dimensionless coefficients associated with these groups remain substantially unaltered with respect to the (1,1) interactions implicitly indicates the concrete possibility of a simultaneous combined binding of As(III) by the thiol and the carboxylic groups. After all, this kind of chelation modality of arsenic has also been observed in systems such as thiolactic and thiomalic acids when an ab initio treatment of the solvent is employed [14], even though important nuclear quantum effects [33] were overlooked in that analysis. Finally, Figure 5c shows that under the idealized conditions reproduced in these calculations--completely neglecting the key role played by the pH--the ground-state separation between the nitrogen and the arsenic atoms was larger than that between the oxygen atom of the carboxylic group and As(III). Moreover, once rescaled, these distances reveal that the coefficient \(c\) associated with N was equal to 1.51, whereas that of O was 1.40, as summarized in Table 3. \begin{table}
\begin{tabular}{c c c} \hline _c=dIr\({}_{VdW}\)_ & **(1,1) Interaction** & **(2,1) Interaction** \\ \hline
**SH** & 1.24 & 1.23–1.24 \\ \hline
**COOH** & 1.39 & 1.38–1.40 \\ \hline
**NH\({}_{2}\)** & 1.34 & 1.51–1.69 \\ \hline \end{tabular}
\end{table} 
Table 3: Coefficients associated with the binding capabilities of the different functional groups here investigated (SH, COOH, NH\({}_{2}\)) in the cysteine molecule and for the (1,1) and (2,1) interaction with As(III).