## 3 Results and Discussion
### ESI-MS of Uranyl Complexes with Glycine, Aspartic Acid, Cysteine, and Histidine
#### 3.2.2 Anionic Uranyl-Amino Acid Complexes  
The main identified anionic uranyl-amino acid complexes, [UO\({}_{2}\)(AA-H)\({}_{3}\)]\({}^{-}\), [UO\({}_{2}\)(Cl)(AA-H)\({}_{2}\)]\({}^{-}\), and [UO\({}_{2}\)(Cl)\({}_{2}\)(AA-H)]\({}^{-}\), were examined by CID. Table 2 summarizes the CID results in negative ion mode. Fragmentation of [UO\({}_{2}\)(AA-H)\({}_{3}\)]\({}^{-}\) led to the elimination of a neutral amino acid for Asp, Cys, and His, yielding [UO\({}_{2}\)(AA-2H)(AA-H)]\({}^{-}\) (see Figures S17-S19), which presumably contain a doubly deprotonated AA and conserve the +VI oxidation state of uranium. In contrast, CID of [UO\({}_{2}\)(Gly-H)\({}_{3}\)]\({}^{-}\) did not show the elimination of Gly but instead showed the elimination of (Gly-H), generating uranyl(V) in [UO\({}_{2}\)(Gly-H)\({}_{2}\)]\({}^{-}\), as shown in Figure 5. This distinctive behavior of the Gly complex likely reflects the absence of a sidechain group in Gly. In accord with gas-phase CID, solution acidities of amino acids (see Table S4) show higher acidities of sidechain groups (p\(K_{\rm c}\)) versus amino groups (p\(K_{\rm b}\)) for Asp, Cys, and His. The formation of a uranyl(V)-Gly complex is substantiated by the formation of an O\({}_{2}\)-addition product, [UO\({}_{2}\)(O\({}_{2}\))(Gly-H)\({}_{2}\)]\({}^{-}\), from reaction with background O\({}_{2}\) on the timescale of CID, a result analogous to O\({}_{2}\) addition previously observed for other anionic uranyl(V) complexes [48]. The reaction of O\({}_{2}\) with cationic and anionic uranyl(V) complexes to yield uranyl(VI) superoxides is well known [49; 50; 51; 52]. The rate of O\({}_{2}\) addition to [UO\({}_{2}\)(Gly-H)\({}_{2}\)]\({}^{-}\) was measured as \(k\) = 15 s\({}^{-1}\), which is comparable to rates previously reported under similar conditions for [U\({}^{\rm V}\)O\({}_{2}\)(X)\({}_{2}\)]\({}^{-}\) complexes comprising bidentate ligands [48]. The [U\({}^{\rm V}\)O\({}_{2}\)(Gly-H)\({}_{2}\)]\({}^{-}\) complex was also prone to hydrolysis by background water to form [U\({}^{\rm V}\)O\({}_{2}\)(OH)(Gly-H)\({}_{2}\)]\({}^{-}\), a phenomenon previously reported for other uranyl(V) complexes [52]. \begin{table}
\begin{tabular}{l c c c c} \hline \hline \multirow{2}{*}{**Precursor Complex**} & \multicolumn{4}{c}{**Neutral Losses (\%)**} \\ \cline{2-5}  & **Gly** & **Asp** & **Cys** & **His** \\ \hline
[UO\({}_{2}\)(Cl)\({}_{2}\)(AA-H)]\({}^{-}\) & -(AA-H) (55) -CO\({}_{2}\) (45) & -HCl (100) & -HCl (100) & -HCl (100) \\ \hline
[UO\({}_{2}\)(Cl)(AA-H)\({}_{2}\)]\({}^{-}\) & -(AA-H) (100) & -HCl (100) & -HCl (100) & -AA (25) -HCl (75) \\ \hline
[UO\({}_{2}\)(AA-H)\({}_{3}\)]\({}^{-}\) & -(AA-H) (100) & -AA (100) & -AA (100) & -AA (100) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: CID of anionic uranyl–amino acid complexes. Figure 5: CID mass spectrum of [UO\({}_{2}\)(Gly-H)\({}_{3}\)]\({}^{-}\). Interestingly, CID product [UO\({}_{2}\)(His-2H)(His-H)]\({}^{-}\) reacted quickly with background water to produce [UO\({}_{2}\)(OH)(His-H)\({}_{2}\)]\({}^{-}\) during the experiment, while no such hydrolysis was observed for Asp and Cys complexes (see Figures S17-S19). Reactions of [UO\({}_{2}\)(AA-2H)(AA-H)]\({}^{-}\) complexes with water are supposed to yield [UO\({}_{2}\)(OH)(AA-H)\({}_{2}\)]\({}^{-}\) and not [UO\({}_{2}\)(AA-2H)(AA-H)(H\({}_{2}\)O)]\({}^{-}\) as the second removed proton is clearly less acidic than the first; although there are no gas-phase data to substantiate this assumption, solution data may provide this indication (see p\(K_{\rm a}\) and p\(K_{\rm c}\) in Table S4). Estimated reaction rates with water, \(k\)[His] (fast, \(\sim\)11 s\({}^{-1}\)) >> \(k\)[Asp] \(\approx\)\(k\)[Cys] (no reaction up to 10 s), may suggest that the binding strength of doubly deprotonated ligands to uranyl is lower for histidine versus aspartic acid or cysteine. These differences in rates may as well reflect kinetic barriers due to differences in coordination of the doubly deprotonated amino acids.