## 3 Results and Discussion
### ESI-MS of Uranyl Complexes with Glycine, Aspartic Acid, Cysteine, and Histidine
#### 3.2.1 Cationic Uranyl-Amino Acid Complexes  
The main cationic uranyl-amino acid complexes identified were [UO\({}_{2}\)(AA-H)(AA)\({}_{2}\)]\({}^{+}\) and [UO\({}_{2}\)(Cl)(AA)\({}_{2}\)]\({}^{+}\); [UO\({}_{2}\)(AA-H)(AA)]\({}^{+}\) and [UO\({}_{2}\)(Cl)(AA)]\({}^{+}\) were minor species not examined by CID. Table 1 summarizes the CID results in positive ion mode.  
For all four amino acids, CID of [UO\({}_{2}\)(AA-H)(AA)\({}_{2}\)]\({}^{+}\) led to the elimination of a neutral amino acid followed by immediate water addition on the \(\sim\)40 ms CID timescale, forming [UO\({}_{2}\)(AA-H)(AA)(H\({}_{2}\)O)]\({}^{+}\) for all AAs except His. Figure 3 shows a representative mass spectrum for the case of [UO\({}_{2}\)(Gly-H)(Gly)\({}_{2}\)]\({}^{+}\), while Figures S7-S9 present spectra for the other amino acids. Additional observed fragmentation channels were CO\({}_{2}\)-loss for the Gly and Asp complexes and NH\({}_{3}\)-loss for the Cys complex.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Precursor Complex**} & \multicolumn{4}{c}{**Neutral Losses (\%)**} \\ \cline{2-5}  & **Gly** & **Asp** & **Cys** & **His** \\ \hline \multirow{2}{*}{[UO\({}_{2}\)(Cl)(AA)\({}_{2}\)]\({}^{+}\)} & -HCl (50) & -HCl (100) & -HCl (50) & -HCl (100) \\  & -HCl-CO\({}_{2}\) (50) & -HCl-NH\({}_{3}\) (50) & -HCl (100) \\ \hline \multirow{2}{*}{[UO\({}_{2}\)(AA-H)(AA)\({}_{2}\)]\({}^{+}\)} & -AA (50) & -CO\({}_{2}\) (10) & -NH\({}_{3}\) (55) & \multirow{2}{*}{-AA (100)} \\  & -AA-CO\({}_{2}\) (50) & -AA (75) & -AA (25) & -AA (100) \\ \hline \hline \end{tabular}
\end{table}
Table 1: CID of cationic uranyl–amino acid complexes.  
CID of all [UO\({}_{2}\)(Cl)(AA)\({}_{2}\)]\({}^{+}\) except the His case led to the elimination of HCl with the immediate formation of [UO\({}_{2}\)(AA-H)(AA)(H\({}_{2}\)O)]\({}^{+}\). These were again, as commented above, considered water adducts and not complexes with hydroxide and two neutral AAs. Figure 4 shows the CID mass spectrum of [UO\({}_{2}\)(Gly-H)(Gly)\({}_{2}\)]\({}^{+}\), while Figures S10-S12 present spectra for the remaining cases. Additional fragmentation channels were CO\({}_{2}\)-loss for the Gly complex and NH\({}_{3}\)-loss for the Cys complex.  
For uncoordinated protonated amino acids, loss of NH\({}_{3}\) is typical of sulfur-containing ones such as cysteine, and loss of H\({}_{2}\)O+CO is prevalent for most others [40; 41; 42; 43]. Under the present experimental conditions, these were also the main fragmentations observed for protonated Gly, Asp, Cys, and His (see Figures S13-S16 in the SI). Loss of CO\({}_{2}\) (decarboxylation) is a common feature of gas-phase metal complexes with carboxylate ligands [44], and studies with such uranyl complexes have shown decarboxylation as an effective route to remarkable uranyl species [45; 46; 47].  
Formation of [UO\({}_{2}\)(AA-H)(AA)(H\({}_{2}\)O)]\({}^{+}\) by prompt hydration of [UO\({}_{2}\)(AA-H)(AA)]\({}^{+}\) for AA = Gly, Asp, and Cys, but not His, was compared through estimates of the reaction rates: \(k\)[Gly] \(\approx\)\(k\)[Cys] (very fast, \(>\)70 s\({}^{-1}\)) \(>\)\(k\)[Asp] (fast, \(\sim\)10 s\({}^{-1}\)) \(>>\)\(k\)[His] (no reaction up to 10 s). This comparison suggests that the [UO\({}_{2}\)(AA-H)(AA)]\({}^{+}\) complexes are under-coordinated for Gly, Asp, and Cys, but not for His, possibly pointing to an important role for the imidazole sidechain group of histidine in coordinating uranyl.  
Figure 4: CID mass spectrum of [UO\({}_{2}\)(Cl)(Gly)\({}_{2}\)]\({}^{+}\).  
Figure 3: CID mass spectrum of [UO\({}_{2}\)(Gly-H)(Gly)\({}_{2}\)]\({}^{+}\).