## 3 Results and Discussion
### ESI-MS of Uranyl Complexes with Glycine, Aspartic Acid, Cysteine, and Histidine
#### 3.3.2 Competitive CID and Reactivity of Anionic Uranyl Mixed Amino Acid Complexes  
Table 4 summarizes the results of competitive CID experiments in negative ion mode. It was anticipated that complexes with one chloride and two deprotonated amino acids, [UO\({}_{2}\)(Cl)(AA-H)(AA\({}^{\prime}\)-H)]\({}^{-}\), could illuminate the relative binding strengths of different AAs to uranyl cations. However, as CID elimination of HCl was dominant, as for [UO\({}_{2}\)(Cl)(AA-H)\({}_{2}\)]\({}^{-}\), these experiments were not particularly el \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multirow{3}{*}{**Precursor Complex**} & \multicolumn{6}{c}{**Neutral Losses (\%)**} \\ \cline{2-7}  & \multicolumn{6}{c}{**AA/AA\({}^{\prime}\)**} \\ \cline{2-7}  & **Asp/Gly** & **Cys/Gly** & **His/Gly** & **Asp/Cys** & **Asp/His** & **His/Cys** \\ \hline \multirow{3}{*}{[UO\({}_{2}\)(Cl)(AA-H)(AA\({}^{\prime}\)-H)]\({}^{-}\)} & -HCl (90) & -HCl (45) & -HCl (50) & -HCl (100) & -HCl (100) & -HCl (100) \\  & -AA\({}^{\prime}\) (10) & -AA\({}^{\prime}\) (55) & -AA\({}^{\prime}\) (50) & -HCl (100) & -HCl (100) & -HCl (100) \\ \hline \multirow{3}{*}{[UO\({}_{2}\)(AA-H)\({}_{2}\)(AA\({}^{\prime}\)-H)]\({}^{-}\)} & -AA\({}^{\prime}\) (100) & -AA\({}^{\prime}\) (100) & -AA (35) & -AA (10) & -AA (45) & -AA (85) \\  & -AA\({}^{\prime}\) (65) & -AA\({}^{\prime}\) (90) & -AA\({}^{\prime}\) (55) & -AA\({}^{\prime}\) (50) & -AA\({}^{\prime}\) (15) \\ \hline \multirow{3}{*}{[UO\({}_{2}\)(AA-H)(AA\({}^{\prime}\)-H)\({}_{2}\)]\({}^{-}\)} & -AA\({}^{\prime}\) (100) & -AA\({}^{\prime}\) (100) & -AA\({}^{\prime}\) (100) & -AA\({}^{\prime}\) (100) & -AA (15) \\  & -AA\({}^{\prime}\) (85) & -AA\({}^{\prime}\) (85) & -AA\({}^{\prime}\) (85) & -AA\({}^{\prime}\) (85) & -AA\({}^{\prime}\) (85) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Competitive CID of anionic uranyl-amino acid complexes. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multirow{3}{*}{**Precursor Complex**} & \multicolumn{6}{c}{**Neutral Losses (\%)**} \\ \cline{2-7}  & \multicolumn{6}{c}{**AA/AA\({}^{\prime}\)**} \\ \cline{2-7}  & **Asp/Gly** & **Cys/Gly** & **His/Gly** & **Asp/Cys** & **Asp/His** & **His/Cys** \\ \hline \multirow{3}{*}{[UO\({}_{2}\)(AA-H)(AA\({}^{\prime}\))\({}_{2}\)]\({}^{+}\)} & \multirow{3}{*}{-AA\({}^{\prime}\) (100)} & -NH\({}_{3}\) (10) & \multirow{3}{*}{-AA\({}^{\prime}\) (55)} & \multirow{3}{*}{-AA\({}^{\prime}\) (100)} & \multirow{3}{*}{-AA\({}^{\prime}\) (100)} & \multirow{3}{*}{-AA\({}^{\prime}\) (100)} & \multirow{3}{*}{-AA\({}^{\prime}\) (100)} \\  & & -AA\({}^{\prime}\) (50) & -AA\({}^{\prime}\) (55) & -AA\({}^{\prime}\) (55) & & \\  & & -AA\({}^{\prime}\)-NH\({}_{3}\) (25) & -AA\({}^{\prime}\)-CO\({}_{2}\) (45) & & & \\  & & -AA\({}^{\prime}\)-NH\({}_{3}\)-CO\({}_{S55). When Gly-H was present, as in [UO\({}_{2}\)(Cl)(AA-H)(Gly-H)]\({}^{-}\), loss of Gly but not (Gly-H), as in [UO\({}_{2}\)(Cl)(Gly-H)\({}_{2}\)]\({}^{-}\), was observed, showing altered behavior due to the presence of an amino acid with a sidechain predisposed to deprotonation. CID of [UO\({}_{2}\)(AA-H)\({}_{2}\)(AA\({}^{\prime}\)-H)]\({}^{-}\) and [UO\({}_{2}\)(AA-H)(AA\({}^{\prime}\)-H)\({}_{2}\)]\({}^{-}\) resulted in the loss of a neutral amino acid in all cases (see Figures S56-S67), with an overall decreasing loss trend of Gly >> His \(\approx\) Cys > Asp. This ordering probably reflects the coordination strength of the residual doubly deprotonated amino acid ligand, which evidently increases from Gly, with no sidechain to deprotonate, to Asp, with a rather acidic sidechain. Reactivity of [UO\({}_{2}\)(AA-2H)(AA\({}^{\prime}\)-H)]\({}^{-}\) from CID with background water was in essential accord with the previous observation that only [UO\({}_{2}\)(His-2H)(His-H)]\({}^{-}\) reacted efficiently, to produce [UO\({}_{2}\)(OH)(His-H)\({}_{2}\)]\({}^{-}\), while no hydrolysis was observed for Asp and Cys complexes.