## 3 Results and Discussion
### ESI-MS of Uranyl Complexes with Glycine, Aspartic Acid, Cysteine, and Histidine
#### 3.2.2 Anionic Uranyl-Amino Acid Complexes
CID of [UO\({}_{2}\)(Cl)(AA-H)\({}_{2}\)]\({}^{-}\) led to the elimination of HCl for Asp, Cys, and His, again yielding [UO\({}_{2}\)(AA-2H)(AA-H)]\({}^{-}\) (see Figures S20-S22). Once more, [UO\({}_{2}\)(His-2H)(His-H)]\({}^{-}\) reacted rapidly with background water to produce [UO\({}_{2}\)(OH)(His-H)\({}_{2}\)]\({}^{-}\), while no reaction (up to 10 s) was observed for the Asp and Cys complexes. Intriguingly, CID of [UO\({}_{2}\)(Cl)(His-H)\({}_{2}\)]\({}^{-}\) also led to the elimination of His, concurrent with prompt reaction with water to give [UO\({}_{2}\)(Cl)(OH)(His-H)]\({}^{-}\), suggesting weaker binding of deprotonated histidine to uranyl compared to deprotonated aspartic acid or cysteine. CID of [UO\({}_{2}\)(Cl)(Gly-H)\({}_{2}\)]\({}^{-}\) was dissimilar to the corresponding complexes comprising the other amino acids, but in accord with results for [UO\({}_{2}\)(Gly-H)\({}_{3}\)]\({}^{-}\), as shown in Figure 6. Loss of (Gly-H) was the only observed fragmentation, forming a new uranyl(V) complex, [UO\({}_{2}\)(Cl)(Gly-H)]\({}^{-}\), which reacted with O\({}_{2}\) to yield [UO\({}_{2}\)(Cl)(O\({}_{2}\))(Gly-H)]\({}^{-}\) and H\({}_{2}\)O to produce [UO\({}_{2}\)(Cl)(OH)(Gly-H)]\({}^{-}\). The rate of O\({}_{2}\) addition to [UO\({}_{2}\)(Cl)(Gly-H)]\({}^{-}\) (\(k\) = 11 s\({}^{-1}\)) was slightly lower than that above for [UO\({}_{2}\)(Gly-H)\({}_{2}\)]\({}^{-}\) (\(k\) = 15 s\({}^{-1}\)). CID of [UO\({}_{2}\)(Cl)\({}_{2}\)(AA-H)]\({}^{-}\) mainly eliminated HCl for Asp, Cys, and His, forming [UO\({}_{2}\)(AA-2H)(Cl)]\({}^{-}\) (see Figures S23-S25), which reacted with background water to give [UO\({}_{2}\)(Cl)(OH)(AA-H)]\({}^{-}\) with very different rates: \(k\)[His] (very fast, \(>\)70 s\({}^{-1}\)) > \(k\)[Cys] (fast, \(\sim\)36 s\({}^{-1}\)) >> \(k\)[Asp] (slow, \(\sim\)1 s\({}^{-1}\)). Loss of (AA-H) followed by prompt hydrolysis to form [UO\({}_{2}\)(Cl)\({}_{2}\)(OH)]\({}^{-}\) was observed for all three complexes, but was clearly more dominant for the His complex. As observed in Figure 7, CID of [UO\({}_{2}\)(Cl)\({}_{2}\)(Gly-H)]\({}^{-}\) eliminated (Gly-H) to form a uranyl(V) species, [UO\({}_{2}\)(Cl)\({}_{2}\)]\({}^{-}\), which hydrolyzed to give [UO\({}_{2}\)(Cl)\({}_{2}\)(OH)]\({}^{-}\). In contrast to the other uranyl(V) complexes discussed above, reaction with background O\({}_{2}\) was not observed, in accord with previous results showing inefficient O\({}_{2}\) addition to [UO\({}_{2}\)(Cl)\({}_{2}\)]\({}^{-}\) [48]. Figure 6: CID mass spectrum of [UO\({}_{2}\)(Cl)(Gly-H)\({}_{2}\)]\({}^{-}\). Another important fragmentation channel for [UO\({}_{2}\)(Cl)\({}_{2}\)(Gly-H)]\({}^{-}\) was decarboxylation to yield [UO\({}_{2}\)(Cl)\({}_{2}\)(NHCH\({}_{3}\))]\({}^{-}\), which presumably comprises a methylamide. ESI-MS, Competitive CID, and Reactivity of Mixed Glycine, Aspartic Acid, Cysteine, and Histidine Complexes with Uranyl  
ESI-MS complexes comprising two or three amino acids--i.e., cationic [UO\({}_{2}\)(AA-H)(AA)\({}_{2}\)]\({}^{+}\) and [UO\({}_{2}\)(Cl)(AA)\({}_{2}\)]\({}^{+}\) and anionic [UO\({}_{2}\)(AA-H)\({}_{3}\)]\({}^{-}\) and [UO\({}_{2}\)(Cl)(AA-H)\({}_{2}\)]\({}^{-}\)- open up the possibility of mixed amino acid complexes that can be examined by CID to assess relative ligand affinities. Such complexes were prepared (see Figures S26-S37) using solutions with two amino acids (AA and AA\({}^{\prime}\)) in a 1:1 ratio; signal intensities were predictably lower than those for ESI-MS of solutions with only one amino acid. In positive ion mode, CID of complexes with one chloride and amino acids AA and AA\({}^{\prime}\), [UO\({}_{2}\)(Cl)(AA)(AA)\({}^{\prime}\)]\({}^{+}\), is expected to result in the elimination of HCl, as observed for [UO\({}_{2}\)(Cl)(AA)\({}_{2}\)]\({}^{+}\); as such processes would not be particularly illuminating, CID experiments were not pursued.