## 4 Conclusions  
ESI-MS proved to be an efficient method for producing amino acid complexes of uranyl(VI) in the gas phase, with illustrative results presented for glycine, aspartic acid, cysteine, and histidine. CID and water reactivity provided insight into amino acid coordination and relative binding affinities. In negative ion mode ESI-MS, the comparative intensities of [UO\({}_{2}\)(AA-H)\({}_{3}\)]\({}^{-}\), [UO\({}_{2}\)(Cl)(AA-H)\({}_{2}\)]\({}^{-}\), and [UO\({}_{2}\)(Cl)\({}_{2}\)(AA-H)]\({}^{-}\) apparently correlate with gas-phase acidities of the amino acids. In positive ion mode, relative intensities of [UO\({}_{2}\)(AA-H)(AA)\({}_{2}\)]\({}^{+}\), [UO\({}_{2}\)(Cl)(AA)\({}_{2}\)]\({}^{+}\), and [UO\({}_{2}\)(Cl)\({}_{2}\)(AA)]\({}^{+}\) did not reveal such a clear trend. Glycine complexes showed distinctive behavior, with apparent weaker binding attributed to the absence of a sidechain. CID in negative ion mode demonstrated these differences in that dissociation of [UO\({}_{2}\)(AA-H)\({}_{3}\)]\({}^{-}\) did not yield [UO\({}_{2}\)(AA-2H)(AA-H)]\({}^{-}\). CID of chloride/AA complexes in both ion modes displayed dominant HCl elimination, except for the case of glycine. Further insight was provided by the reactivity of CID products with background water. A rapid reaction was observed for [UO\({}_{2}\)(AA-H)(AA)]\({}^{+}\) yielding [UO\({}_{2}\)(AA-H)(AA)(H\({}_{2}\)O)]\({}^{+}\) for AA = Gly, Asp, and Cys, but not His. This suggests that [UO\({}_{2}\)(AA-H)(AA)]\({}^{+}\) are under-coordinated for Gly, Asp, and Cys, but not His, probably due to uranyl coordination by the imidazole sidechain of histidine. [UO\({}_{2}\)(AA-2H)(AA-H)]\({}^{-}\) produced by CID reacted with water in the rate order His >> Cys \(\sim\) Asp, which suggests that the binding strength of doubly deprotonated histidine to uranyl is lower than that for the other amino acids. Competitive CID of uranyl species with mixed AAs provided additional insights. In positive ion mode, CID and subsequent reaction with water suggested that the strength of uranyl-neutral AA interactions follows the order His > Asp > Cys > Gly, which parallels the amino acid proton affinities. In negative ion mode, CID of [UO\({}_{2}\)(AA-H)\({}_{2}\)(AA\({}^{\prime}\)-H)]\({}^{-}\) and [UO\({}_{2}\)(AA-H)(AA\({}^{\prime}\)-H)\({}_{2}\)]\({}^{-}\) revealed decreasing dissociation in the order Gly >> His \(\approx\) Cys > Asp, which presumably reflects enhanced binding of doubly deprotonated amino acids to uranyl in the reverse order. Understanding the interactions of uranyl ions with biomolecules is essential in environmental/radiological protection and toxicology. Gas-phase ion chemistry experiments such as those described herein, involving amino acids as elementary components of peptides and proteins, can provide qualitative information on the basic properties of key species. They may serve as a foundation for more elaborate studies addressing structure and energetics, using spectroscopic and computational techniques, and can be especially relevant when radioactive species are involved. The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app13063834/s1](https://www.mdpi.com/article/10.3390/app13063834/s1)]([https://www.mdpi.com/article/10.3390/app13063834/s1](https://www.mdpi.com/article/10.3390/app13063834/s1)), Additional ESI and CID mass spectra obtained in the study; schemes with structures of the studied amino acids; tables with summaries of ESI-MS results; tables with gas-phase and solution data for the studied amino acids. **Author Contributions:** [PERSON] and [PERSON] conducted the experiments and analyzed the results; [PERSON] and [PERSON] conceived and designed the study; [PERSON] wrote the original draft; [PERSON], [PERSON], and [PERSON] reviewed and edited the manuscript. All authors have read and agreed to the published version of the manuscript.