## 3 Results and Discussion  
### ESI-MS of Uranyl Complexes with Glycine, Aspartic Acid, Cysteine, and Histidine  
ESI-MS of mixtures of uranyl(VI) chloride and glycine (Gly), aspartic acid (Asp), cysteine (Cys), or histidine (His) in water/ethanol generated monopositive and mononegative complexes containing the UO\({}_{2}\)\({}^{2+}\) ion coordinated by one to three neutral (AA) or deprotonated ((AA-H)\({}^{-}\)) amino acids (Scheme S1 shows the structures of studied amino acids). The general formulas of the main complexes obtained are as follows: in the positive ion mode, [UO\({}_{2}\)(AA-H)(AA)\({}_{2}\)]\({}^{+}\), [UO\({}_{2}\)(AA-H)(AA)]\({}^{+}\), [UO\({}_{2}\)(Cl)(AA)\({}_{2}\)]\({}^{+}\), and [UO\({}_{2}\)(Cl)(AA)]\({}^{+}\); in the negative ion mode, [UO\({}_{2}\)(AA-H)\({}_{3}\)]\({}^{-}\), [UO\({}_{2}\)(Cl)(AA-H)\({}_{2}\)]\({}^{-}\), and [UO\({}_{2}\)(Cl)\({}_{2}\)(AA-H)]\({}^{-}\). Figures 1 and 2 show illustrative spectra for the cases of Cys in positive ion mode and His in negative ion mode, respectively, and Figures S1-S6 present spectra for the remaining cases in both ion modes. Tables S1 and S2 summarize the main results of ESI-MS in positive and negative ion modes, respectively, including \(m/z\) assignments. In some cases, in the positive ion mode, water-addition products were also observed (see Figures 1 and S1-S3). As precursor [UO\({}_{2}\)(AA-H)(AA)]\({}^{+}\) complexes were also present in the spectra, water-addition products were considered water adducts and not complexes with one hydroxide and two neutral AAs. Furthermore, simple water addition can prevail as the gas-phase acidity of H\({}_{2}\)O is 200-280 kJ mol\({}^{-1}\) larger than that of the AA (see Table S3 for a compilation of gas-phase data for the studied amino acids [32]; the gas-phase acidity of water is 1633 kJ mol\({}^{-1}\)[33]); this could compensate differences in relative binding strengths of OH\({}^{-}\) vs. (AA-H)\({}^{-}\) and H\({}_{2}\)O vs. AA to uranyl (there are no available data for this evaluation). Figure 1: ESI-MS of a uranyl chloride + cysteine solution (1:4) in positive ion mode. Uranyl complexes containing cystine (Cis) were identified in spectra obtained from UO\({}_{2}\)Cl\({}_{2}\)+Cys solutions, as cystine easily forms through condensation of two cysteine molecules that covalently link via a disulfide bond (see Figure S6; see also Scheme S2 that shows the structure of cystine). Comprising a soft ionization method, ESI-MS may directly reveal solution speciation of metal complexes. However, due to the intrinsically complex mechanism of electrospray, as well as perturbations during ion transfer from ambient to low pressures, and other factors, gas-phase species from ESI do not necessarily directly represent those in solution [34, 35, 36, 37]. As the focus is on gas-phase uranyl-amino acid complexes, the issue of solution speciation was mostly sidestepped. It is notable that in negative ion mode the relative abundance of [UO\({}_{2}\)(AA-H)\({}_{3}\)]\({}^{-}\) for the four different amino acids parallels their gas-phase acidities [32]. The most acidic, aspartic acid, displays [UO\({}_{2}\)(Asp-H)\({}_{3}\)]\({}^{-}\) as the base peak in the spectrum, whereas the least acidic, glycine, shows only minor [UO\({}_{2}\)(Gly-H)\({}_{3}\)]\({}^{-}\); cysteine and histidine are intermediate (see Figures 2 and S4-S6, and also Table S3). Gas- and condensed-phase properties are frequently dissimilar, with the acidity of amino acids being no exception as solution p\(K_{3}\)[38] indicates histidine as the most acidic, glycine again as the least acidic, and aspartic acid and cysteine as intermediate (see Figures 2 and S4-S6; Table S4 compiles solution data for the amino acids). Uranyl(VI), the linear U(VI) dioxide cation UO\({}_{2}\)\({}^{+}\), coordinates ligands in the equatorial plane with typical coordination numbers between 3 and 6, depending on ligand size, dentity, and binding strength.