Examining Interactions of Uranyl(VI) Ions with Amino Acids in the Gas Phase

Gas-phase experiments, using electrospray ionization quadrupole ion trap mass spectrometry (ESI-QIT/MS), were conducted to probe basic interactions of the uranyl(VI) ion, UO22+, with selected natural amino acids, namely, L-cysteine (Cys), L-histidine (His), and L-aspartic acid (Asp), which strongly bind to metal ions. The simplest amino acid, glycine (Gly), was also studied for comparison. Cys, His, and Asp have additional potentially coordinating groups beyond the amino and carboxylic acid functional groups, specifically thiol in Cys, imidazole in His, and a second carboxylate in Asp. Gas-phase experiments comprised collision-induced dissociation (CID) of uranyl-amino acid complexes and competitive CID to assess the relative binding strength of different amino acids in the same uranyl complex. Reactivity of selected uranyl-amino acid complexes with water provided further insights into relative stabilities. In positive ion mode, CID and ensuing reactions with water suggested that uranyl-neutral AA binding strength decreased in the order His > Asp > Cys > Gly, which is similar to amino acid proton affinities. In negative ion mode, CID revealed a decreasing dissociation tendency in the order Gly >> His approximate to Cys > Asp, presumably reflecting a reverse enhanced binding to uranyl of the doubly deprotonated amino acids formed in CID.

Automated summary

Gas-phase experiments were conducted to study the basic interactions between the uranyl(VI) ion and selected amino acids using electrospray ionization quadrupole ion trap mass spectrometry. The results revealed that the binding strength of neutral amino acids to uranyl decreased in the order His > Asp > Cys > Gly, similar to their proton affinities. In negative ion mode, the dissociation tendency of the amino acids was found to be Gly >> His ≈ Cys > Asp, indicating enhanced binding to uranyl of the doubly deprotonated amino acids formed in CID.