Formation of metal complex ions from amino acid in the presence of Li⁺, Na⁺ and K⁺ by electrospray ionization: metal replacement of hydrogen in the ligands

Abstract

Alkali metal cations easily form complexes with proteins in biological systems; understanding amino acid clusters with these cations can provide useful insight into their behaviors at the molecular level including diagnosis and therapy of related diseases. For the purpose of characterization of basic interaction between amino acids and alkali metal, each of the 20 naturally occurring amino acids were ionized in the presence of lithium, sodium and potassium cations by electrospray ionization, and the resulting product ions were analyzed. We focus our attention on the gas phase alkali metal ion-proton exchanged complexes in current study, specifically complexes with serine, threonine, asparagine and glutamine, which share characteristic pattern unlike other amino acids. All amino acids generated [M + H](+) and [M + Na](+) ions, where M stands for the neutral amino acid. Serine, threonine, asparagine and glutamine generated cluster ions of [nM - nH + (n + 1)Na](+) and [nM - (n - 1)H + (n - 1)Na + K](+) , where n = 1-7. While the (M - H + Li) and (M - H + K) species were not observed, the neutral (M - H + Na) species formed by proton-sodium cation exchange had a highly stable cyclic structure with ketone and amine ligand sites, suggesting that (M - H + Na) serves as a building block in cluster ion formation. Cluster ion intensity distributions of [nM - nH + (n + 1)Na](+) and [nM - (n - 1)H + (n - 1)Na + K](+) showed a magic number at n = 3 and 4, respectively. Extensive B3LYP-DFT quantum mechanical calculations were carried out to elucidate the geometry and energy of the cluster ions, and they provided a reasonable explanation for the stability and structure of the cluster ions.

Contrasting preferences of N and P substituted heteroaromatics towards metal binding: probing the regioselectivity of Li+ and Mg2+ binding to (CH)6−m−nNmPn

“…4 The metal complexation of the bioactive ligands has been used as an analytical tool to enhance the characterization of bioactive molecules which is otherwise complicated. 5 Spectroscopic data and the X-ray analysis show that the N7 atom of the purine or N3 of pyrimidine was the preferential site of metal binding during the metal-DNA interaction. 6 Therefore the study of these metal ions interaction with N substituted heteroaromatics is of fundamental importance to understand various biological processes.…”

Conformations and Metal Ion Affinities of Glutamine Binding with Alkali and Alkaline Earth Metal Cations: an ab initio Study

“…However, some metalated amino acids may exhibit so called salt bridge (SB) structures as their low energy conformations [11,18,21,22,27], while SB or zwitterions structures are often absent for free amino acids. Therefore, the aforementioned trial structures are only reliable for determining low energy charge solvated (CS) structures of metalated amino acid.…”

Electrospray Ionization Mass Spectrometry of Non-Covalent Complexes Formed between N-Alkylimidazolium-Containing Zwitterionic Sulfonates and Protonated Bases

“…6 Attention has also been paid to small-molecule association in solution. Numerous papers have described the formation of complexes of different macrocycles (crown ethers, cryptands, glymes), 7 cyclodextrins, 8 calixarene, 9 aminoacids 10 and guanidinium ions 11 with various guests.…”