Differentiation of Isomeric HydroxypyridineN-Oxides Using Metal Complexation and Electrospray Ionization Mass Spectrometry

Abstract

Differentiation between two isomers of hydroxypyridine N-oxide according to the metal cation adducts generated by electrospray ionization (ESI) was investigated for different metal cations, namely Mg (II), Al (III), Ca (II), Sc (III), Fe (III), Co (II), Ni (II), Cu (II), Zn (II), Ga (III), besides the diatomic cation VO(IV). Protonated molecules of the isomeric hydroxypyridine N-oxides as well as the singly/doubly charged adducts formed from neutral or deprotonated ligands and a doubly/ triply charged cation were produced in the gas phase using ESI, recording mass spectra with different metal ions for each isomer. While complex formation was successful for 2-hydroxypyridine N-oxide with trivalent ions, in the case of 3-hydroxypyridine N-oxide, only peaks related to the protonated molecule were present. On the other hand, divalent cations formed specific species for each isomer, giving characteristic spectra in every case. Hence, differentiation was possible irrespective of the metal cation utilized. In addition, quantum chemical calculations at the B3LYP/6-31+G(d,p) level of theory were performed in order to gain insight into the different complexation of calcium(II) with the isomers of hydroxypyridine N-oxide. The relative stability in the gas phase of the neutral complexes of calcium made up of two ligands, as well as the singly charged and doubly charged complexes, was investigated. The results of these calculations improved the understanding of the differences observed in the mass spectra obtained for each isomer.

Differentiation of cyclosporin A from isocyclosporin A by liquid chromatography/electrospray ionization mass spectrometry with post-column addition of divalent metal salt

“…Among the different metal ions, Ca 2+ , Cu 2+ , and Zn 2+ were chosen because they are endogenous metals that are known to form complexes with pure CsA. CuSO 4 was used instead of CuCl 2 because it has the same capacity to produce metal adducts, but it does not form adducts with the counterion Cl . The chromatographic conditions were chosen based on previous analysis of complex mixtures of natural fungal extracts containing cyclosporins.…”

“…In the same work, CsA and isoCsA could be distinguished by liquid chromatography (LC) since both compounds have different retention times. [9] Differentiation of isomers by the use of metal complexation and electrospray was previously achieved, for example in hydroxypyridine N-oxides [10] and polyaromatic hydrocarbons. [11] The same strategy was explored for the differentiation of CsA and isoCsA by metal complexation, based on the ability of cyclosporins to form stable adducts with Na + , Mg 2+ , Ca 2+ , Cu 2+ and Zn 2+ in solution.…”

A mass spectrometry-based method for differentiation of positional isomers of monosubstituted pyrazineN-oxides using metal ion complexes

“…A similar complexation behavior has been reported previously for analogous ligands, namely 2-and 3-hydroxypyridine N-oxides [13] with the same metal cations. Hence, differences between the two regioisomers had been established, considering the ability of the isomer substituted in position 2-to form stable chelates acting as a bidentate ligand upon deprotonation, thereby forming mainly singly charged ion adducts.…”

“…The present work extends the previous investigation on the dissociation behavior [14] and metal complexation [13] of two isomeric hydroxypyridine N-oxides by examining a larger set of regioisomeric pyrazine N-oxides with different substituents on the aromatic ring. Distinction between 2-and 3-substituted pyrazine N-oxides was studied employing the ESI mass spectra of each compound in the presence of different metal cations, i.e.…”

“…Metal ions of alkali, alkaline earth and transition metals have been extensively used in ESI for the study of many types of compounds, and were also suitable for the analysis of isomeric compounds by means of metal complexation strategies . For instance, two regioisomeric hydroxypyridine N ‐oxides could be differentiated with various metal cations by ion adduct generation using ESI‐MS.…”