Department of Materials and Environmental Chemistry, Stockholm University, Svante Arrhenius väg 16, 106 91 Stockholm, Sweden.
Institute of Chemistry, University of Tartu, Ravila 14a, Tartu 50411, Estonia.
Anal Chem. 2022 Aug 2;94(30):10601-10609. doi: 10.1021/acs.analchem.2c00257. Epub 2022 Jul 21.
The structural annotation of isomeric metabolites remains a key challenge in untargeted electrospray ionization/high-resolution mass spectrometry (ESI/HRMS) metabolomic analysis. Many metabolites are polyfunctional compounds that may form protomers in electrospray ionization sources and therefore yield multiple peaks in ion mobility spectra. Protomer formation is strongly structure-specific. Here, we explore the possibility of using protomer formation for structural elucidation in metabolomics on the example of caffeine, its eight metabolites, and structurally related compounds. It is observed that two-thirds of the studied compounds formed high- and low-mobility species in high-resolution ion mobility. Structures in which proton hopping was hindered by a methyl group at the purine ring nitrogen (position 3) yielded structure-indicative fragments with collision-induced dissociation (CID) for high- and low-mobility ions. For compounds where such a methyl group was not present, a gas-phase equilibrium could be observed for tautomeric species with two-dimensional ion mobility. We show that the protomer formation and the gas-phase properties of the protomers can be related to the structure of caffeine metabolites and facilitate the identification of the structural isomers.
在非靶向电喷雾电离/高分辨率质谱（ESI/HRMS）代谢组学分析中，对同分异构体代谢物的结构注释仍然是一个关键挑战。许多代谢物是多功能化合物，它们可能在电喷雾电离源中形成前体，因此在离子淌度谱中产生多个峰。前体形成具有强烈的结构特异性。在这里，我们以咖啡因、其 8 种代谢物和结构相关的化合物为例，探索了在前体形成在代谢组学中用于结构阐明的可能性。研究表明，三分之二的研究化合物在高分辨率离子淌度中形成了高迁移率和低迁移率的物质。嘌呤环氮（3 位）上的甲基阻碍质子迁移的结构产生具有碰撞诱导解离（CID）的结构指示性片段，用于高和低迁移率离子。对于没有这种甲基的化合物，可以观察到互变异构体在二维离子淌度中的气相平衡。我们表明，前体形成和前体的气相性质可以与咖啡因代谢物的结构相关，并有助于鉴定结构异构体。
