Molecular Horizons and School of Chemistry and Molecular Bioscience, University of Wollongong, Wollongong, New South Wales 2522, Australia.
Illawarra Health and Medical Research Institute, Wollongong, New South Wales 2522, Australia.
J Am Soc Mass Spectrom. 2022 Feb 2;33(2):347-354. doi: 10.1021/jasms.1c00331. Epub 2022 Jan 11.
Understanding how neutral molecules become protonated during positive-ion electrospray ionization (ESI) mass spectrometry is critically important to ensure analytes can be efficiently ionized, detected, and unambiguously identified. The ESI solvent is one of several parameters that can alter the dominant site of protonation in polyfunctional molecules and thus, in turn, can significantly change the collision-induced dissociation (CID) mass spectra relied upon for compound identification. Ciprofloxacin─a common fluoroquinolone antibiotic─is one such example whereby positive-ion ESI can result in gas-phase [M + H] ions protonated at either the keto-oxygen or the piperazine-nitrogen. Here, we demonstrate that these protonation isomers (or ) of ciprofloxacin can be resolved by differential ion mobility spectrometry and give rise to distinctive CID mass spectra following both charge-directed and charge-remote mechanisms. Interaction of mobility-selected protomers with methanol vapor (added via the throttle gas supply) was found to irreversibly convert the piperazine -protomer to the keto--protomer. This methanol-mediated proton-transport catalysis is driven by the overall exothermicity of the reaction, which is computed to favor the -protomer by 93 kJ mol (in the gas phase). Conversely, gas phase interactions of mobility-selected ions with acetonitrile vapor selectively depletes the -protomer ion signal as formation of stable [M + H + CHCN] cluster ions skews the apparent protomer population ratio, as the -protomer is unaffected. These findings provide a mechanistic basis for tuning protomer populations to ensure faithful characterization of multifunctional molecules by tandem mass spectrometry.
理解中性分子在正离子电喷雾电离（ESI）质谱中如何质子化对于确保分析物能够有效地离子化、检测和明确识别至关重要。ESI 溶剂是可以改变多官能团分子中质子化主要位置的几个参数之一，因此可以显著改变用于化合物鉴定的碰撞诱导解离（CID）质谱。环丙沙星——一种常见的氟喹诺酮类抗生素——就是这样一个例子，在正离子 ESI 中，[M + H]+离子可以在酮氧或哌嗪氮上质子化。在这里，我们证明这些环丙沙星的质子化异构体（或）可以通过差分离子迁移谱分辨，并在电荷导向和电荷远程机制下产生独特的 CID 质谱。发现经流动选择的前体与甲醇蒸气（通过节流气体供应添加）相互作用会不可逆地将哌嗪前体转化为酮-前体。这种甲醇介导的质子迁移催化是由反应的总体放热驱动的，计算表明，在气相中，-前体的优势为 93 kJ mol（。相反，经流动选择的离子与乙腈蒸气在气相中的相互作用会选择性地耗尽 -前体离子信号，因为稳定的[M + H + CHCN]簇离子的形成会使表观前体种群比发生倾斜，因为 -前体不受影响。这些发现为调节前体种群提供了一个机制基础，以确保通过串联质谱对多功能分子进行忠实的特征描述。
