Effects of ESI conditions on kinetic trapping of the solution-phase protonation isomer of p-aminobenzoic acid in the gas phase.
Effects of ESI conditions on kinetic trapping of the solution-phase protonation isomer of p-aminobenzoic acid in the gas phase.
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DOI：
10.1016/j.ijms.2016.09.022
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发表时间：
2017-07
影响因子：
1.8
通讯作者：
Polfer NC
中科院分区：
文献类型：
--
作者：
Patrick AL;Cismesia AP;Tesler LF;Polfer NC
研究了电喷雾电离（ESI）溶剂和源温度对对氨基苯甲酸（PABA）的溶液相优先（N -质子化；即胺）异构体与气相优先（O -质子化；即酸）异构体相对丰度的影响。当从质子性溶剂（即甲醇/水）中对PABA进行电喷雾时，根据计算和先前的研究，所记录的红外多光子解离（IRMPD）光谱与O -质子化的光谱一致。当使用非质子性溶剂（即乙腈）时，记录到不同的光谱，并归属于N -质子化异构体。由于胺是溶液中优先的质子化位点，这表明在某些条件下会发生异构化。利用O -质子化异构体诊断波段（NH₂伸缩模式）的光解离来量化每种异构体对离子信号的相对贡献随ESI条件的变化。对于甲醇和乙腈的混合物，O -质子化气相结构的相对贡献随着甲醇含量的增加而增加。然而，用甲醇代替水会导致向O -质子化结构异构化的显著减少。发现源温度（即加热的去溶剂化毛细管的温度）在决定异构化程度方面起着关键作用，较高的温度会使气相结构的存在增加。这些结果与质子桥机制一致，在该机制中，ESI液滴温度取决于吸热去溶剂化和来自毛细管的辐射加热，可能决定异构化产率。
The effects of electrospray ionization (ESI) solvent and source temperature on the relative abundance of the preferred solution-phase (N-protonated; i.e. amine) versus preferred gas-phase (O-protonated; i.e., acid) isomers of p-aminobenzoic acid (PABA) were investigated. When PABA was electrosprayed from protic solvents (i.e., methanol/water), the infrared multiple photon dissociation (IRMPD) spectrum recorded was consistent with that for O-protonation, according to both calculations and previous studies. When aprotic solvent (i.e., acetonitrile) was used, a different spectrum was recorded and was assigned to the N-protonated isomer. As the amine is the preferred protonation site in solution, this suggests that an isomerization takes place under certain conditions. Photodissociation at the diagnostic band for the O-protonated isomer (NH2 stretching mode) was used to quantify the relative contributions of each isomer to ion signal as a function of ESI conditions. For mixtures of methanol and acetonitrile, the relative contribution of the O-protonated gas-phase structure increased as a function of methanol content. Yet, substituting methanol for water resulted in a marked decrease of isomerization to the O-protonated structure. The source temperature (i.e., temperature of a heated desolvation capillary) was found to play a key role in determining the extent of isomerization, with higher temperatures yielding increased presence of gas-phase structures. These results are consistent with a protic bridge mechanism, in which the ESI droplet temperatures, dependent on endothermic desolvation and radiative heating from the capillary, may determine the isomerization yield.
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