Distinguishing Cis and Trans Isomers in Intact Complex Lipids Using Electron Impact Excitation of Ions from Organics Mass Spectrometry
Distinguishing Cis and Trans Isomers in Intact Complex Lipids Using Electron Impact Excitation of Ions from Organics Mass Spectrometry
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DOI：
10.1021/acs.analchem.6b04734
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发表时间：
2017-07-18
影响因子：
7.4
通讯作者：
Baker, Paul R. S.
中科院分区：
文献类型：
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作者：
Baba, Takashi;Campbell, J. Larry;Baker, Paul R. S.
我们提出了一种基于质谱的方法，利用电子碰撞激发离子从有机物（EIEIO）质谱法的完整的复杂脂质混合物内的顺式和反式双键异构体的识别。EIEIO涉及用具有516 eV动能的电子照射单电荷脂质离子。由此产生的EIEIO光谱可用于辨别顺式和反式双键异构体的凭借在相邻的双键的碳碳单键的断裂模式的差异。对于反式双键，这些特征片段包括独特的闭壳和开壳（自由基）产物。为了解释这种断裂模式的反式双键，我们提出了一个反应机制，涉及激发的双键π电子，然后由氢原子重排。使用EIEIO方法分析几种脂质标准品，包括这些标准品的混合物。在EIEIO之前，这些混合物中的一些脂质物质通过使用差示迁移率光谱法（DMS）与其异构体形式分离。例如，通过该DMSEIEIO工作流程鉴定了三酰基甘油和磷脂酰胆碱的混合顺式和反式形式。通过这种气相分离和随后的碎片化相结合的方法，我们可以消除对用于鉴定的真实标准的需要。当DMS不能完全分离顺式和反式异构体时，如鞘磷脂的情况，我们依赖于上述诊断EIEIO片段峰来确定混合样品中反式双键异构体的相对贡献。我们还将DMSEIEIO方法应用于从反刍动物（牛）中提取的天然样品，这些样品是包括乳制品在内的典型西方饮食中反式脂肪酸的常见来源。
We present a mass spectrometry-based method for the identification of cis and trans double-bond isomers within intact complex lipid mixtures using electron impact excitation of ions from organics (EIEIO) mass spectrometry. EIEIO involves irradiating singly charged lipid ions with electrons having kinetic energies of 516 eV. The resulting EIEIO spectra can be used to discern cis and trans double-bond isomers by virtue of the differences in the fragmentation patterns at the carboncarbon single bonds neighboring the double bonds. For trans double bonds, these characteristic fragments include unique closed-shell and open-shell (radical) products. To explain this fragmentation pattern in trans double bonds, we have proposed a reaction mechanism involving excitation of the double bonds pi electrons followed by hydrogen atom rearrangement. Several lipid standards were analyzed using the EIEIO method, including mixtures of these standards. Prior to EIEIO, some of the lipid species in these mixtures were separated from their isomeric forms by using differential mobility spectrometry (DMS). For example, mixed cis and trans forms of triacylglycerols and phosphatidylcholines were identified by this DMSEIEIO workflow. With this combined gas-phase separation and subsequent fragmentation, we could eliminate the need for authentic standards for identification. When DMS could not separate cis and trans isomers completely, as was the case with sphingomyelins, we relied upon the aforementioned diagnostic EIEIO fragment peaks to determine the relative contribution of the trans double-bond isomer in the mixed samples. We also applied the DMSEIEIO methodology to natural samples extracted from a ruminant (bovine), which serve as common origins of trans fatty acids in a typical Western diet that includes dairy products.