Xu Yichuan, Brenna J Thomas
Division of Nutritional Sciences, Cornell University, Ithaca, New York 14853, USA.
Anal Chem. 2007 Mar 15;79(6):2525-36. doi: 10.1021/ac062055a. Epub 2007 Feb 6.
We report a method to elucidate the structure of triacyl-glycerols (TAGs) containing monoene or diene fatty acyl groups by atmospheric pressure covalent adduct chemical ionization (APCACI) tandem mass spectrometry using acetonitrile as an adduct formation reagent. TAGs were synthesized with the structures ABB and BAB, where A is palmitate (C16:0) and B is an isomeric C18 monoene unsaturated at position 9, 11, or 13 or an isomeric diene unsaturated at positions 9 and 11, 10 and 12, or 9 and 12. In addition to the species at m/z 54 observed in previous CI studies of fatty acid methyl esters, we also found that ions at m/z 42, 81, and 95 undergo covalent reaction with TAGs containing double bonds to yield ions at m/z 40, 54, 81, and 95 units greater than that of the parent TAG: [M + 40]+, [M + 54]+, [M + 81]+, and [M + 95]+ ions. When collisionally dissociated, these ions fragment to produce two or three diagnostic ions that locate the double bonds in the TAG. In addition, ions [RCH=C=O + 40]+ and [RCH=C=O + 54]+ formed from collisional dissociation are of strong abundance in MS/MS spectra, and collisional activation of these ions produces two intense confirmatory diagnostic ions in the MS3 spectra. Fragment ions reflecting neutral loss of an sn-1-acyl group from [M + 40]+ and [M + 54]+ are more abundant than those reflecting neutral loss of an sn-2-acyl group, analogous to previous reports for protonated TAGs. The position of each acyl group on the glycerol backbone is thus determined by the relative abundances of these ions. Under the conditions in our instrument, the [M + 40]+ adduct is at the highest signal and also yields all information about the double bond position and TAG stereochemistry. With the exception of geometries about the double bonds, racemic TAG isomers containing two monoenes or dienes and a saturate can be fully characterized by APCACI-MS/MS/MS.
我们报道了一种通过大气压共价加合物化学电离（APCACI）串联质谱法，以乙腈作为加合物形成试剂来阐明含有单烯或二烯脂肪酰基的三酰基甘油（TAGs）结构的方法。合成了结构为ABB和BAB的TAGs，其中A为棕榈酸酯（C16:0），B为在9、11或13位异构的C18单烯不饱和脂肪酸，或在9和11、10和12或9和12位异构的二烯不饱和脂肪酸。除了在先前脂肪酸甲酯的化学电离研究中观察到的m/z 54处的离子外，我们还发现m/z 42、81和95处的离子会与含双键的TAGs发生共价反应，生成比母体TAG的质荷比大40、54、81和95个单位的离子：[M + 40]+、[M + 54]+、[M + 81]+和[M + 95]+离子。当这些离子发生碰撞解离时，会裂解产生两个或三个诊断离子，从而确定TAGs中的双键位置。此外，碰撞解离形成的离子[RCH=C=O + 40]+和[RCH=C=O + 54]+在二级质谱图中丰度很高，对这些离子进行碰撞活化会在三级质谱图中产生两个强烈的确认诊断离子。与先前关于质子化TAGs的报道类似，反映从[M + 40]+和[M + 54]+中中性丢失sn-1-酰基的碎片离子比反映中性丢失sn-2-酰基的碎片离子更丰富。因此，甘油主链上每个酰基的位置由这些离子的相对丰度决定。在我们仪器的条件下，[M + 40]+加合物信号最强，并且能提供有关双键位置和TAG立体化学的所有信息。除了双键的几何构型外，含有两个单烯或二烯以及一个饱和脂肪酸的外消旋TAG异构体可以通过APCACI-MS/MS/MS进行全面表征。
