Positive chemical ionization (PCI)-like behavior of estradiol in matrix-assisted laser desorption/ionization.
Positive chemical ionization (PCI)-like behavior of estradiol in matrix-assisted laser desorption/ionization.
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基质辅助激光解吸/电离中雌二醇的正化学电离 (PCI) 样行为。
DOI：
10.1002/rcm.4178
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发表时间：
2009
期刊：
影响因子：
--
通讯作者：
Giese,RogerW
中科院分区：
文献类型：
--
作者：
Wang,Poguang;Kremsky,JonathanN;Giese,RogerW
通常，只有不稳定的分析物在基质辅助激光解吸电离质谱（MALDI-MS）中发生显著的快速碎片化，1如抗坏血酸2或维生素B12。3基本上，这是因为MALDI通常使用中等激光通量，是一种相对温和的电离/解吸技术。因此，我们感兴趣的是，在常规MALDI条件下，以α-氰基-4-羟基肉桂酸（CCA）作为基质，在m/z 255处观察到β-雌二醇（E2，Mr= 272，结构见图1）的相对丰富的碎片离子，如图2A所示。我们想知道m/z 255处的离子是否来自质子化E2（m/z 273）的水损失，或者来自自由基阳离子（m/z 272）的羟基自由基损失，以及这是如何发生的。(Note图2A中m/z 273处的峰大部分（81%）是m/z 272处的自由基阳离子的13 C形式，部分（19%）是质子化的E2。）我们的注意力立即转向前一种途径，因为在70 eV下E2的电子电离产生的OH损失可以忽略不计。4此外，使m/z 272经受源后衰变仅提供了m/z 255处的可忽略的产物离子，如图2B所示。原则上，质子化的E2可能以四种方式失去水：在C3或C17处的消除反应（OH沿着相邻H一起失去），或在这些位点中的任一个处的非消除反应。在源后衰变实验中，我们观察到m/z 255处的离子保留了芳基-OH：来自m/z 255的主要产物离子是m/z 159，芳基-OH部分是该离子的一部分（图3A）。这使我们的注意力转向了C17处的水损失。在这个碳原子上失去水的消除机制通过观察到当17β-雌二醇-16，16，17-d3时，m/z 255完全被m/z 258取代，排除了17β-雌二醇-16，16，17-d3（结构如图1所示，由Prime Organics，Woburn，MA，USA制备）代替E2进行测试，如图2C所示。(We还通过源后衰变研究了17β-雌二醇-16，16，17-d3，如图3C所示，以加强我们对图A3 A所示离子的分配。）通过观察到当使用2，5-二羟基苯甲酸基质时，m/z 255处的产物离子也容易形成，排除了CCA特有的机制，如图2D所示。(Note在后一个光谱中，m/z 273.044处的强峰是基质离子，而m/z 273.171是自由基阳离子的13 C和质子化分子的组合。）类似的雌激素，5α-雄甾烷-17 β-醇，在甲烷的正化学电离（PCI）中容易失去OH。5一般来说，醇（至少是那些具有五个或更多碳原子的醇）在PCI中容易失去OH。6我们观察到5β-雄甾烷-17 β-醇（Mr= 276，
Ordinarily only labile analytes undergo significant prompt fragmentation in matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS), 1 such as oligonucleotides2 or vitamin B12. 3 Basically this is because MALDI, which commonly uses moderate laser fluences, is a relatively gentle ionization/desorption technique.. It was therefore of interest for us to observe a relatively abundant fragment ion at m/z 255 from β-estradiol (E2, Mr= 272, structure in Figure 1), under routine MALDI conditions, with α-cyano-4-hydroxycinnamic acid (CCA) as matrix, as seen in Figure 2A. We wondered whether the ion at m/z 255 came from loss of water from protonated E2 (m/z 273), or from loss of a hydroxyl radical from the radical cation (m/z 272), and how this took place.(Note that the peak at m/z 273 in Figure 2A is mostly (81%) the 13C form of the radical cation at m/z 272, and partly (19%) protonated E2.) Our attention immediately turned to the former pathway since electron ionization of E2 at 70 eV gives negligible loss of OH. 4 Further, subjecting m/z 272 to post-source decay furnished only a negligible product ion at m/z 255, as seen in Figure 2B.In principle, protonated E2 might lose water in four ways: an elimination reaction (loss of OH along with an adjacent H) at C3 or C17, or a non-elimination reaction at either of these sites. In a post-source decay experiment we observed that the ion at m/z 255 retains the aryl-OH: the major product ion from m/z 255 is m/z 159, and the aryl-OH moiety is part of this ion (Figure 3A). This turned our attention to the loss of water at C17. An elimination mechanism for the loss of water at this carbon atom (in conjunction with protonation at the aryl-OH to yield an ion of m/z 255) was ruled out by the observation that m/z 255 is completely replaced by m/z 258 when 17β-estradiol-16, 16, 17-d3 (structure shown in Figure 1, prepared by Prime Organics, Woburn, MA, USA) is tested instead of E2, as seen in Figure 2C.(We also studied 17β-estradiol-16, 16, 17-d3 by post-source decay, as shown in Figure 3C, to strengthen the assignments that we made for the ions shown in Figure A3A.) A mechanism peculiar to CCA was ruled out by the observation that the product ion at m/z 255 also forms readily when using a matrix of 2, 5-dihydroxybenzoic acid, as seen in Figure 2D.(Note that in the latter spectrum the intense peak at m/z 273.044 is a matrix ion, while m/z 273.171 is the combination of the 13C of the radical cation and the protonated molecule.) The analogous estrogen, 5α-androstan-17β-ol, readily loses OH in positive chemical ionization (PCI) with methane. 5 Alcohols in general (at least those with five or more carbon atoms) readily lose OH in PCI. 6 We observed that 5β-androstan-17β-ol (Mr= 276,
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