Jaskolla Thorsten W, Onischke Kristin, Schiller Jürgen
Institute of Hygiene, University of Münster, Robert-Koch-Str. 41, 48149, Münster, Germany.
Rapid Commun Mass Spectrom. 2014 Jun 30;28(12):1353-63. doi: 10.1002/rcm.6910.
In the last decades the interest in lipids as important components of membranes has considerably increased. Nowadays, lipids are often routinely analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). In this regard, many relevant aspects are so far unknown, e.g., gas-phase stabilities, adduct formation and fragmentation. To fill this gap, MALDI matrix salts are presented which allow for simplified lipid analysis and elucidation of the underlying gas-phase fragmentation mechanisms.
MALDI-TOF MS was used due to its beneficial properties for lipid investigations, e.g., high sensitivity, simple sample preparations, and a high tolerance to contaminants. The lipid hydrolysis, ionization and fragmentation properties of synthesized near neutral Na(+) and NH4 (+) salts of the commonly used MALDI matrix 2,5-dihydroxybenzoic acid were compared to that of DHB free acid itself as well as to base addition to DHB during dried-droplet sample preparation.
Many lipid classes such as sterols, triacylglycerols, phosphatidylcholines and -ethanolamines undergo initial protonation with subsequent prompt partial up to quantitative fragmentation when analyzed with classical acidic matrices by MALDI-TOF MS. Neutral matrix salts can prevent initial analyte fragmentation by suppression of analyte protonation. Additionally, intramolecular gas-phase fragmentation reactions can be inhibited due to analyte stabilization by cation chelation. Base addition during sample preparation leads not only to in situ generation of matrix salts but also to analyte hydrolysis.
Neutral DHB salts avoid separation of lipid species into several ionization states when used as matrices in MALDI-TOF MS. This allows for simplified lipid spectra interpretation. Due to the high cationization efficiency of DHB matrix salts, certain lipid classes become detectable which cannot be analyzed easily using standard acidic DHB.
在过去几十年中，人们对脂质作为膜的重要组成部分的兴趣显著增加。如今，脂质通常通过基质辅助激光解吸/电离飞行时间质谱（MALDI-TOF MS）进行常规分析。在这方面，许多相关方面目前仍不清楚，例如气相稳定性、加合物形成和碎片化。为了填补这一空白，本文介绍了MALDI基质盐，其可简化脂质分析并阐明潜在的气相碎片化机制。
使用MALDI-TOF MS是因为其在脂质研究方面具有诸多优点，例如高灵敏度、简单的样品制备以及对污染物的高耐受性。将合成的常用MALDI基质2,5-二羟基苯甲酸的近中性Na(+)和NH4(+)盐的脂质水解、电离和碎片化特性与DHB游离酸本身以及在干滴样品制备过程中向DHB中添加碱的情况进行了比较。
许多脂质类别，如甾醇、三酰甘油、磷脂酰胆碱和乙醇胺，在用经典酸性基质通过MALDI-TOF MS分析时会经历初始质子化，随后迅速发生部分直至定量的碎片化。中性基质盐可通过抑制分析物质子化来防止初始分析物碎片化。此外，由于阳离子螯合使分析物稳定，分子内气相碎片化反应也可受到抑制。样品制备过程中添加碱不仅会导致基质盐的原位生成，还会导致分析物水解。
中性DHB盐在用作MALDI-TOF MS的基质时，可避免脂质种类分离为多种电离状态。这使得脂质谱图的解释更加简化。由于DHB基质盐的高阳离子化效率，某些脂质类别变得可检测，而使用标准酸性DHB则难以分析这些脂质。
