Department of Chemistry, Purdue University, West Lafayette, IN, 47907, USA.
J Am Soc Mass Spectrom. 2017 Jun;28(6):1091-1098. doi: 10.1007/s13361-017-1684-1. Epub 2017 May 12.
Laser-induced acoustic desorption (LIAD) allows for desorption of neutral nonvolatile compounds independent of their volatility or thermal stability. Many different ionization methods have been coupled with LIAD. Hence, this setup provides a better control over the types of ions formed than other mass spectrometry evaporation/ionization methods commonly used to characterize biomolecules, such as ESI or MALDI. In this study, the utility of LIAD coupled with electron ionization (EI) was tested for the analysis of common amino acids with no derivatization. The results compared favorably with previously reported EI mass spectra obtained using thermal desorption/EI. Further, LIAD/EI mass spectra collected for hydrochloride salts of two amino acids were found to be similar to those measured for the neutral amino acids with the exception of the appearance of an HCl ion. However, the hydrochloride salt of arginine showed a distinctly different LIAD/EI mass spectrum than the previously published literature EI mass spectrum, likely due to its highly basic side chain that makes a specific zwitterionic form particularly favorable. Finally, EI mass spectra were measured for seven small peptides, including di-, tri-, and tetrapeptides. These mass spectra show a variety of ion types. However, a type ions are prevalent. Also, electron-induced dissociation (EID) of protonated peptides has been reported to form primarily a type ions. In addition, the loss of small neutral molecules and side-chain cleavages were observed that are reminiscent of other high-energy fragmentation methods, such as EID. Finally, the isomeric dipeptides LG and IG were found to produce drastically different EI mass spectra, thus allowing differentiation of the leucine and isoleucine amino acids in these dipeptides. Graphical Abstract ᅟ.
激光诱导声解吸（LIAD）可使中性非挥发性化合物解吸，而与它们的挥发性或热稳定性无关。许多不同的电离方法已与 LIAD 耦合。因此，与其他常用于生物分子特征分析的质谱蒸发/电离方法（如 ESI 或 MALDI）相比，这种设置可以更好地控制形成的离子类型。在这项研究中，LIAD 与电子电离（EI）的联用被测试用于分析未经衍生化的常见氨基酸。结果与先前报道的使用热解吸/EI 获得的 EI 质谱相媲美。此外，对于两种氨基酸的盐酸盐，LIAD/EI 质谱与中性氨基酸的质谱相似，除了出现 HCl 离子外。然而，精氨酸的盐酸盐与以前发表的文献 EI 质谱表现出明显不同的 LIAD/EI 质谱，可能是由于其高度碱性的侧链使其形成特别有利的特定两性离子形式。最后，对七种小肽（包括二肽、三肽和四肽）进行了 EI 质谱测量。这些质谱显示出多种离子类型。然而，a 型离子是主要的。此外，还观察到质子化肽的电子诱导解离（EID）主要形成 a 型离子。此外，观察到小分子中性分子的损失和侧链的断裂，这让人联想到其他高能碎裂方法，如 EID。最后，发现异构体二肽 LG 和 IG 产生了截然不同的 EI 质谱，从而能够区分这些二肽中的亮氨酸和异亮氨酸氨基酸。
