Research Progress of Free Radical Chemistry-Tandem Mass Spectrometry for Fine Structure Analysis of Lipids

As one of the six major nutrients, lipids play important roles in various biological processes, including cell membrane construction, signal transduction, and energy metabolism. The diverse functions of lipids are intricately linked to the distinct structures. More than 48 000 molecular lipid structures have been curated in the database which can be further categorized into 8 categories and more than one hundred lipid classes and subclasses. The diverse structures of lipids pose challenges for their lipidomic analysis, especially the co-existence of isomers and isobar. Traditional tandem mass spectrometry (MS/MS) techniques based on collision-induced dissociation (CID) can provide the informations of molecular species and acyl chain composition of various types of lipids in a sensitive fashion. However, they typically fall short in providing detailed structural information, including C=C positions, sn-positions, functional group substitutions and their locations on the fatty acyl chains. In recent years, mass spectrometry methods capable of distinguishing lipid isomers at various structural levels have been developed, among which radical chemistry plays important role in both gas-phase ion activation and chemical derivatization. This article summarized the MS/MS techniques for lipid structural analysis at detailed structural levels via harnessing the power of both radical chemistry and tandem mass spectrometry in the past decade. Radical-induced dissociation (RID) refers to a group of gas-phase dissociation methods which can generate intrachain C-C cleavages in the fatty acyl chain and thus produce fragmentation patterns useful for identification of chain modification. Ultraviolet photodissociation (UVPD) and CID triggered radical-directed dissociation, and electron impact excitation of ions from organics (EIEIO) are the major RID methods. By coupling RID with separation methods, such as reversed-phase liquid chromatography (RPLC), identification and quantitation of lipid isomers consisting of methyl branching, hydroxy group, and cyclopropane modification and their locations have been achieved. The two major derivatization methods utilizing radical chemistry for C=C bond modification were discussed, including radical initiated epoxidation reactions and the Paternò-Büchi reaction. Furthermore, the future direction of combining radical chemistry and tandem mass spectrometry for deep lipidotyping was discussed, which included the coupling with advanced separation methods, integrated workflows for multi-level structure analysis, and development of automated annotation tools for data analysis.