Perspective on Emerging Mass Spectrometry Technologies for Comprehensive Lipid Structural Elucidation

Abstract

Lipids and metabolites are of interest in many clinical and research settings because it is the metabolome that is increasingly recognized as a more dynamic and sensitive molecular measure of phenotype. The enormous diversity of lipid structures and the importance of biological structure–function relationships in a wide variety of applications makes accurate identification a challenging yet crucial area of research in the lipid community. Indeed, subtle differences in the chemical structures of lipids can have important implications in cellular metabolism and many disease pathologies. The speed, sensitivity, and molecular specificity afforded by modern mass spectrometry has led to its widespread adoption in the field of lipidomics on many different instrument platforms and experimental workflows. However, unambiguous and complete structural identification of lipids by mass spectrometry remains challenging. Increasingly sophisticated tandem mass spectrometry (MS/MS) approaches are now being developed and seamlessly integrated into lipidomics workflows to meet this challenge. These approaches generally either (i) alter the type of ion that is interrogated or (ii) alter the dissociation method in order to improve the structural information obtained from the MS/MS experiment. In this Perspective, we highlight recent advances in both ion type alteration and ion dissociation methods for lipid identification by mass spectrometry. This discussion is aimed to engage investigators involved in fundamental ion chemistry and technology developments as well as practitioners of lipidomics and its many applications. The rapid rate of technology development in recent years has accelerated and strengthened the ties between these two research communities. We identify the common characteristics and practical figures of merit of these emerging approaches and discuss ways these may catalyze future directions of lipid structural elucidation research.

Relative Quantitation of Unsaturated Phosphatidylcholines Using 193 nm Ultraviolet Photodissociation Parallel Reaction Monitoring Mass Spectrometry

“…Apart from comprising the cellular membrane, these lipids influence cellular functions by modulating membrane protein structure, compartmentalizing organelles, and serving as signaling molecules. − Consequently, lipid composition is carefully regulated to ensure proper cellular function. Dysregulation in lipid profiles has been associated with several diseases such as cancer, diabetes, neurodegeneration and cardiovascular disease. − Accordingly, deciphering lipid structures has become imperative to understanding the relationship between disease state and aberrant lipid profiles, promising to inform disease biomarkers, potential therapeutic targets, and disease mechanisms. − Thus, the development of deep lipidomic strategies that provide both fine structural detail and quantitation at the lipidomic scale has become a major endeavor to unravel how disease and lipid composition are intertwined.…”

“…Major advances in lipidomic studies have largely been catalyzed by mass spectrometry (MS) and tandem mass spectrometry (MS/MS) based strategies. High sensitivity, resolution, and throughput afforded by modern MS technologies have enabled the routine quantitation and characterization of glycerophospholipids to provide insight into the structure and function of lipids. ,− MS-based strategies largely fall into direct introduction (“shotgun”), separation, or imaging workflows, with each approach featuring unique advantages such as rapid analyses, enhanced molecular depth, or spatial resolution, respectively, which together offer tremendous versatility for a range of applications. ,− From traditional collision-based MS/MS strategies, several crucial structural features can be discerned for glycerophospholipids, including the identity of the glycerophospholipid class as denoted by the phospho-headgroup esterified at position sn-3 of the glycerol backbone as well as the linkages and total number of carbons and double bonds in the acyl chains linked at positions sn-1 and sn-2 of the glycerol backbone . Although these features are informative, structural characterization at this level is typically incomplete, failing to reveal subtle yet important structural features such as double bond position and stereospecific numbering (sn) stereochemistry .…”

Identification of Isomeric Lipids by UV Spectroscopy of Noncovalent Complexes with Aromatic Molecules

“…Full structural identification of isomeric lipids is therefore vitally important in many fields of life science, drug design, and the food industry. Tandem mass spectrometry (MS) of bare and chemically tagged lipids and chromatographic and, more recently, ion mobility separations coupled to MS are among the current analytical techniques to identify isomeric lipids. − Despite all the advances of these powerful methods, the isomer identifications for many types of lipids remain challenging . This makes the development of new orthogonal analytical approaches, such as spectroscopy-based, highly desired.…”

Fully Automatized Detection of Phosphocholine-Containing Lipids through an Isotopically Labeled Buffer Modification Workflow

“…As such, the information obtained following MS lipidomics can range from the individuation of the lipid classes and subclasses to the evaluation of the stereochemical properties depending on the performance of the analytical platform. 6 The structural variability of PLs has a significant impact on data collection. While PAs present a free phosphate group, which favors deprotonation to form [M − H] − adducts in the negative ion mode, PCs have a cationic quaternary ammonium that promotes ionization in the positive ion mode, generating intense [M + H] + ions, and hinders the formation of deprotonated molecules in the negative mode.…”

“…Shotgun mass spectrometry (MS) and liquid chromatography coupled to MS (LC–MS) are the foremost techniques for PL analysis, either in a targeted or untargeted fashion. , In untargeted lipidomics, the structural information on the identified lipids is highly dependent on the analytical methods, comprising both data collection and data handling. As such, the information obtained following MS lipidomics can range from the individuation of the lipid classes and subclasses to the evaluation of the stereochemical properties depending on the performance of the analytical platform . The structural variability of PLs has a significant impact on data collection.…”