Cryogenic infrared spectroscopy provides mechanistic insight into the fragmentation of phospholipid silver adducts

Studying the Intrinsic Reactivity of Chromanes by Gas-Phase Infrared Spectroscopy

Tandem mass spectrometry is routinely used for the structural analysis of organic molecules, but many fragmentation reactions are not well understood. Because several potential structures can correspond to a measured mass, the assignment of product ions is ambiguous using mass spectrometry alone. Here, we combine mass spectrometry with high-resolution gas-phase infrared spectroscopy and computational chemistry tools to identify product ion structures and derive collision-induced fragmentation mechanisms of the chromane derivatives Trolox and Methyltrolox. We find that protonated Trolox and Methyltrolox fragment identically via dehydration and decarbonylation, while deprotonated ions display substantially diverging reactivities. For deprotonated Methyltrolox, we observe unusual radical fragmentation reactions and suggest a [1,2]-Wittig rearrangement involving aryl migration in the gas phase. Overall, the combined experimental and theoretical approach presented here revealed complex proton dynamics and intramolecular rearrangement reactions, which expand our understanding on structure-reactivity relationships of isolated molecules in different protonation states.

Identification and Imaging of Prostaglandin Isomers Utilizing MS3 Product Ions and Silver Cationization

Prostaglandins (PGs) are important lipid mediators involved in physiological processes, such as inflammation and pregnancy. The pleiotropic effects of the PG isomers and their differential expression from cell types impose the necessity for studying individual isomers locally in tissue to understand the molecular mechanisms. Currently, mass spectrometry (MS)-based analytical workflows for determining the PG isomers typically require homogenization of the sample and a separation method, which results in a loss of spatial information. Here, we describe a method exploiting the cationization of PGs with silver ions for enhanced sensitivity and tandem MS to distinguish the biologically relevant PG isomers PGE2, PGD2, and Δ12-PGD2. The developed method utilizes characteristic product ions in MS3 for training prediction models and is compatible with direct infusion approaches. We discuss insights into the fragmentation pathways of Ag+ cationized PGs during collision-induced dissociation and demonstrate the high accuracy and robustness of the model to predict isomeric compositions of PGs. The developed method is applied to mass spectrometry imaging (MSI) of mouse uterus implantation sites using silver-doped pneumatically assisted nanospray desorption electrospray ionization and indicates localization to the antimesometrial pole and the luminal epithelium of all isomers with different abundances. Overall, we demonstrate, for the first time, isomeric imaging of major PG isomers with a simple method that is compatible with liquid-based extraction MSI methods.

Determination of Polar Lipids in Wheat and Oat by a Complementary Approach of Hydrophilic Interaction Liquid Chromatography and Reversed-Phase High-Performance Liquid Chromatography Hyphenated with High-Resolution Mass Spectrometry

Cereals contain lipids that fulfill important physiological roles and are associated with stress in the plant. However, many of the specific biological roles of lipids are yet unknown. Comprehensive analysis of these polar lipid categories in whole grain wheat and oat, cereals highly relevant also in nutrition, was performed. Hydrophilic interaction liquid chromatography (HILIC) and reversed-phase high-performance liquid chromatography (RP-HPLC) coupled with high-resolution mass spectrometry using electrospray ionization in both positive and negative ionization mode was used. Exploiting the different separation mechanisms, HILIC was used as a screening method for straightforward lipid class assignment and enabled differentiation of isomeric lipid classes, like phosphatidylethanolamine and lyso-N-acylphosphatidylethanolamine, while RP-HPLC facilitated separation of constitutional isomers. In combination with data-dependent MS/MS experiments, 67 lipid species belonging to nine polar lipid classes could be identified. Furthermore, with both ionization modes, fatty acyl chains directly connected to the lipid headgroups could be assigned. This work focused on the four lipid classes N-acylphosphatidylethanolamines, acyl-monogalactosyldiacylglycerols, digalactosyldiacylglycerols, and monogalactosyldiacylglycerols as they were less studied in detail in the past. Applying the complementary approach, the relative lipid species compositions in these lipid classes was investigated in detail.

Formation and Tandem Mass Spectrometry of Doubly Charged Lipid-Metal Ion Complexes

Phospholipids are major components of most eukaryotic cell membranes. Changes in metabolic states are often accompanied by phospholipid structure variations. The structural changes of phospholipids are the hallmark of disease states, or specific lipid structures have been associated with distinct organisms. Prime examples are microorganisms that synthesize phospholipids with, for example, different branched chain fatty acids. Assignment and relative quantitation of structural isomers of phospholipids that arise from attachment of different fatty acids to the glycerophospholipid backbone are difficult with routine tandem mass spectrometry or with liquid chromatography without authentic standards. In this work, we report on the observation that all investigated phospholipid classes form doubly charged lipid-metal ion complexes during electrospray ionization (ESI) and show that these complexes can be used to assign lipid classes and fatty acid moieties, distinguish isomers of branched chain fatty acids, and relatively quantify these isomers in positive-ion mode. Use of water free methanol and addition of divalent metal salts (100 mol %) to ESI spray solutions afford highly abundant doubly charged lipid-metal ion complexes (up to 70 times of protonated compounds). Higher-energy collisional dissociation and collision-induced dissociation of doubly charged complexes yield a diverse set of lipid class-dependent fragment ions. In common for all lipid classes is the liberation of fatty acid-metal adducts that yield fragment ions from the fatty acid hydrocarbon chain upon activation. This ability is used to pinpoint sites of branching in saturated fatty acids and is showcased for free fatty acids as well as glycerophospholipids. The analytical utility of doubly charged phospholipid-metal ion complexes is demonstrated by distinguishing fatty acid branching-site isomers in phospholipid mixtures and relatively quantifying the corresponding isomeric compounds.

Differentiation of tetracyclines and their 4-epimers by mass spectrometry of the alkali metal adduct ions

Tetracyclines (TCs) are a family of broad-spectrum antibiotics. During the manufacturing process or storage, epimerization of tetracyclines could occur, leading to 4-epimers which are nearly inactive. From an analytical point of view, isomers are often difficult to distinguish. Previously, four pairs of TCs (oxytetracycline, tetracycline, doxycycline, chlortetracycline and their respective 4-epimers) were differentiated by mass spectrometry (MS) through protonated ions. However, they do not follow common rules and so it is still quite difficult to differentiate between them. In order to solve this, the four pairs were differentiated in the current study by collision induced dissociation (CID) spectra of the alkali adduct ions, including lithium, sodium and potassium. In the spectra of the sodium adducts, all studied tetracyclines showed a tendency to form [M+Na-NH₃]⁺ ions, while the 4-epimers liked to form [M+Na-NH₃-H₂O]⁺ ions. Meanwhile, energy resolved mass spectrometry (ERMS) showed that all four 4-epimers' sodium adducts had the tendency to fragment at higher energy points. In the CID spectra of lithium adducts of TCs, a similar trend was observed for three pairs, except for doxycycline. For potassium adducts, the fragmentation was found to be less discriminative. As was derived from the 3D model, the four pairs all interact with the alkali metal through the dimethyl amino group at the C-4 position. The lithium adduct species also bound through the hydroxyl group at the C-5 position. If the TCs did not have a hydroxyl group at the C-5 position, they bound with the hydroxyl group at the C-6 position. For the same TC, with an increase of the diameter of the metal ion, the loss of H₂O decreased gradually. As sodium adduct ions are common during the ionization process, TCs and their 4-epimers could be differentiated rapidly by ERMS of the sodium adduct ions.

The Influence of the Electron Density in Acyl Protecting Groups on the Selectivity of Galactose Formation

The stereoselective formation of 1,2-cis-glycosidic bonds is a major bottleneck in the synthesis of carbohydrates. We here investigate how the electron density in acyl protecting groups influences the stereoselectivity by fine-tuning the efficiency of remote participation. Electron-rich C4-pivaloylated galactose building blocks show an unprecedented α-selectivity. The trifluoroacetylated counterpart with electron-withdrawing groups, on the other hand, exhibits a lower selectivity. Cryogenic infrared spectroscopy in helium nanodroplets and density functional theory calculations revealed the existence of dioxolenium-type intermediates for this reaction, which suggests that remote participation of the pivaloyl protecting group is the origin of the high α-selectivity of the pivaloylated building blocks. According to these findings, an α-selective galactose building block for glycosynthesis is developed based on rational considerations and is subsequently employed in automated glycan assembly exhibiting complete stereoselectivity. Based on the obtained selectivities in the glycosylation reactions and the results from infrared spectroscopy and density functional theory, we suggest a mechanism by which these reactions could proceed.

Quantitative determination of sn-positional phospholipid isomers in MSn using silver cationization

Glycerophospholipids are one of the fundamental building blocks for life. The acyl chain connectivity to the glycerol backbone constitutes different sn-positional isomers, which have great diversity and importance for biological function. However, to fully realize their impact on function, analytical techniques that can identify and quantify sn-positional isomers in chemically complex biological samples are needed. Here, we utilize silver ion cationization in combination with tandem mass spectrometry (MSⁿ) to identify sn-positional isomers of phosphatidylcholine (PC) species. In particular, a labile carbocation is generated through a neutral loss (NL) of AgH, the dissociation of which provides diagnostic product ions that correspond to acyl chains at the sn-1 or sn-2 position. The method is comparable to currently available methods, has a sensitivity in the nM–µM range, and is compatible with quantitative imaging using mass spectrometry in MS⁴. The results reveal a large difference in isomer concentrations and the ion images show that the sn-positional isomers PC 18:1_18:0 are homogeneously distributed, whereas PC 18:1_16:0 and PC 20:1_16:0 show distinct localizations to sub-hippocampal structures.