<br/>The location of double bonds (C=C) in lipids is determined by their biosynthetic pathways and it can be used to reveal metabolic alterations in biological systems. Recently, positional lipid isomers, which differ only in the C=C location, have been recognized as potential biomarkers of numerous diseases including diabetes and cancer. However, these subtle structural differences in lipids remain invisible to traditional mass spectrometry imaging (MSI) techniques, in which ion images typically represent composite distributions of coexisting isomers. Herein, we have developed an online chemical derivatization method based on the singlet oxygen (SO) reaction that allows the identification of C=C positions in different lipid classes. The SO reaction is coupled to electrospray ionization (ESI) for the analysis of lipid extracts and nanospray desorption ionization (nano-DESI) for in situ spatial profiling and imaging of tissue sections. The reaction is promoted using a green laser pointer (532 nm) which is directed to the tip of the capillary and by using a photosensitizer added directly to the solvent. On-the-fly photooxidation of lipids into their corresponding lipid hydroperoxide (LOOH) is achieved promptly after laser excitation. Collision-induced dissociation (CID) of LOOHs generates fragment ions indicative of the position of the hydroperoxide group and hence C=C bond in the unreacted lipid. For instance, we found that species containing FA 18:1 such as PC 16:0_18:1(LOOH) at <i>m/z</i> 814.5 contain a pair of positional isomers with C=C location at C9 and C11. Using this workflow, we successfully characterized the C=C locations of 14 lipid species in a gastrocnemius muscle lipid extract using the ESI setup. Nano-DESI MSI experiments were performed using rat cerebellum tissue sections as a model system. Ratio images of C9 and C11 isomers from PC 16:0_18:1(LOOH) reveal that the C11 isomer is enriched in the grey matter whereas the C9 isomer is enriched in the white matter. These findings demonstrate that coupling of SO reaction to nano-DESI improves the molecular specificity of MSI experiments thereby unravelling the unique distributions of positional isomers that would be otherwise plotted as a mixture. The information obtained in these experiments will be essential to understanding the physiological role of lipid isomers in human health and disease, which still presents a significant challenge.

Rapid profiling and imaging of positional lipid isomers through online photochemical derivatization of C=C bonds

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.