Rapid Profiling of Isomeric Unsaturated Lipids Through Online Photochemical Derivatization of C=C Bonds

Unraveling the complexity of the lipidome requires the development of novel approaches for the structural characterization of lipid species with isomeric resolution. Herein, we introduce an online photochemical approach for lipid isomer identification through selective derivatization of double bonds by reaction with singlet oxygen. Lipid hydroperoxide products are generated promptly after laser irradiation. Fragmentation of these species in a mass spectrometer produces diagnostic fragments, which reveal the C=C locations in the unreacted lipids. This approach uses an inexpensive light source and photosensitizer making it easy to incorporate into any lipidomics workflow. We demonstrate the utility of this approach for the shotgun profiling of C=C locations in different lipid classes present in tissue extracts using electrospray ionization (ESI) and for spatially-resolved analysis of lipids in tissue sections using nanospray desorption electrospray ionization (nano-DESI). These results provide a path for both rapid profiling and ambient imaging of positional isomers in biological samples.