Lipids are the major components of cell membranes and play various roles in maintaining cellular functions. Complete lipid structure elucidation is difficult since lipids are structurally diverse and often contain mixtures of isomers. These structural isomers can perform dramatically different roles in lipid metabolism and pathogenesis, which attract increasing attention to lipid structure identification. Although commercial mass spectrometers coupled with tandem MS can aid lipid assignment of headgroup and fatty acyl chain length isomers, it fails in analyzing positions and geometries of carbon-carbon double-bonds (C=C bonds) and sn-position in lipids. These limitations motivate us to develop novel MS-based methodologies for lipid structure characterization at the isomer level. Nanoelectrospray ionization (nESI), performed by using a wire-in-a-capillary emitter, has been extensively used in the MS analysis of biological samples. Besides, nESI itself can also be considered as an electrochemical cell, in which the metal wire serves as an anode when a positive voltage is applied to the metal wire. The unique role of nESI emitter as an electrochemical cell can be utilized to oxidize the lipid C=C bonds. Herein, we developed an on-demand electrochemical epoxidation of lipid C=Cs and incorporated it into the standard nESI-MS workflow for identification of lipid C=C position(s). The electro-epoxidation of lipids can be switched on/off by simply tuning the spray voltage and fragmentation of the epoxidized products in tandem MS can generate diagnostic ions which indicate the C=C position(s). When a wire-in-a-capillary nESI emitter is fitted with a metallic anode, metals can be ionized by electrochemical corrosion and transported by ESI. We proposed to use in-situ produced cobalt ions (Co2+) by anodic corrosion of Co electrode in the nESI process to form complexes with lipids at sn-positions, so that characteristic ions can be formed upon fragmentation in tandem MS to differentiate lipid sn-isomers. It is noteworthy that the formation of Co-lipid complexes in this method is also voltage-dependent, which allows us to cascade it with electro-epoxidation of lipids for identifying C=C bond positional isomers simultaneously. The unique feature of this setup lies in the sequential combination of localization of C=C bonds, elucidation of sn-positions, and collection of native lipid information simply by tuning the nESI voltages within a single experimental run.

On-demand electrochemical strategies in mass spectrometry for lipids structure characterization

ACS Spring 2022

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