Trimethylation Enhancement Using 13C-Diazomethane (13C-TrEnDi): Gas-Phase Charge Inversion of Modified Phospholipid Cations for Enhanced Structural Characterization

摘要

Methylation of phospholipids (PL) leads to increased uniformity in positive electrospray ionization (ESI) efficiencies across the various PL sub-classes. This effect is realized in the approach referred to as “trimethylation enhancement using ¹³C-diazomethane” (¹³C-TrEnDi), which results in the methyl esterification of all acidic sites and the conversion of amines to quaternary ammonium sites. Collision induced dissociation (CID) of these cationic, modified lipids enables class identification by forming distinctive head-group fragments based on the number of ¹³C atoms incorporated during derivatization. However, there are no distinctive fragment ions in positive mode that provide fatty acyl information for any of the modified lipids. Gas-phase ion/ion reactions of ¹³C-TrEnDi-modified PE, PS, PC, and SM cations with dicarboxylate anions are shown to charge-invert the positively charged phospholipids to the negative mode. An electrostatically-bound complex anion is shown to fragment predominantly via a novel head-group dication transfer to the reagent anion. Fragmentation of the resulting anionic product yields fatty acyl information, in the case of the glycerophospholipids (PE, PS, and PC), via ester bond cleavage. Analogous information is obtained from modified SM lipid anions via amide bond cleavage. Fragmentation of the anions generated from charge inversion of the ¹³C-TrEnDi modified phospholipids was also found to yield lipid class information without having to perform CID in positive mode. The combination of ¹³C-TrEnDi-modification of lipid mixtures with charge inversion to the negative ion mode retains the advantages of uniform ionization efficiency in the positive ion mode with the additional structural information available in the negative ion mode without requiring the lipids to be ionized directly in both ionization modes.