Fatty acid neutral losses observed in tandem mass spectrometry with collision-induced dissociation allows regiochemical assignment of sulfoquinovosyl-diacylglycerols.

A full characterization of sulfoquinovosyldiacylglycerols (SQDGs) in the lipid extract of spinach leaves has been achieved using liquid chromatography/electrospray ionization-linear quadrupole ion trap mass spectrometry (MS). Low-energy collision-induced dissociation tandem MS (MS/MS) of the deprotonated species M - H was exploited for a detailed study of sulfolipid fragmentation. Losses of neutral fatty acids from the acyl side chains (i.e. M - H - RCOOH) were found to prevail over ketene losses (M - H - R'CHCO) or carboxylates of long-chain fatty acids (RCOO), as expected for gas-phase acidity of SQDG ions. A new concerted mechanism for RCOOH elimination, based on a charge-remote fragmentation, is proposed. The preferential loss of a fatty acids molecule from the sn-1 position (i.e. M - H - R(1)COOH) of the glycerol backbone, most likely due to kinetic control of the gas-phase fragmentation process, was exploited for the regiochemical assignment of the investigated sulfolipids. As a result, 24 SQDGs were detected and identified in the lipid extract of spinach leaves, their number and variety being unprecedented in the field of plant sulfolipids. Moreover, the prevailing presence of a palmitic acyl chain (16:0) on the glycerol sn-2 position of spinach SQDGs suggests a prokaryotic or chloroplastic path as the main route for their biosynthesis.