Novel Aza-Paternò-Büchi Reaction Allows Pinpointing Carbon–Carbon Double Bonds in Unsaturated Lipids by Higher Collisional Dissociation

The evaluation of double bond positions in fatty acyl
chains has
always been of great concern given their significance in the chemical
and biochemical role of lipids. Despite being the foremost technique
for lipidomics, it is difficult in practice to obtain identification
beyond the fatty acyl level by the sole high-resolution mass spectrometry.
Paternò–Büchi reactions of fatty acids (FAs)
with ketones have been successfully proposed for pinpointing double
bonds in FAs in combination with the collision-induced fragmentation
technique. In the present paper, an aza-Paternò–Büchi
(aPB) reaction of lipids with 6-azauracil (6-AU) was proposed for
the first time for the determination of carbon–carbon double
bonds in fatty acyl chains using higher collisional dissociation in
the negative ion mode. The method was optimized using free FA and
phospholipid analytical standards and compared to the standard Paternò–Büchi
reaction with acetone. The introduction of the 6-AU moiety allowed
enhancing the ionization efficiency of the FA precursor and diagnostic
product ions, thanks to the presence of ionizable sites on the derivatizing
agent. Moreover, the aPB derivatization allowed the obtention of deprotonated
ions of phosphatidylcholines, thanks to an intramolecular methyl transfer
from the phosphocholine polar heads during ionization. The workflow
was finally applied for pinpointing carbon–carbon double bonds
in 77 polar lipids from an yeast (Saccharomyces cerevisiae) extract.