title: GC-ECNICI-MS/MS of eicosanoids as pentafluorobenzyl-trimethylsilyl (TMS) derivatives: Evidence of CAD-induced intramolecular TMS ether-to-ester rearrangement using carboxy-18O-labelled eicosanoids and possible implications in quantitative analysis.
abstract: GC-MS and GC-MS/MS of pentafluorobenzyl (PFB) ester trimethylsilyl (TMS) ether (PFB-TMS) derivatives of hydroxylated long-chain fatty acids including arachidonic acid metabolites, the eicosanoids, in the electron-capture negative-ion chemical ionization (ECNICI) mode are the most sensitive and accurate approaches to quantify carboxyl groups-containing compounds in complex biological fluids such as plasma and urine. Under ECNICI conditions, PFB-TMS derivatives of eicosanoids ionize to form very few ions, with the carboxylates [M-PFB]-being typically the most intense. Less intense ions may be additionally formed by consecutive neutral loss (NL) of trimethylsilanol (TMSOH, 90Da) groups ([M-PFB-(TMSOH)n]-). By using [1,1-18O2]- and [1,ω-18O2]-eicosanoids, we studied ion processes following collisionally activated dissociation (CAD) of the precursor ions [M-PFB]-. We found that CAD resulted in formation of product ions due to NL of a TMS18OH (92Da) group in monocarboxylic and of a PFB18OH (200Da) group in dicarboxylic eicosanoids. TMS18OH NL implies an intra-molecular transfer of the TMS group from hydroxyl groups to their carboxylate anions [M-PFB]-. From a mechanistic point of view, this rearrangement may explain formation of unique product ions in GC-MS/MS of eicosanoids under ECNICI conditions. From the quantitative point of view, quantification by GC-MS/MS of product ions due to [M-PFB-(TMSOH)n]-and [M-PFB-TMS18OH-(TMSOH)n-1]-would reveal incorrect data, if [1,1-18O2]-eicosanoids are used as internal standards and if no correction for the18O-loss is performed. In18O-labelled dicarboxylic eicosanoids, such as the major urinary metabolite (MUM) of E prostaglandins, i.e., [1,ω-18O2]-PGE-MUM), no TMS ester/TMS ether rearrangement was observed. Yet,18O-loss occurred upon CAD of [M-PFB]-due to NL of PFB18OH (200Da). In both cases the extent of18O-loss needs to be determined and considered for accurate quantification of monocarboxylic acids such as 8-isoprostaglandin F2α(8-iso-PGF2α) and dicarboxylic eicosanoids such as PGE-MUM.
authors.affiliation: Centre of Pharmacology and Toxicology, Hannover Medical School, Carl-Neuberg-Str. 1, D-30625, 30623 Hannover, Germany. Electronic address: tsikas.dimitros@mh-hannover.de.
authors.forename: Dimitrios
authors.initials: D
authors.lastname: Tsikas
mesh_terms: D015777:Eicosanoids / Q000032:analysis*; D004952:Esters / Q000737:chemistry; D005464:Fluorobenzenes / Q000737:chemistry*; D008401:Gas Chromatography-Mass Spectrometry / Q000379:methods*; D006801:Humans; D010103:Oxygen Isotopes / Q000032:analysis / Q000652:urine; D011466:Prostanoic Acids / Q000032:analysis / Q000652:urine; D053719:Tandem Mass Spectrometry / Q000379:methods; D014297:Trimethylsilyl Compounds / Q000737:chemistry*
chemical_list: D015777:Eicosanoids; D004952:Esters; D005464:Fluorobenzenes; D010103:Oxygen Isotopes; D011466:Prostanoic Acids; D014297:Trimethylsilyl Compounds; C037830:11 alpha-hydroxy-9,15-dioxo-2,3,4,5,20-pentanor-19-carboxyprostanoic acid
keywords: Collisionally-activated dissociation; Derivatization; Electron-capture negative-ion chemical ionization; Oxygen-18; Pentafluorobenzyl bromide; Trimethylsilyl