Analytical Sciences

Studies on High-energy Collision-induced Dissociation of Endogenous Cannabinoids: 2-Arachidonoylglycerol and N-Arachidonoylethanolamide in FAB-Mass Spectrometry

Hiroko F. KASAI, Masayoshi TSUBUKI, and Toshio HONDA

Faculty of Pharmaceutical Sciences, Hoshi University, Ebara 2-4-41, Shinagawa, Tokyo 142-8501, Japan

Analysis of 2-arachidonoylglycerol (2-AG) and N-arachidonoylethanolamide (anandamide) via alkali or alkaline earth metal-adduct high-energy collision-induced dissociation (CID) in fast-atom bombardment (FAB) ionization-mass spectrometry (MS) is described. The CID-MS/MS of the [2-AG+Li]⁺ or [2-AG+Na]⁺ ion undergoes charge-remote fragmentation (CRF), which is useful for the determination of the double-bond positions in the hydrocarbon chain, while the CID-MS/MS of the [2-AG-H+Cat]⁺ (Cat = Mg²⁺, Ca²⁺, Ba²⁺) ion provides an abundant fragment ion of the cationized arachidonic acid species, which is derived from cleaving the ester bond via a McLafferty-type rearrangement in addition to structurally informative CRF ions in small amounts. On the other hand, the CID-MS/MS spectra of anandamide cationized with both alkali metal (Li⁺ or Na⁺) and alkaline earth metal (Mg²⁺, Ca²⁺, or Ba²⁺) show CRF patterns: the spectra obtained in lithium or sodium adduct are more clearly visible than those in magnesium, calcium, or barium adduct. The McLafferty rearrangement is not observed with metal-adduct anandamide. The characteristics in each mass spectrum are useful for the detection of these endogenous ligands. m-Nitrobenzyl alcohol (m-NBA) is the most suitable matrix. A lithium-adduct [2-AG+Li]⁺ or [anandamide+Li]⁺ ion is observed to be the most abundant in each mass spectrum, since the affinity of lithium for m-NBA is lower than that for other matrices examined.

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