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Profiling of Cholesteryl Esters by Coupling Charge Tagging Paternò-Büchi Reaction and Liquid Chromatography-Mass Spectrometry.

The profile of cholesteryl esters (CEs) is increasingly used in metabolic disease monitoring due to the roles of CE in regulating the cholesterol level. While electrospray ionization-tandem mass spectrometry is routinely applied for the identification and quantitation of CE, it has a limitation of not being able to provide the location of carbon-carbon double bond (C = C) within unsaturated fatty acyls. In this study, we paired offline 2-acetylpyridine (2-AP) Paternò-Büchi (PB) reaction and reversed-phase liquid chromatography tandem mass spectrometry (RPLC-MS/MS) to achieve highly sensitive and structural informative CE analysis from complex mixtures. The 2-AP PB reactions of CE standards provided 20-30% conversion but resulted in enhanced ion signal relative to that of intact CE detected as ammonium adduct ions. MS/MS of 2-AP derivatized CE via collision-induced dissociation produced two abundant diagnostic ions for each C = C in a fatty acyl, leading to both sensitive identification and quantitation of C = C location isomers. Twelve saturated and twenty-seven unsaturated CEs were profiled in pooled human plasma; of the latter group, relative quantitation of 6 groups of C=C location isomers was achieved. A dehydrocholesteryl ester, DHE 18:2(Δ9,12), was confidently differentiated from co-existing compositional isomers: CE 18:3 (Δ9,12,15) and CE 18:3 (Δ6,9,12). The above results represented improved CE coverage at C = C location level than those reported by gas-chromatography MS or acetone PB-MS/MS methods.