Abstract
Alkylresorcinols (5-alkyl-1,3-dihydroxybenzenes, ARs) are bioactive phenolic lipid compounds which are particularly abundant in rye and partly other cereals. In this study on ARs, whole rye grain extracts were gained with cyclohexane/ethyl acetate (46/54, w/w). Silylated extracts were used to develop a gas chromatography with mass spectrometry method in the selected ion monitoring mode (GC/MS-SIM) for the sensitive detection of conventional ARs along with keto-substituted (oxo-AR) and ring-methylated ARs (mAR) with 5-alkyl chain lengths of 14 to 27 carbon atoms and 0 to 4 double bonds in one run. Analysis was performed by countercurrent chromatographic (CCC) fractionation using the solvent system n-hexane/ethyl acetate/methanol/water (9/1/9/1, v/v/v/v). Subsequent GC/MS-(SIM) analysis of 80 silylated CCC fractions enabled the detection of 74 ARs in the sample. The CCC elution of the ARs followed the equivalent chain length (ECL) rule in which one double bond compensated the effect of two (additional) carbon atoms. Novel or rarely reported ARs were detected in virtually all classes, i.e. saturated AR (AR14:0), even-numbered monounsaturated AR isomers (AR16:1-AR26:1), triunsaturated ARs (AR25:3), oxo-ARs (AR17:0 oxo, AR19:1 oxo, AR21:2 oxo, AR23:2 oxo) and odd-numbered methyl-ARs (mAR15:0-mAR23:0). Positions of the double bonds of monounsaturated ARs and oxo-ARs were determined with the help of dimethyl disulfide (DMDS) derivatives.Graphical abstract
Highlights
Alkylresorcinols (ARs) are amphiphilic 1,3-dihydroxybenzene derivatives characterized by a hydrocarbon chain on C-5 of the resorcinol backbone (Fig. 1) [1]
To establish the individual time windows, it was considered that isobaric ARs eluted in the order methylated ARs (mAR) < AR < oxo-AR [7, 30] and unsaturated ARs eluted between mAR and saturated ARs
The resulting gas chromatography with mass spectrometry (GC/MS)-selected ion monitoring (SIM) method enabled the additional detection of low amounts of AR27:0, AR 27:1, AR17:2, AR19:2 and even-numbered ARs (AR16:0-AR 22:0) along with traces of AR17:0 oxo, AR19:0 oxo, AR21:0 oxo and AR23:0 oxo in the silylated SPE fraction 4 of the lipid class separation
Summary
Alkylresorcinols (ARs) are amphiphilic 1,3-dihydroxybenzene (resorcinol) derivatives characterized by a hydrocarbon chain on C-5 of the resorcinol backbone (Fig. 1) [1]. High-performance liquid chromatography with tandem mass spectrometry (HPLC/MSMS) [6] and gas chromatography with mass spectrometry (GC/MS) analyses [7] showed that ~ 85% of the ARs in rye feature a saturated alkyl chain (ARn:0) [8]. The most relevant representatives are odd-numbered (AR17:0-AR25:0) along with low shares of AR15:0 and AR27:0 and even-numbered homologues (AR16:0-AR24:0) [6, 9]. The remaining share of ~ 15% constitutes mainly of mono- (ARn:1), di- (ARn:2) and tri- (ARn:3) unsaturated hydrocarbon chains (alkenylresorcinols). Hydroxyl (ARn:1-OH) and keto (ARn:0 oxo) groups can be present as substituents on the alkyl chain [6, 7, 10]
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