Geometrical and positional isomers of unsaturated furan fatty acids in food

Abstract

Furan fatty acids (FuFA) are important antioxidants found in low concentrations in many types of food. In addition to conventional FuFA which normally feature saturated carboxyalkyl and alkyl chains, a few previous studies indicated the FuFA co‐occurrence of low shares of unsaturated furan fatty acids (uFuFA). For their detailed analysis, the potential uFuFA were enriched by centrifugal partition chromatography (CPC) or countercurrent chromatography (CCC) followed by silver ion chromatography from a 4,7,10,13,16,19‐docosahexaenoic acid ethyl ester oil, a 5,8,11,14,17‐eicosapentaenoic acid ethyl ester oil and a latex glove extract. Subsequent gas chromatography with mass spectrometry (GC/MS) analysis enabled the detection of 16 individual uFuFA isomers with a double bond in conjugation with the central furan moiety. In either case, four instead of two uFuFA isomers previously reported in food, respectively, were detected by GC/MS. These isomers showed characteristic elution and abundance patterns in GC/MS chromatograms which indicated the presence of two pairs of cis/trans‐isomers (geometrical isomers).

Enrichment and structural assignment of geometric isomers of unsaturated furan fatty acids

“…Thirty-six disposable latex gloves (150 g) were cut to small pieces, and three portions of 50 g of each were extracted three times with 900 mL n- hexane, respectively. The n- hexane extracts were combined, evaporated, transesterified with 1% sulfuric acid in methanol for 4 h, and the formed FAMEs were extracted with n- hexane, dried with sodium sulfate, and concentrated on the rotary evaporator [ 8 ].…”

“…Silver ions (Ag + ) coated on silica gel form charge–transfer complexes with double bonds of the analytes, and therefore, this method basically separates FAMEs according to the number, configuration, and partly to the position of double bonds [ 35 ]. In the present study (see previous section), CCC fractions 25–37 were pooled, concentrated to ~ 1 mL, and subjected silver ion chromatography (5 g of 1% deactivated silver nitrate silica gel (20% AgNO 3 , iron-free)) according to Müller et al (2023) [ 8 ] with the following modifications: (i) Ag + -fraction I ( n -hexane/diethyl ether, 99.5:0.5, v/v; targeted compounds: saturated FAMEs) was omitted, and the fractionation started with Ag + -fraction II ( n -hexane/diethyl ether, 97:3, v/v, 70 mL, target compounds: saturated FuFA-ME) on a preconditioned column using Ag + -eluent II. (ii) Ag + -eluent III consisted of 50 mL n -hexane/ethyl acetate, 80:20, v/v (instead of n -hexane/diethyl ether, 80:20, v/v, target compounds: u9M5 isomers).…”

“…Individual FuFAs, shown on the right side, were abbreviated according to Vetter et al [ 7 ]. b Structure of two E -configured unsaturated furan fatty acids (uFuFAs), i.e., E -9-(3-methyl-5-pentylfuran-2-yl)-non-8-enoic acid methyl ester (9:1M5-ME) and E -9-(3-methyl-5-pent-1-enylfuran-2-yl)-nonanoic acid methyl ester (9M5:1-ME) [ 8 ] …”

“…1 b) [ 9 – 12 ]. Moreover, Jandke et al (1988) [ 13 ] and recently Müller et al (2023) [ 8 ] not only detected two but four uFuFA isomers, which produced strong evidence for the existence of Z - and E -isomers [ 8 , 13 ]. Noteworthy and valid for five isomeric groups of uFuFA (u9D5 [ 13 ], u11D3, u11D5, u11M5, and u9M5 [ 8 ]), two isomers were predominant, and two were only low abundant, which might explain why two of the isomers had been overlooked in most of the previous studies that reported the presence of uFuFAs in fish (oil) [ 8 , 10 , 14 , 15 ], beef liver [ 16 ], olive oil [ 11 ], or herbs [ 12 ], and in laboratory studies [ 13 , 16 ].…”

“…Later, it was found that 9M5 was also high abundant in laboratory latex gloves made from Hevea brasiliensis from which it could be isolated in mg amounts [ 23 ]. More recently, it was found that this matrix also contained traces of uFuFAs that were structurally related to 9M5 (u9M5 isomers) [ 8 ]. However, the obtained uFuFA amounts were too small and the purity too poor for a subsequent structure elucidation by NMR.…”