Re-characterization of three conjugated linolenic acid isomers by GC–MS and NMR

Conjugated linolenic acids (CLN) refer to a group of octadecatrienoic acids with three conjugated double bonds. Minor positional and geometrical differences among CLN isomers make their separation and identification difficult. We have used GC–MS and NMR to study three common CLN isomers namely α-eleostearic acid, β-eleostearic acid and punicic acid, finding that some signals of olefinic carbon atoms in NMR spectra were mistakenly assigned in the literature. The present study was therefore undertaken to re-characterize the location of CC double bonds and assign the chemical signals of proton and carbon atoms using 1H NMR, 13C NMR, 1H–1H two-dimensional correlation spectra (1H–1H COSY) and 13C–1H two-dimensional correlation spectra (13C–1H COSY). The geometrical structure of double bonds in these three CLN isomers was identified using homonuclear decoupling technique.