On-line liquid-chromatography–nuclear magnetic resonance spectroscopy–mass spectrometry coupling for the separation and characterization of secoisolariciresinol diglucoside isomers in flaxseed

Two secoisolariciresinol diglucoside (SDG) diastereomers were extracted from flaxseed and liberated through alkaline hydrolysis. Anion-exchange and reversed-phase chromatography were successfully employed to purify the hydrolyzed flaxseed extract. On-line LC–NMR–MS analyses revealed the structure of the isolated and purified SDG diastereomers, [2R,2′R]-2,3-bis[(4-hydroxy-3-methoxyphenyl)-methyl]-1,4-butanediyl-bis-β-glucopyranoside the predominant flaxseed lignan and [2R,2′S]-2,3-bis[(4-hydroxy-3-methoxyphenyl)methyl]-1,4-butanediyl-bis-β-glucopyranoside, a previously incompletely characterized minor flaxseed lignan. Circular dichroism (CD) analyses confirmed the presence of two distinguished optically active compounds present in the flaxseed extract.