Targeted screening and quantitative analyses of antioxidant compounds in aged-garlic extract

Previous studies suggest the existence of unidentified antioxidants in aged garlic extract (AGE), which are responsible for the potent antioxidant property of the whole AGE. Therefore, potential candidate antioxidants were synthesized followed by a targeted LC–MS/MS screening, resulting in the identification of 18 compounds in AGE. Several hits showed antioxidant activity comparable to the reference compounds. Subsequently, all the identified compounds were quantitated by means of a stable isotope dilution assay or external standard calibration in combination with LC–MS/MS. From the results of the antioxidant assays and quantitative analysis, (–)-(2R,3S)-dihydrodehydrodiconiferyl alcohol (1), (+)-(2S,3R)-dehydrodiconiferyl alcohol (2), threo-guaiacylglycerol-β-O-4′-coniferyl ether (4), N-trans-feruloyltyramine (14), and N-trans-coumaroyltyramine (16) as well as S-allyl-l-cysteine were highlighted to be the major antioxidants in AGE.