title: Coupling HPLC-SPE-NMR with a microplate-based high-resolution antioxidant assay for efficient analysis of antioxidants in food--validation and proof-of-concept study with caper buds.
abstract: This work describes the coupling of a microplate-based antioxidant assay with a hyphenated system consisting of high-performance liquid chromatography-solid-phase extraction-nuclear magnetic resonance spectroscopy, i.e., HPLC-SPE-NMR/high-resolution antioxidant assay, for the analysis of complex food extracts. The applicability of the microplate-based antioxidant assay for high-resolution screening of common food phenolics as well as parameters related to their trapping efficiency, elution behavior, and recovery on/from SPE cartridges are described. It was found that the microplate-based high-resolution antioxidant assay is an attractive and easy implementable alternative to direct on-line screening methods. Furthermore, it was shown that Resin SH and Resin GP SPE material are superior to RP C18HD for trapping of phenolic compounds. Proof-of-concept study was performed with caper bud extract, revealing the most important antioxidants to be quercetin, kaempferol, rutin, kaempferol-3-O-β-rutinoside and N(1),N(5),N(10)-triphenylpropenoyl spermidine amides. Targeted isolation of the latter, and comprehensive NMR experiments showed them to be N(1),N(10)-di-(E)-caffeoyl-N(5)-p-(E)-coumaroyl spermidine, N(1)-(E)-caffeoyl-N(5),N(10)-di-p-(E)-coumaroyl spermidine, N(10)-(E)-caffeoyl-N(1),N(5)-di-p-(E)-coumaroyl spermidine, and N(1),N(5),N(10)-tri-p-(E)-coumaroyl spermidine amides.
authors.affiliation: Department of Basic Sciences and Environment, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871 Frederiksberg, Denmark.
authors.forename: Stefanie
authors.initials: S
authors.lastname: Wiese
authors.forename: Sileshi G
authors.initials: SG
authors.lastname: Wubshet
authors.forename: John
authors.initials: J
authors.lastname: Nielsen
authors.forename: Dan
authors.initials: D
authors.lastname: Staerk
mesh_terms: D000975:Antioxidants / Q000032:analysis* / Q000302:isolation & purification*; D031484:Capparis / Q000737:chemistry*; D002851:Chromatography, High Pressure Liquid / Q000379:methods*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D010936:Plant Extracts / Q000032:analysis* / Q000302:isolation & purification*; D052616:Solid Phase Extraction / Q000379:methods*
chemical_list: D000975:Antioxidants; D010936:Plant Extracts
keywords: Antioxidant activity; Caper bud; Flavonoids; HPLC-SPE-NMR; High-resolution screening; Phenolic acids; Spermidine