title: NMR assignment of iso-alpha-acids from isomerised extracts of Humulus lupulus L. cones.
abstract: Iso-alpha-acids are known to contribute to the characteristic bitter taste of beer. Six iso-alpha-acids were isolated from a carbon dioxide extract of the cones of Humulus lupulus L. by centrifugal partition chromatography followed by separation through beta-cyclodextrin binding. This method overcame the low yield issue of most isolation procedures that results from the low stability of these compounds to light and oxygen. Their full identification was performed using one- and two-dimensional NMR spectrometry, including (1)H- and (13)C-NMR, (1)H-(1)H COSY, HMQC and HMBC and electrospray ionisation mass spectrometry. The results confirmed the structures of the isolated compounds as trans-isocohumulone, cis-isocohumulone, trans-isohumulone, cis-isohumulone, trans-isoadhumulone and cis-isoadhumulone. Epimers can be easily distinguished by observing the chemical shift differences of the H-5, H-1''', H-2''' and C-5 signals and the different splitting pattern of H-5 and H-2''.
authors.affiliation: Division of Pharmacognosy, Section Metabolomics, Institute of Biology, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands.
authors.forename: Alfi
authors.initials: A
authors.lastname: Khatib
authors.forename: Erica G
authors.initials: EG
authors.lastname: Wilson
authors.forename: Hye Kyong
authors.initials: HK
authors.lastname: Kim
authors.forename: Moses
authors.initials: M
authors.lastname: Supardi
authors.forename: Young Hae
authors.initials: YH
authors.lastname: Choi
authors.forename: Robert
authors.initials: R
authors.lastname: Verpoorte
mesh_terms: D000143:Acids / Q000737:chemistry; D003512:Cyclohexanones / Q000737:chemistry*; D005638:Fruit / Q000737:chemistry; D027582:Humulus / Q000737:chemistry* / Q000378:metabolism; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D010936:Plant Extracts / Q000737:chemistry*; D013729:Terpenes / Q000737:chemistry*
chemical_list: D000143:Acids; D003512:Cyclohexanones; D010936:Plant Extracts; D013729:Terpenes