Fate of flavins in sensitized photodegradation of isohumulones and reduced derivatives: studies on formation of radicals via EPR combined with detailed product analyses.
Photodegradation of isohumulones accounts for formation of the lightstruck flavor in beer. The reactions involved are mediated by riboflavin, a natural photosensitizer present in beer in ppb quantities. The results of an investigation of this sensitized degradation process are presented herein. Product analyses and electron paramagnetic resonance spectroscopy, in steady-state as well as in time-resolved mode, offer extensive insight into the photophysical and photochemical details of the degradation mechanism. In contrast to energy transfer and Norrish type I alpha-cleavage reactions that take place on direct irradiation of isohumulones, the sensitization pathway proceeds via one-electron redox chemistry involving the excited triplet state of riboflavin and derivatives. The flavin semiquinone radical thus formed could be readily detected, either by steady state or by time-resolved electron paramagnetic resonance spectroscopy. Superimposed signals in the spectra revealed the presence of radical fragments derived from isohumulones or tetrahydroisohumulones, which, on recombination with riboflavin semiquinone radicals, produced stable reaction products that were identified by HPLC-MS. However, no superimposed signals were observed on sensitized irradiation of dihydroisohumulones.
Heyerick
Arne
A
Laboratory of Pharmacognosy and Phytochemistry, Ghent University, Faculty of Pharmaceutical Sciences, Harelbekestraat 72, B-9000, Ghent, Belgium. Arne.Heyerick@UGent.be
Huvaere
Kevin
K
De Keukeleire
Denis
D
Forbes
Malcolm D E
MD
D003517:Cyclopentanes / Q000737:chemistry* / Q000528:radiation effects*; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D005415:Flavins / Q000737:chemistry* / Q000528:radiation effects*; D005609:Free Radicals / Q000032:analysis / Q000138:chemical synthesis; D008872:Microwaves; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D010777:Photochemistry / Q000379:methods; D010782:Photolysis / Q000528:radiation effects*; D012680:Sensitivity and Specificity
D003517:Cyclopentanes; D005415:Flavins; D005609:Free Radicals; C081209:isohumulone