Journal
JOURNAL OF AGRICULTURAL AND FOOD CHEMISTRY
Volume 57, Issue 22, Pages 11014-11023Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jf903000c
Keywords
Iso-alpha-acids; tricyclocohumol; tricyclocohumene; isotricyclocohumene; tetracyclocohumol; epitetracyclocohumol; stable isotope labeling
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The typical bitterness of fresh beer is well-known to decrease in intensity and to change in quality with increasing age. This phenomenon was recently shown to be caused by the conversion of bitter tasting trans-iso-alpha-acids into lingering and harsh bitter tasting tri- and tetracyclic degradation products such as tricyclocohumol, tricyclocohumene, isotricyclocohumene, tetracyclocohumol, and epitetracyclocohumol. Interestingly, the formation of these compounds was shown to be trans-specific and the corresponding cis-iso-alpha-acids were found to be comparatively stable. Application of O-18 stable isotope labeling as well as quantitative model studies combined with LC-MS/MS experiments, followed by computer-based molecular dynamics simulations revealed for the first time a conclusive mechanism explaining the stereospecific transformation of trans-iso-alpha-acids into the tri- and tetracyclic degradation products. This transformation was proposed to be induced by a proton-catalyzed carbon/carbon bond formation between the carbonyl atom C(1') of the isohexenoyl moiety and the alkene carbon C(2 '') of the isoprenyl moiety of the trans-iso-alpha-acids.
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