18O stable isotope labeling, quantitative model experiments, and molecular dynamics simulation studies on the trans-specific degradation of the bitter tasting iso-alpha-acids of beer.
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 18O 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.
Food Chemistry and Molecular Sensory Science, Technische Universitat Munchen, Lise-Meitner Strasse 34, D-85354 Freising, Germany.
Daniel
D
Intelmann
Oliver
O
Demmer
Nina
N
Desmer
Thomas
T
Hofmann
D001515:Beer / Q000032:analysis*; D003512:Cyclohexanones / Q000737:chemistry*; D053138:Cyclohexenes / Q000737:chemistry*; D005511:Food Handling / Q000379:methods; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D007553:Isotope Labeling; D008958:Models, Molecular; D010103:Oxygen Isotopes; D013237:Stereoisomerism; D013649:Taste; D013696:Temperature; D013729:Terpenes / Q000737:chemistry*
D003512:Cyclohexanones; D053138:Cyclohexenes; D010103:Oxygen Isotopes; D013729:Terpenes; C442487:cohumulone; C442488:adhumulone; C007966:humulon