Title: Mechanistic insights into the oxidation of the sesquiterpenesbeta-caryophyllene and alpha-humulene
Other Titles: Mechanistische inzichten in de oxidatie van de sesquiterpenenbeta-caryofylleen en alfa-humeleen
Authors: Steenackers, Bart
Issue Date: 16-Apr-2015
Abstract: Hops (Humulus lupulus L.) are one of the four basic raw materials of the beer brewing process. The secondary metabolites of hops strongly contribute to the organoleptic properties of beer: hop acids are the precursors of the main bittering principles of beer and hop oil components contribute to the aroma of beer. The essential oil of hops consists of a complex mixture of terpenoids with the monoterpene myrcene and the sesquiterpenes beta-caryophyllene and alpha-humulene as major constituents. Not the terpenic hydrocarbons themselves but their oxygenated derivatives are the major contributors to the aroma of beer. Oxygenated monoterpenoids are generally associated with floral and citrusy notes whereas oxygenated sesquiterpenoids are linked to woody and spicy aroma types of beer. Oxygenated sesquiterpenoids, identified in beer and/or hop oil, primarily consist of beta-caryophyllene- and alpha-humulene-derived epoxides and their respective rearrangement and hydrolysis products. Furthermore, when beta-caryophyllene- and alpha-humulene are exposed to simulated wort boiling conditions, they are epoxidized with remarkably high selectivities. Given the importance of oxygenated derivatives of beta-caryophyllene and alpha-humulene for the aroma of beer, the oxidation of beta-caryophyllene and alpha-humulene is studied in this work, with special attention for the spontaneous epoxidation of these sesquiterpenes.
First, the reactivity of beta-caryophyllene and alpha-humulene in the presence of various oxidants was explored. Selective, uncatalyzed epoxidation of these sesquiterpenes occurs in the presence of peracids, alkyl hydroperoxides, hydrogen peroxide and even molecular oxygen. Beta-caryophyllene is epoxidized to beta-caryophyllene oxide whereas humulene monoepoxide II is the major oxidation product of alpha-humulene.
The mechanism of the uncatalyzed epoxidation of beta-caryophyllene and alpha-humulene with hydrogen peroxide was then studied. The combined data from a kinetic study, a computational study and the screening of different olefinic substrates point to an activation mechanism of hydrogen peroxide based on hydrogen bonding with the solvent and the increased reactivity of beta-caryophyllene and alpha-humulene in electrophilic oxidation systems.
Next, we attempted to elucidate the mechanism of the selective aerobic epoxidation of beta-caryophyllene. The formed products, observed solvent and additive effects and the data obtained from an electrochemical study are consistent with an oxidation chain mechanism, initiated by electron transfer from beta-caryophyllene to molecular oxygen. The cation radical intermediate carries the oxidation chain in which dioxetanes are formed as primary oxidation products. The deoxygenation of these dioxetanes by beta-caryophyllene constitutes a selective epoxidation pathway. When the beta-caryophyllene-based cation radical is formed under inert atmosphere, beta-caryophyllene is selectively isomerized instead.
Finally, the photosensitized oxidation of beta-caryophyllene and alpha-humulene was studied. Methylene blue-sensitized photooxidation results in selective formation of beta-caryophyllene- and alpha-humulene-derived hydroperoxides through type II photooxidation chemistry. Calculated Foote reactivity indices are consistent with the observed increased reactivity of these sesquiterpenes in electrophilic oxidation systems. 9,10-Dicyanoanthracene-sensitized oxidation results in parallel epoxidation and hydroperoxidation of beta-caryophyllene and alpha-humulene, indicative of competing type I and type II photooxidation. Epoxidation of these sesquiterpenes by type I photosensitized oxidation is consistent with the proposed mechanism for the aerobic epoxidation of beta-caryophyllene.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Centre for Surface Chemistry and Catalysis
Bioengineering Technology TC, Technology Campuses Ghent and Aalst
Technologiecluster Bioengineering Technologie

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