Photochemical Production of Ethylene from Methionine and Its Analogues in the Presence of Flavin Mononucleotide

Revisiting the mechanism responsible for the light-struck flavor in white wines and Champagnes

“…The mechanism proposed bears resemblance to that previously put forth by F. A. Yang et al in 1967, in which the formation of volatile sulfur products is also accompanied by the production of ethylene as evidenced using 14 C-labelled Met (Huvaere et al, 2006;Yang, Ku, & Pratt, 1967). In both mechanisms, the initial step involves oxidative electron transfer quenching of 3 Rf * by Met to generate a thiol-centered cation radical that undergoes subsequent decarboxylation and hydrolysis to give methional.…”

“…We may expect that the dimerization constant for Met would lie in-between these two values, indicating that its formation is thermodynamically favored under the experimental conditions found in white wines and Champagnes. (Lieberman, Kunishi, Mapson, & Wardale, 1965;Yang et al, 1967). Additionally, we have observed the formation of glycine by GC / MS (see supplementary information)…”

“…The mechanisms proposed by Maujean and Seguin (Maujean et al, 1983) and by Yang et al (Yang et al, 1967) share a common pathway leading to the formation of methional even though the latter was not detected by either group, reportedly owing to its instability under the reaction conditions. They then diverge in that Yang et al propose a second PET reaction between 3 Rf * and methional, followed by elimination to form MeSH, ethylene, and formic acid.…”

“…Scheme 1. Comparison of the mechanisms proposed by Maujean and Seguin (Maujean et al, 1983) and by Yang et al (Yang et al, 1967) for the formation of volatile sulfur species upon irradiation of Rf solutions containing Met. Bimolecular reactions between intermediates present at very low concentrations are highlighted in red.…”

Flavin-induced photodecomposition of sulfur-containing amino acids is decisive in the formation of beer lightstruck flavor

“…As this intermediate was not detected after photooxidation of homocysteine, it was attributed to deprotonation of the sulfur-centered radical cation with formation of H 3 N + -CH(COO − )CH 2 CH • -SCH 3 . Still, methanethiol and dimethyl disulfide were identified as major reaction products and, moreover, decarboxylation was unambiguously proven, as 13 CO 2 (m/z 45) was produced after photooxidation of methionine-carboxy-13 C. According to Yang et al, degradation products include ethylene, methane, and ammonia as well, 46 but, since the lower mass limit was set by saturation of the samples with nitrogen, these compounds escaped detection by headspace GC-MS.…”