cis-trans Isomerization Products from S-(cis-l-Propenyl)-L-cysteine and Its Sulfoxide Irradiated in Aqueous Solutions

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Abstract
In connection with food-flavor deterioration accompanied by irradiation, the formation mechanism of the degradation products from S-(cis-l-propenyl)-L-cysteine (PeCS) and its sulfoxide (PeCSO) (isomers of the naturally-occurring lachrymatory precursors of onion) irradiated by γ-ray in oxygen-free aqueous solutions was investigated. From the comparisons of mass-spectral fragmentations, gas-chromatographic retention time and IR spectra with reference compounds, the major volatile products from γ-irradiated PeCS were identified as n-propyl cis-l-propenyl sulfide, n-propyl trans-l-propenyl sulfide, cis, cis-di-l-propenyl sulfide and cis, trans-di-l-propenyl sulfide and those from γ-irradiated PeCSO were identified as di-l-propenyl sulfides (cis-cis and cis-trans). A new cis-trans isomerization mechanism of the 1-propenyl thiyl radicals was suggested and it was elucidated to give a mixture of about 33% cis- and 67% trans-l-propenyl thiyl radicals.

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