S -Alkenyl Cysteine Sulfoxide and Its Antioxidant Properties from Allium cepa var. tropeana (Red Onion) Seeds

A new cysteine sulfoxide, (S(S)R(C))-S-(3-pentenyl)-L-cysteine sulfoxide (1), was identified from the seeds of Allium cepa var. tropeana, together with the known methiin, etiin, alliin, isoalliin, propiin, and butiin. The structure of compound 1 was established by analysis of its physical and spectroscopic data. The antioxidant activity of an extract containing cysteine sulfoxides and compound 1 was evaluated using the FRAP and DPPH tests.