Abstract:  Objective: To study the effects of geometrical isomerisation on singlet-oxygen quenching by lycopene in vitro. Method: In this study, induced by iodine and with further separation and purification by C30-HPLC, the certain amounts of 13Z- and 9Z-isomers were prepared from the all E-isomer of natural lycopene. Ability to extinguish singlet-oxygen generated from reaction between NaClO and H2O2 by each isomer was then compared by count photons generated from the reaction. Result: Ability to quench singlet-oxygen by each isomer is ranked in following order: allE-isomer>13Z-isomer>9 Z-isomer. Conclusion: It is therefore considered that Z-isomerisation appeared by both sides of double bond in the molecule of lycopene can reduce the ability to extinguish singlet-oxygen of the molecule. Additionally, the closer to the center of the molecule this isomerisation, the more decrease of ability to extinguish singlet-oxygen by the molecule.

Key words: lycopene, isomerisation, singlet-oxygen quenching