Study On The Isomerization Of Lycopene And The Separating Method Of All-trans/Cis Isomers

Lycopene is a kind of carotenoids with 11 conjugated and 2 un-conjugated double bonds with which it express strong anti-oxidant activity and know as bioactive in lowering the risks of prostate cancer, breast cancer and cardiovascular diseases. Because there are 11 conjugated double bonds in its structure, lycopene has many geometric-isomers and the most common forms are all-trans,9-cis,5-cis and 13-cis. About 90% of natural lycopene in the diet is in all-trans configuration, but when we check the lycopene in our body, we find that cis-lycopene is the predominant, showing cis-isomers may be more active than all-trans ones. Focussing on the relationship between geometric and bioactivity, the author studied the HPLC protocol to separate main lycopene isomer and the technology to isomerize lycopene. The main contents are described as follows:The authors studied the methodology of lycopene isomers detection via COSMOSIL Cholester-HPLC system. The optimized conditions were as follows:A mixture of tetrahydrofuran and Acetonitrile (10:90, v/v) was used as mobile phase and its flow rate was 1mL/min, the injection volume was 20μL, and the separating was monitored at 472nm. Confirmatory test was done and the result showed that this HPLC system was effective in lycopene isomers detection with ideal resolution and fairly good repeatability.Five kinds of isomers were got after isomerization and three of them can be identified as all-trans,9-cis,13-cis, the others were uncertain because there was not enough data to verify. Oxidative reaction and degradation reaction were disabled during isomerization.All-trans lycopene can undergoe geometrical isomerization into cis-isomers with thermal treatment. Influences of five isomerization method including ethyl acetate refluxing, high temperature treatment, microwave assisted refluxing, ultrasound/microwave assisted refluxing and refluxing-crystal were investigated. The results showed that the proportion of cis-isomers of 6% oleoresin samples were higher than that of 90% lycopene samples after ethyl acetate refluxing. Although Microwave and ultrasound-microwave assisted refluxing had no significant difference with each other, both showed significance influence compared with ethyl acetate refluxing, proportion of cis-isomers reached more than 54% after 5 h treatment. A product containing 70-80% of cis-lycopene was got by refluxing-crystal method with conditions as follows:concentration of lycopene was 100mg/100 mL, refluxing time was 5-8 h, crystallizing temperature was 0℃-4℃, and the crystallizing time was 12 h.An ambient of low temperature, dark, absence of oxygen and dissolved in solvent was necessary in order to keep lycopene with high content of cis-isomers. A product containing 50.03% of cis-lycopene was got after 25d storing under-20℃and in darkness.