Iron(II)-induced isomerization of (all-E)-xanthophyll pigments lutein, zeaxanthin, and beta cryptoxanthin in acetone

摘要

Lutein, zeaxanthin, and beta-cryptoxanthin are xanthophyll pigments that own conjugated double bonds; some factors can convert (all-E)-xanthophylls to their (Z)-isomers such as high temperature, illumination, and oxidants. In the present work, the Fe(II)-induced isomerization of (all-E)-lutein, (all-E)-zeaxanthin, and (all-E)-beta-cryptoxanthin in acetone were analyzed by HPLC coupled with DAD and APcI-MS. The three (all-E)-xanthophylls were baseline separated and their (Z)-isomers were identified bythe chromatographic retention, UV/vis spectra and positive mass spectrometry with reference values reported in the literature. The results showed that Fe(II) exhibited a rapidly isomerization induction of (all-E)-xanthophylls to their (Z)-isomers exceeding a 8/1 mass ratio of Fe(II)/pigments. The (13-Z)-lutein and (13′-Z)-lutein were identified as the major (Z)-isomers of (all-E)-lutein treated with FeSO_4·7H_2O, and (13-Z)-zeaxanthin was identified as the major isomerization product of (all-E)-zeaxanthin. Furthermore, a type of (Z)-beta-cryptoxanthin was also detected. Increasing either mass ratio of Fe(II)/pigments or the incubation time of FeSO_4·7H_2O and (all-E)-xanthophylls may affect on the stabilities of (Z)-isomers derived from (all-E)-xanthophylls.