Spectral Characterization, Isomerization Behavior, and Antioxidant Activity of ζ-Carotene Geometric Isomers: From...

ζ-Carotene is a key biosynthetic intermediate of lycopene that is widely distributed in plants and fungi but difficult to purify, which has limited detailed physicochemical and biological studies. Here, we established a purification strategy that affords four ζ-carotene geometric isomers (all-E-, 9Z, 9Z,9'Z, and 9Z,13Z,9'Z-isomers) in high purity and elucidated their structures fully using nuclear magnetic resonance spectroscopy. Photochemical experiments showed that, unlike many carotenoids that photoisomerize toward the all-E-form, ζ-carotene yields a photostationary mixture dominated by the 9Z-isomer. For the major Z-isomers, (9Z)- and (9Z,9'Z)-ζ-carotene, we determined solvent-dependent spectral properties, color parameters, and ultraviolet (UV)-shielding capacities, revealing remarkably strong UV-A shielding. These Z-isomers also exhibited potent singlet oxygen quenching and pronounced inhibition of fluorescent advanced glycation end-product formation, together with detectable hydroxyl radical scavenging. Our findings provide fundamental data supporting ζ-carotene as a promising natural UV-A filter and skin health-promoting functional ingredient.