Spectral Characterization, Isomerization Behavior, and Antioxidant Activity of ζ-Carotene Geometric Isomers: From Biosynthetic Intermediates to Functional Ingredients

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 -, 9 Z, 9 Z,9′ Z, and 9 Z,13 Z,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 9 Z -isomer. For the major Z -isomers, (9 Z )- and (9 Z,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.