Comprehensive characterization of purified astaxanthin isomers: Structure, spectral properties, stability, and...

Astaxanthin Z-isomers differ from the naturally predominant all-E-isomer in their physicochemical properties; however, their isomer-specific properties remain unclear. We purified six Z-isomers, including di-Z-isomers, and investigated their UV-Vis absorption ability, stability, crystallinity, solubility, and antioxidant activity. Z-Isomerization resulted in hypsochromic shifts and decreased molar absorption coefficients; response factors at 474 nm, referenced to the all-E-isomer, varied between isomers (e.g., 9Z-isomer ≈ 1.1; 13Z-isomer ≈ 1.4; di-Z-isomers ≈ 1.1-1.8), supporting an accurate quantification. The 9Z-isomer showed the highest thermal stability, photostability, and free-radical scavenging among Z-isomers. Astaxanthin Z-isomers showed higher solubility in olive oil than the all-E-isomer (e.g., all-E-isomer ≈ 12.7 mg L^-1; 15Z-isomer ≈ 3930.1 mg L^-1; 9Z,13Z-isomer ≈ 2839.4 mg L^-1), consistent with reduced crystallinity upon Z-isomerization. In contrast, symmetric di-Z-isomers showed relatively low solubility among the Z-isomers (e.g., 13Z,13'Z-isomer ≈ 760.9 mg L^-1), likely reflecting their higher crystallinity. These findings enable accurate quantification and isomer-selective applications.