Abstract: Z-isomerization of carotenoids usually change their properties, so it is necessary to comprehensively analyze the physicochemical properties of all-E and Z-astaxanthin (AST). AST with high proportions of Z-isomer were prepared by illumination followed by silica gel column chromatography, and the physicochemical properties of all-E- and Z-AST were measured and compared using an X-ray diffractometer, a differential scanning calorimeter, a scanning electron microscope and a colorimeter, and their in vitro antioxidant activity and in vivo antioxidant activity in Caenorhabditis elegans were evaluated. Meanwhile, the mechanisms underlying the differences in color and antioxidant activity between the two isomers of AST were revealed using quantum chemistry. The results showed that as the proportion of Z-isomer rose, the crystallinity of AST declined, its morphology changed from regular to amorphous, and its solubility in organic solvents and vegetable oils increased. The solubility of 96% Z-AST was 491.2 and 59.9 times as high as that of all-E AST in ethanol and olive oil, respectively. Moreover, the scavenging effect of Z-AST on hydroxyl radical and superoxide anion radical and its capacity to enhance resistance to oxidative stress in C. elegans were significantly higher than those of all-E-AST (P < 0.05). Furthermore, the results of quantum chemistry showed that after Z-isomerization of AST, the absorption intensity at the maximum absorption wavelength (λmax) attenuated significantly, and the molar absorption coefficient decreased, and the red value of AST dropped significantly. The antioxidant capacity of AST was improved by an increase in the length of hydroxyl bonds in the terminal rings and changes in the ionization potential and electron affinity. These results provide a theoretical basis for revealing the differences between all-E and Z-AST and guiding the application of AST in health field.

Key words: astaxanthin; all-E-isomer; Z-isomer; physicochemical properties; quantum chemistry