Assessment of the antioxidant activities of representative optical and geometric isomers of astaxanthin against singlet oxygen in solution by a spectroscopic approach.

Astaxanthin (AST) is a natural antioxidant and has been widely applied as a food supplement. While astaxanthin has many isomers, there are few studies comparing its physicochemical properties. In this work, we were concerned about their antioxidant activities against external oxidative stresses, and specifically, the singlet oxygen (O) quenching capacities of the representative optical and geometric isomers of astaxanthin were examined. Methylene blue (MB) was used as the photosensitizer to produce O, and 1,3-diphenylisobenzofuran (DPBF) was used to probe O. Our results showed that the O quenching capacities of the optical isomers, including 3S,3'S, 3R,3'S, and 3R,3'R all-trans-astaxanthin, are identical. In contrast, the O quenching capacity of cis-astaxanthin is higher than that of all-trans-astaxanthin. As such, this work provides an effective spectroscopic approach to assessing the antioxidant activities of various forms of astaxanthin against singlet oxygen, and demonstrates the remarkable difference among the geometric isomers.