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

[Display omitted] •Astaxanthin isomer antioxidant activities against 1O2 can be assessed by using DPBF.•The astaxanthin 1O2 quenching capacity is three orders of magnitude greater than that of vitamin E.•The 1O2 quenching capacities of the optical isomers of all-trans-astaxanthin are identical.•The cis-astaxanthin 1O2 quenching capacity is greater than that of all-trans-astaxanthin. 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 (1O2) quenching capacities of the representative optical and geometric isomers of astaxanthin were examined. Methylene blue (MB) was used as the photosensitizer to produce 1O2, and 1,3-diphenylisobenzofuran (DPBF) was used to probe 1O2. Our results showed that the 1O2 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 1O2 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.