Preparation, characterization and antioxidant activity of astaxanthin esters with different molecular structures

Astaxanthin (Asta) is widely used in the nutraceutical and food industry because of its strong antioxidant properties. However, natural Asta mainly exists in the esterified form with various fatty acid chains, making it difficult to understand the particular molecular structure of astaxanthin esters (Asta-Es) that have better antioxidant capacity. In this study, Asta-Es with different molecular structures was systematically prepared, and identified by using thin-layer chromatography (TLC), ultraviolet-visible (UV-visible), high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS), and proton nuclear magnetic resonance (¹ H-NMR). In addition, their antioxidant properties were evaluated by 2,2'-diphenylpicrylhydrazyl (DPPH) and ABTS scavenging activity.

Fourteen Asta-Es with different molecular structures were systematically synthesized. This study used a simple and efficient method for the separation and purification of astaxanthin monoester (Asta-ME) and astaxanthin diester (Asta-DE) with high purity (86%-94%) by silica gel column chromatography. 13-cis-Asta-E and 9-cis-Asta-E were firstly identified from Asta-E. The DPPH clearance rates and ABTS scavenging rates of Asta-C4:0, Asta-C8:0, Asta-C12:0, and Asta-C18:0 were relatively close, but the DPPH and ABTS scavenging rates of Asta-C18:0, Asta-C18:1, Asta-C18:2, and Asta-C22:6 increased gradually. Among all Asta-Es, Asta-C22:6/C22:6 showed the highest antioxidant capacity, with the DPPH and ABTS scavenging rates of 77.22 ± 3.29% and 51.84 ± 1.65%, respectively.

In this study, it was concluded that chemically synthesized Asta-Es contained cis-astaxanthin ester and polyunsaturated fatty acid chain increased the antioxidant activity of Asta, showing less effect of the length of fatty acid chain. These results provide useful information for the production and use of highly efficient Asta-E as functional food and pharmaceutical ingredients. © 2020 Society of Chemical Industry.