Thermal‐ and Photo‐Induced Isomerization of All‐<i>E</i>‐ and <i>Z</i>‐Isomer‐Rich Xanthophylls: Astaxanthin and Its Structurally‐Related Xanthophylls, Adonirubin, and Adonixanthin
Masaki Honda, Toshiyasu Sowa, Y. Kawashima
Abstract
Abstract The effects of heating and photo‐irradiation on the stability of all‐ E ‐isomer‐rich and Z ‐isomer‐rich xanthophylls, astaxanthin and its structurally related xanthophylls, adonirubin, and adonixanthin, are investigated. The xanthophylls with high Z ‐isomer content are prepared from their high‐purity all‐ E ‐isomers by thermal isomerization and filtering techniques, that is, total Z ‐isomer ratios of adonirubin, astaxanthin, and adonixanthin are 80.9%, 89.5%, and 72.5%, respectively. The all‐ E ‐ and Z ‐isomer‐rich xanthophylls dissolved in ethanol are stored at 4, 30, and 50 °C in the dark and at 30 °C under photo‐irradiation using a fluorescent light for 21 days. In the all‐ E ‐isomer‐rich xanthophylls, as the storage temperature increases, the total Z ‐isomer ratio becomes higher, whereas in the Z ‐isomer‐rich xanthophylls, the all‐ E ‐isomer ratio becomes higher. Photo‐irradiation slightly promotes Z ‐isomerization in (all‐ E )‐xanthophylls, but highly promotes all‐ E ‐isomerization in Z ‐isomer‐rich xanthophylls. In addition, photo‐irradiation prevents thermal Z ‐isomerization of (all‐ E )‐xanthophylls. Moreover, it is found that some xanthophyll Z ‐isomers such as (9 Z )‐astaxanthin are more stable than that of the other Z ‐isomers against heating and photo‐irradiation. These findings can contribute not only to establishing suitable storage conditions for Z ‐isomer‐rich xanthophylls, but also to developing control techniques for the E / Z ‐isomer ratio of the xanthophylls. Practical Applications : The fundamental data on the stability of xanthophyll isomers against heating and photo‐irradiation and finding stable xanthophyll Z ‐isomers are very important to develop xanthophyll materials rich in the Z ‐isomers. Moreover, this study clearly shows that the heat treatment enhances the Z ‐isomerization of xanthophylls, whereas the photo‐irradiation enhances the all‐ E ‐isomerization and prevents thermal Z ‐isomerization of them. This information can be utilized in technology for arbitrarily controlling E / Z ‐isomerization of xanthophylls.