The Effect of the Solvent Nature and Lighting on Isomerization and Oxidative Degradation of Astaxanthin

Astaxanthin and its esters, which are classified as secondary carotenoids, are synthesized under stress conditions in Haematococcus pluvialis cells that undergo transformation from vegetative cells to hematocysts. Stress factors can affect not only the rate and the direction of synthesis, but also the conformational changes in the pigment molecule, causing conversion of trans-isomers to cis-isomers. The aim of this study was to evaluate the effects of different factors (the polarity of the medium and light) on the process of trans–cis isomerization of astaxanthin. Investigation of the formation of 9-cis- and 13-cis-isomers from all-trans astaxanthin solutions at 50°C showed that medium polarity solvents stabilized cis-isomers more efficiently than highly polar solvents. Nonpolar solvents did not contribute to the stabilization of cis-isomers. The presence of π-bonds in the solvent molecule also stimulated isomerization. Analysis of the effects of the solvent refractive index on polarizability of astaxanthin revealed differences between the dependencies for polar and nonpolar solvents. Supposedly, solvents of medium polarity facilitate the process of trans–cis astaxanthin isomerization by decreasing the energy of the torque and stabilizing the cis-conformation of the molecule due to diminished dipole–dipole repulsion between the methyl groups in the polyene chain and the ionone ring. Exposure of astaxanthin solutions to light at 25°C induced formation of cis-isomers, which were more abundant within the first 4 h of exposure than after 6 h or later. Exposure to irradiation for longer periods caused astaxanthin decomposition and formation of degradation and oxidation products.