Isomerization of trans-Astaxanthin to cis-Isomers in Organic Solvents

The isomerization of trans-astaxanthin to cis-isomers in organic solvents was investigated. trans-Astaxanthin was dissolved in dimethyl sulfoxide, dichloromethane, chloroform, acetone, methanol, acetonitrile, and a mixture of dichloromethane and methanol (25:75) respectively, and heated at 35 °C followed by analyzing cis- and trans-astaxanthins in the solutions using HPLC. The isomerization rates of trans-astaxanthin were dependent on the solvent, and the following order was found: dichloromethane > chloroform > the mixture of dichloromethane and methanol (25:75) > methanol > acetonitrile > acetone > dimethyl sulfoxide. In different solvents, the relative contents of 9-cis- and 13-cis-astaxanthins formed during isomerization were different. In all solvents, 13-cis-isomer was the main cis-isomer from trans-astaxanthin. trans-Astaxanthin dissolved in dichloromethane or chloroform was very readily isomerized to cis-isomers, especially for dichloromethane, in which a maximum isomerization percentage was found and an equilibrium practically was reached after an appropriate time interval. Results also indicated that a higher temperature could promote markedly the isomerization of trans-astaxanthin. Keywords: Astaxanthin; cis−trans isomerization; organic solvent