Kinetics for the reversible isomerization reaction of trans-astaxanthin

The isomerization of cis-astaxanthins in organic solvents, a reverse reaction of the isomerization of trans-astaxanthin, was investigated. HPLC analysis revealed that cis-astaxanthins could be also isomerized to produce trans-astaxanthin and the other cis-astaxanthin. The results showed that the isomerization of trans-astaxanthin to cis-astaxanthin was a reversible reaction and followed first-order reversible reaction kinetics. The effect of temperature on the isomerization reaction of trans-astaxanthin, dissolved in dimethyl sulfoxide at 20–70°C or in a mixture of dichloromethane and methanol (25:75) at 10°C, respectively, was also studied. The results indicated that increasing temperature could markedly increase the reaction rate of trans-astaxanthin isomerization. Temperature-dependence of the isomerization rate constants of trans-astaxanthins could be described by the Arrhenius equation with activation energy (Ea) of 105.8±4.2 kJ/mol.