Kinetic and Thermodynamic Study of the Thermally Induced ()-Isomerization of the -Carotenoid Rhodoxanthin.

The hitherto scarcely investigated -carotenoid rhodoxanthin possesses high potential for coloration in the food and beverage industry using technofunctional formulations prepared thereof. Hence, we studied ()-isomerization pathways of rhodoxanthin, including seven ()-isomers comprising ()-configured double bonds at unusual exocyclic and inner polyene chain positions. A mathematical approach was developed to deduce kinetic and thermodynamic parameters of six parallel equilibrium reactions interconnecting (all-)-rhodoxanthin with mono-, di-, and tri-()-isomers using multiresponse modeling. At 40-70 °C in ethyl acetate, reaction rate constants regarding the rotation from (all-)- to (6)-rhodoxanthin were 11-14 times higher than those of the common ()-isomerization reaction at C-13,14 of the non--structured carotenoid canthaxanthin. Moreover, the equilibrium reaction between (all-)- and (6)-rhodoxanthin was strongly product favored as indicated by negative Gibbs energies (-1.6 to -2.2 kJ mol), which is unusual for carotenoids within the studied temperatures. Overall, this study provides novel insights into structure-related dependencies of ()-isomerization reaction kinetics and thermodynamics of polyenes.