Kinetic and Thermodynamic Study of the Thermally Induced (<i>E/Z</i>)-Isomerization of the <i>retro</i>-Carotenoid Rhodoxanthin

Abstract

The hitherto scarcely investigated <i>retro</i>-carotenoid rhodoxanthin possesses high potential for coloration in the food and beverage industry using technofunctional formulations prepared thereof. Hence, we studied (<i>E/Z</i>)-isomerization pathways of rhodoxanthin, including seven (<i>E/Z</i>)-isomers comprising (<i>Z</i>)-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-<i>E</i>)-rhodoxanthin with mono-, di-, and tri-(<i>Z</i>)-isomers using multiresponse modeling. At 40-70 °C in ethyl acetate, reaction rate constants regarding the rotation from (all-<i>E</i>)- to (6<i>Z</i>)-rhodoxanthin were 11-14 times higher than those of the common (<i>E/Z</i>)-isomerization reaction at C-13,14 of the non-<i>retro</i>-structured carotenoid canthaxanthin. Moreover, the equilibrium reaction between (all-<i>E</i>)- and (6<i>Z</i>)-rhodoxanthin was strongly product favored as indicated by negative Gibbs energies (-1.6 to -2.2 kJ mol^-1), which is unusual for carotenoids within the studied temperatures. Overall, this study provides novel insights into structure-related dependencies of (<i>E/Z</i>)-isomerization reaction kinetics and thermodynamics of polyenes.