Effect of carbonyls on the reactivity of polyenes in autoxidation

Autoxidation of the biologically active polyenes beta-carotene, canthaxanthin, retinyl acetate, methyl retinoate, retinal and 3,7,11,11-tetramethyl-10,15-dioxo-2,4,6,8-hexadecatetraenaI (diketoretinal) in solid amorphous films was investigated. The course of the process was followed by electronic and IR spectroscopy. The overall activation energies were obtained. It was shown that insertion in the polyene molecule of a carbonyl group conjugated with polyene chain results in a drastic decrease in the reactivity towards molecular oxygen. The results are discussed and explained on the basis of radical stabilization energy, E-S(R), as a driving force of the autoxidation process. Semi-empirical AM1 calculations of DeltaH(f) of some retinyl polyenes and polyenyl radicals were carried out and the C-H bond strengths and E-S(R) values of corresponding polyenyl radicals were evaluated. It was shown that the incorporation of a carbonyl group in the polyene results in a decrease in the overall autoxidation rate due to the decrease in both the initiation and propagation rates. Copyright (C) 2003 John Wiley Sons, Ltd.