Effect of carbonyls on the reactivity of polyenes in autoxidation

Abstract

Autoxidation of the biologically active polyenes b-carotene, canthaxanthin, retinyl acetate, methyl retinoate, retinal and 3,7,11,11-tetramethyl-10,15-dioxo-2,4,6,8-hexadecatetraenal (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 Á ), as a driving force of the autoxidation process. Semiempirical AM1 calculations of DH f of some retinyl polyenes and polyenyl radicals were carried out and the C-H bond strengths and E S (R Á ) 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.

Neurosporaxanthin Overproduction by Fusarium fujikuroi and Evaluation of Its Antioxidant Properties

“…All mechanisms involve the formation of carotenyl radicals (Car • ) and further peroxy-carotenyl radicals (CarOO • ), the stability of which is directly associated with the extension of the electronic conjugated polyene chain (by resonance). Interestingly, Krasnokutskaya et al [68] demonstrated that insertion in the polyene molecule of a carbonyl group results in a drastic decrease in the reactivity towards molecular oxygen (autooxidation), and, consequently, the formation of CarOO • and Car • radicals (this, during the propagation step of lipid oxidation chain reaction). The authors showed that the reactivity of carboxylated carotenoids decreases, due to the lower initiation and propagation rates in the free radical chain reaction [68].…”

“…Interestingly, Krasnokutskaya et al [ 68 ] demonstrated that insertion in the polyene molecule of a carbonyl group results in a drastic decrease in the reactivity towards molecular oxygen (autooxidation), and, consequently, the formation of CarOO• and Car• radicals (this, during the propagation step of lipid oxidation chain reaction). The authors showed that the reactivity of carboxylated carotenoids decreases, due to the lower initiation and propagation rates in the free radical chain reaction [ 68 ]. Notably, ApoC (β-apo-8′-carotenal), a shorter apocarotenoid (C30) with an aldehyde group ( Supplementary Material, Figure S1 ), does not account for such an extended conjugation system as the carboxylated NX, and merely showed equivalent scavenging properties as βC (lacking any conjugated polar group) in liposomal systems here ( Figure 5 A).…”

Non-phenolic radical-trapping antioxidants

“…β‐Carotene is also, as with most carotenoids, a powerful quencher of singlet oxygen, 1 O 2 84 . Peroxyl radicals add rapidly to polyenes when the resulting C‐radicals are strongly stabilized by resonance (see Figure 12), the rate constants being of the order of 10 4 –10 5 m −1 s −1 85,86 …”

Non-phenolic radical-trapping antioxidants

“…[84] Peroxyl radicals add rapidly to polyenes when the resulting C-radicals are strongly stabilized by resonance (see Figure 12), the rate constants being of the order of 10 4 -10 5 M -1 s -1 . [85,86] At low oxygen partial pressure, the b-carotene -OOR radical may quickly quench a second peroxyl radical. In fact, addition of dioxygen to b-carotene -OOR occurs reversibly [81] and is characterized by relatively low rate constants.…”