Alan Mortensen

The kinetics of reaction between trifluoroacetic acid as an acid of medium strength and the carotenoids β-carotene, zeaxanthin, canthaxanthin, and astaxanthin has been examined in detail including the effects of dioxygen, acid concentration, and carotenoid structure. Reaction between acid and carotenoid leads to species absorbing in the red and near-infrared (NIR) spectral regions, intermediates that subsequently disappear. ESR experiments clearly show that these species are not carotenoid radicals, although their NIR absorption is similar to the absorption of carotenoid radical cations. Under most reaction conditions, the disappearance of carotenoids follows pseudo-zero-order kinetics, whereas the reaction order is >1 with respect to acid, and the long-lived (hours) intermediates are suggested to be mono- (700 nm) and diprotonated carotenoid (∼950 nm). Acid induces cis/trans-isomerization via the protonated intermediates, which also decay to nonradical species with shorter conjugated systemsmost probably carotenoid esters. Slow protonization of the methine carbon is the primary step in the degradation, but dioxygen increases the rate as a result of formation of a charge-transfer complex with the carotenoids as indicated by a red-shift of the NIR absorption bands. Carotenoids with carbonyl groups (astaxanthin and canthaxanthin) have slower rates of degradation than β-carotene and zeaxanthin, indicating preferential nondegradative protonation of the carbonyl groups. Keywords: β-Carotene; zeaxanthin; canthaxanthin; astaxanthin; acid degradation

Peroxyl radicals, as model for peroxyl radicals formed during autoxidation of lipids, have been generated in three solvent systems (cyclohexane, tetrahydrofuran and tert-butanol/water) by steady-state and laser flash photolysis, and their reaction with β-carotene studied. Steady-state photolysis experiments showed that alkyl, alkoxyl and alkylperoxyl radicals all react with β-carotene. However, laser flash photolysis experiments indicated that the reaction with peroxyl radicals (second-order rate constant estimated to be less than 10⁶ M⁻¹ s⁻¹) is slower than with alkyl and alkoxyl radicals, and that β-carotene is hence a poor direct scavenger of peroxyl radicals. Scavenging of peroxyl radicals by β-carotene is suggested not to proceed via electron transfer but rather by adduct formation and/or hydrogen abstraction. For different phenoxyl radicals, differences in reactivity towards β-carotene seem to be correlated with standard reduction potential.

Real time detection following laser flash photolysis of transient carotenoid radical cations and tocopheroxyl radicals formed in chloroform and bleaching of the carotenoids has allowed interaction be...