Singlet versus Triplet Reactivity in Photodegradation of C₄₀ Carotenoids

Abstract

Direct observations of triplet excited states of astaxanthin (I), -carotene (II), canthaxanthin (III), and zeaxanthin (IV) in toluene at 25°C following unsensitized laser flash photolysis and using transient absorption spectroscopy have yielded the singlet-triplet intersystem crossing yields 3.7 × 10 -3 (I), 5.4 × 10 -4 (II), 9.7 × 10 -3 (III), and 1.8 × 10 -3 (IV), based on triplet-triplet extinction coefficients obtained in anthracene-sensitized experiments. A carbonyl rather than a hydroxy group distinguished the carotenoids from each other, as further evidenced by rate constants for oxygen quenching of triplet carotenoids, 2.1 × 10 8 (I), 1.1 × 10 8 (II), 2.5 × 10 8 (III), and 0.95 × 10 8 M -1 s -1 (IV), determined under similar conditions and by the fluorescence quantum yields which depend on excitation wavelength (355 versus 430 nm) more significantly for II (factor of 13) and IV (factor of 21) than for I and III (common factor of 2). For II and III, using continuous-wave photolysis, competing oxygen-dependent and oxygen-independent photodegradation was demonstrated, and for the latter, III was shown to degrade both in the singlet manifold with a photodegradation quantum yield Φ 1 ) 7 × 10 -6 and in the triplet with Φ 3 ) 7 × 10 -6 for 366 nm excitation, while II almost exclusively degraded in the singlet with Φ 1 ) 3.8 × 10 -5 . The nature of the excited states of III (and I) with some n, π* character yields (i) more facile transformation between excited states (higher intersystem crossing yield, less wavelength-dependent fluorescence) and (ii) less biradical character (less photolability) compared to excited states of II (and IV) with pure π, π* character.

Effect of gelatin/agar bilayer film incorporated with TiO2 nanoparticles as a UV absorbent on fish oil photooxidation

“…Results showed that the least amount of change in ∆ E was related to the sample (5.31 ± 0.03) with black lid and the sample covered with G/A‐TiO 2 ‐2% (6.16 ± 0.04), respectively (Table S2). On the other hand, without lid samples showed more changes in ∆ E (compared to with lid samples), which is probably due to more oxidation of pigments present in the oil, caused by oxygen transmission through the bilayer films (Nielsen et al ., ). However, the change of ∆ E decreased in samples covered with bilayer film containing low concentration of TiO 2 nanoparticles, which represents the lower UV light transmittance of bilayer films and the significantly lower change in the colour of fish oil caused by photooxidation (Christophersen et al ., ).…”

Carotenoids in different body components of Indian shrimps

“…21 Among the carotenoids, astaxanthin was found to be more effective than β-carotene as an antioxidant 22 and a better agent to destroy free radicals than other carotenoids. 23 The predominance of astaxanthin and its esters in the meat of the shrimps studied indicates that it is a good source of the natural antioxidant astaxanthin, which may provide some health benefits to consumers. Further, as it is also observed that the carotenoid extract from the waste of the four species of shrimp studied contains astaxanthin and its esters as major carotenoids, the waste can form a valuable source of natural carotenoid, which is available for recovery for subsequent use as a pigment source in aquaculture diets.…”

Antioxidant Properties of Carotenoids In Vitro and In Vivo

“…Researchers have reported that capsanthin, which contains 11 conjugated double bonds, a conjugated keto-group and a cyclopentane ring, had higher Antioxidant Properties of Carotenoids 105 ORDER REPRINTS antiphotooxidative activity than b-carotene, which has the same number of double bonds but neither of the functional groups (Chen et al, 1997;Nielsen et al, 1997).…”