Photoactivated Ferric Chloride Oxidation of Carotenoids by Near-UV to Visible Light

Abstract

When canthaxanthin (I) and β-carotene (II) dichloromethane solutions are treated with small amounts of ferric chloride (≤0.26 mol equiv), extensive photodegradation of the remaining unreacted neutral carotenoid occurs upon subsequent irradiation with near-UV to visible light. The rate of this photodegradation is independent of the neutral carotenoid concentration at a given initial FeCl3 concentration and first-order in initial FeCl3 (k 1 = 1.43 and 3.30 min-1 for I and II, respectively). The data are consistent with a mechanism in which Fe(II) is photochemically converted in the presence of CH2Cl2 to Fe(III), which then oxidizes unreacted neutral carotenoids. We also report that canthaxanthin radical cations in dichloromethane form cation dehydrodimers at low temperature or upon irradiation with near-UV to visible light, which have an absorption maximum near 770 nm and a reduction peak at 20 mV in cyclic voltammetry. It is proposed that radical cations associate with parent molecules and then undergo two subsequent steps to form the final cation dehydrodimers. In the presence of supporting electrolytes, canthaxanthin radical cations form ion pairs with the anions PF6 -, resulting in a 10 nm blue shift of the absorption maximum of the radical cations.

Cation radicals of xanthophylls

“…Thus, any blue-shifted peaks or shoulders are very likely attributable to dications. Kispert and co-workers (Gao et al 1997) also observed another NIR peak for canthaxanthin and b-carotene blue-shifted 100 nm compared to the main band, and assigned it to a deprotonated dimeric cation. This species was evident when a vast molar excess (>20 molar equivalents) of the carotenoid over ferric chloride was used.…”

“…The reactions that take place after ferric chloride addition to b-carotene, canthaxanthin, and ethyl all-trans-8¢-apo-bcaroten-8¢-oate have been described previously (Gao et al 1997;Gao and Kispert 2003). Kispert and co-workers (Gao et al 1997) proposed that after addition of the reagent at room temperature, an equilibrium exists between the neutral molecule, the cation radical, a cation radical dimer, a deprotonated dimeric radical, and a deprotonated dimeric cation.…”

“…Kispert and co-workers (Gao et al 1997) proposed that after addition of the reagent at room temperature, an equilibrium exists between the neutral molecule, the cation radical, a cation radical dimer, a deprotonated dimeric radical, and a deprotonated dimeric cation. Another study on b-carotene and ethyl all-trans-8¢-apo-b-caroten-8¢-oate at room temperature by the same group proposed an equilibrium involving the neutral molecule, the cation radical, the dication, and the inorganic species Fe 3+ , Fe 2+ , and Cl - (Gao and Kispert 2003).…”

“…Radical cations of carotenoids may be readily formed chemically by the addition of an oxidizing agent (Ioffe et al 1976;Ding et al 1988;Jeevarajan et al 1996;Gao et al 1997;Wei et al 1997;Krawczyk 1998), electrochemically in a cell having a sufficiently positive voltammetric potential Khaled et al 1990;Khaled et al 1991;He and Kispert 1999a;He and Kispert 1999b;Liu and Kispert 1999;Deng et al 2000;Hapiot et al 2001), photochemically upon light absorption in photosynthetic pigment-protein complexes (Schenck et al 1982;Rivas et al 1993;Polívka et al 2004;Holt et al 2005), or in synthetic covalently linked carotenoid donor/electron acceptor molecular systems (Gust and Moore 1991;Gust et al 1993). …”

“…The optical spectroscopic signature of a carotenoid radical is a strong absorption band in the 800-1,100 nm near-infrared (NIR) region of the electromagnetic spectrum (Sorenson 1965;Adams 1969;Ioffe et al 1976;Ding et al 1988;Jeevarajan et al 1996;Gao et al 1997). In an analysis of the reactivity of carotenoids having various p-electron chain lengths with phenoxyl radicals, Mortensen and Skibsted (1997) reported a correlation between the susceptibility of a carotenoid to degradation and the position of the maximum absorption wavelength of its cation radical NIR spectrum.…”