Photooxidation of Carotenoids in Mesoporous MCM-41, Ni-MCM-41 and Al-MCM-41 Molecular Sieves

Abstract

Photooxidation of β-carotene and canthaxanthin in mesoporous MCM-41, Ni-MCM-41, and Al-MCM-41 molecular sieves was studied by 9-220 GHz electron paramagnetic resonance (EPR) and 9 GHz electron nuclear double resonance (ENDOR). X-ray powder diffraction (XRD) measurements established that the MCM-41 pore size (33 Å) was large enough to accommodate carotenoids. Mesoporous MCM-41 molecular sieves are found to be promising hosts for long-lived photoinduced charge-separation between carotenoid radical cations (Car •+ ) and the MCM-41 framework. Incorporating metal ions into siliceous MCM-41 enhances efficiency of carotenoid oxidation. The photoyield and stability of generated carotenoid radical cations increased in the order MCM < Ni-MCM < Al-MCM. Formation of carotenoid radical cations within the Me-MCM-41 is due to electron transfer between incorporated carotenoid molecules and metal ions, which act as electron acceptor sites. Detected EPR signals of Ni(I) species provide direct evidence for the reduction of Ni(II) ions by carotenoids. The presence of Ni(II) ions in Ni-MCM-41 was verified by 220 GHz EPR spectroscopy. ENDOR measurements revealed that the central C13-CH 3 and C13′-CH 3 groups of both carotenoids in Al-MCM-41 are rapidly rotating, while mobility of the C9-CH 3 and C9′-CH 3 groups is restricted. We propose that carotenoids are bound to the MCM-41 pore walls via the ends of the polyene chain in close proximity to the C9,9′-CH 3 groups.

Formation of Carotenoid Neutral Radicals in Photosystem II

“…EPR and UV/visible studies have established that radicals are stable in this matrix over days to weeks 46. The structure of #Car • in the current study is different from the structure of neutral radicals in other studies in that Car •+ is first formed by electron transfer to the Cu 2+ of the MCM-41 matrix (eq.…”

Effects of antioxidants on the stability of β-Carotene in O/W emulsions stabilized by Gum Arabic

“…Light exposure degrades carotenoids. Photo-oxidation produces carotenoid radical cautions (Mortensen and Skibsted 1996;Konovalova et al 2001). Rapid degradation of β-carotene was observed in the diluted emulsion samples without antioxidants (Fig.…”

Red pepper (Capsicum annuum) carotenoids as a source of natural food colors: analysis and stability—a review

“…Both light and heat influence the stability of carotenoids through radical mediated oxidation reactions. Availability of oxygen, lower pH and presence of pro-oxidants, catalytic metals, oxidative enzymes and unsaturated lipids in food enhance the carotenoid degradation; whereas reduced water activity and presence of antioxidants have protective roles (Boon et al 2010;Henry et al 2000;Konovalova et al 2001;Gao and Kispert 2003). The conditions that favor oxidation reactions generally found to favor the isomerization of naturally predominant all-trans carotenoids to their cis conformations (Boon et al 2010;Gao et al 1996;Gao and Kispert 2003).…”