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Riboflavin-Photosensitized Oxidation Is Enhanced by Conjugation in Unsaturated Lipids

Cardoso, Daniel R., Scurachio, Regina S., Santos, Willy G., Homem-de-Mello, Paula, Skibsted, Leif H.
Journal of agricultural and food chemistry 2013 v.61 no.9 pp. 2268-2275
cholesterol, conjugated linoleic acid, dissociation, ergosterol, esters, eyes, hydrogen, oxidation, photolysis, phytosterols, polyunsaturated fatty acids, riboflavin
Methyl esters of polyunsaturated fatty acids were found to quench triplet-excited riboflavin (³Rib) in efficient bimolecular reactions with rate constants, as determined by laser flash photolysis, linearly depending upon the number of bis-allylic methylene (from 1 to 5). Deactivation of ³Rib is predicted by combining the experimental second-order rate constants k₂ determined for acetonitrile/water (8:2, v/v) at 25 °C with density functional theory (DFT) calculations of bond dissociation energy to have an upper limiting value of 1.22 × 10⁷ L mol–¹ s–¹ for hydrogen abstraction from bis-allylic methylene groups in unsaturated lipid by ³Rib. Still, ergosterol was found to deactivate ³Rib with k₂ = 6.2 × 10⁸ L mol–¹ s–¹, which is more efficient than cholesterol, with 6.9 × 10⁷ L mol–¹ s–¹. Likewise conjugated (9E,11E) methyl linoleate (CLA) reacts with 3.3 × 10⁷ L mol–¹ s–¹, 30 times more efficient than previously found for methyl α-linolenate. Conjugation as in CLA and ergosterol is concluded to enhance ³Rib deactivation, and dietary plant sterols and CLA may accordingly be important macronutrients for eye and skin health, protecting against light exposure through efficient deactivation of ³Rib.