Riboflavin-photosensitized oxidation is enhanced by conjugation in unsaturated lipids.

Methyl esters of polyunsaturated fatty acids were found to quench triplet-excited riboflavin ((3)Rib) 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 (3)Rib 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 × 10(7) L mol(-1) s(-1) for hydrogen abstraction from bis-allylic methylene groups in unsaturated lipid by (3)Rib. Still, ergosterol was found to deactivate (3)Rib with k2 = 6.2 × 10(8) L mol(-1) s(-1), which is more efficient than cholesterol, with 6.9 × 10(7) L mol(-1) s(-1). Likewise conjugated (9E,11E) methyl linoleate (CLA) reacts with 3.3 × 10(7) 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 (3)Rib 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 (3)Rib.