Riboflavin-Photosensitized Oxidation Is Enhanced by Conjugation in Unsaturated Lipids

Abstract

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.

Flavonoids protecting food and beverages against light

“…The high whey protein presence in milk for example ([Sub] ∼ 5–6 g L −1 ) disfavours such approach, as the amount of flavonoids required would affect flavour quality (astringency), colloidal behaviour, and oxidative stability (prooxidant effects). Lipids, on the other hand, are more readily protected as k I of polyunsaturated fatty acid chains is significantly lower (∼10 5 –10 6 L mol −1 s −1 , depending on the number of unsaturations) …”

“…Still, the effective quenching by flavonoids, with larger k Q values of up to four orders of magnitude, may prevent lipid peroxidation as pivotal precursor in stale and rancid off‐flavour formation. Cholesterol, prevailing along with lipids in meat, dairy products, and fish, is much more sensitive to oxidation (6.9 × 10 7 L mol −1 s −1 ) with formation of possibly toxic oxidation products . This rate constant is only slightly below that of rutin, quercetin, and EGCG (1.2 × 10 8 L mol −1 s −1 , 4.6 × 10 8 L mol −1 s −1 , and 1.5 × 10 8 L mol −1 s −1 ), and quenching will thus be effective only if flavonoid concentration exceeds that of cholesterol.…”

“…Lipids, on the other hand, are more readily protected as k I of polyunsaturated fatty acid chains is significantly lower (∼10 5 -10 6 L mol −1 s −1 , depending on the number of unsaturations). 78,79 At higher oxygen concentrations, the flavin-mediated photooxidation mechanism shifts to a type II pathway with singlet oxygen formation outcompeting the quenching of excited sensitiser by flavonoids (vide infra). Indeed the rate constant for quenching of triplet-excited riboflavin by oxygen, k II , is very high (∼10 9 -10 10 L mol −1 s −1 ) 35, 36 which, in combination with the high solubility of oxygen in oils or fatty matrices (∼10 mM), makes conditions ideal for a type II process [Eqn (3)].…”

Mechanism and kinetics of the oxidative damage to ergosterol induced by peroxyl radicals in lipid media: a theoretical quantum chemistry study

“…Concerning the reactivity of sterols toward HER, ergosterol was found to be 19 times more reactive than cholesterol. Indeed, unsaturated cycloalkenes containing conjugated double bonds have been shown to be especially susceptible to oxidation . This enhanced reactivity is due to conformational effects leading to resonance–stable transition states, which in turn allow for more efficient hydrogen atom abstraction …”

Reatividade de lipídeos e metabólitos da cafeína frente a estados excitados de flavinas

“…O teor de riboflavina na pele humana (7,9x10 6 ) e no fluido do olho (4,5x10 6 ) é relevante para as funções de riboflavina como um fotosensibilizador na pele e olhos (1).…”