Light-Induced Oxidation of Unsaturated Lipids as Sensitized by Flavins

Abstract

Triplet-excited riboflavin ((3)RF*) was found by laser flash photolysis to be quenched by polyunsaturated fatty acid methyl esters in tert-butanol/water (7:3, v/v) in a second-order reaction with k approximately 3.0 x 10(5) L mol(-1) s(-1) at 25 degrees C for methyl linoleate and 3.1 x 10(6) L mol(-1) s(-1), with DeltaH(double dagger) = 22.6 kJ mol(-1) and DeltaS(double dagger) = -62.3 J K(-1) mol(-1), for methyl linolenate in acetonitrile/water (8:2, v/v). For methyl oleate, k was <10(4) L mol(-1) s(-1). For comparison, beta-casein was found to have a rate constant k approximately 4.9 x 10(8) L mol(-1) s(-1). Singlet-excited flavin was not quenched by the esters as evidenced by insensitivity of steady-state fluorescence to their presence. Density functional theory (DFT) calculations showed that electron transfer from unsaturated fatty acid esters to triplet-excited flavins is endergonic, while a formal hydrogen atom transfer is exergonic (DeltaG(o)(HAT) = -114.3, -151.2, and -151.2 kJ mol(-1) for oleate, linoleate, and linolenate, respectively, in acetonitrile). The reaction is driven by acidity of the lipid cation radical for which a pK(a) approximately -0.12 was estimated by DFT calculations. Absence of electrochemical activity in acetonitrile during cyclic voltammetry up to 2.0 V versus NHE confirmed that DeltaG(o)(ET) > 0 for electron transfer. Interaction of methyl esters with (3)RF* is considered as initiation of the radical chain, which is subsequently propagated by combination reactions with residual oxygen. In this respect, carbon-centered and alkoxyl radicals were detected using the spin trapping technique in combination with electron paramagnetic resonance spectroscopy. Moreover, quenching of (3)RF* yields, directly or indirectly, radical species which are capable of initiating oxidation in unsaturated fatty acid methyl esters. Still, deactivation of triplet-excited flavins by lipid derivatives was slower than by proteins (factor up to 10(4)), which react preferentially by electron transfer. Depending on the reaction environment in biological systems (including food), protein radicals are expected to interfere in the mechanism of light-induced lipid oxidation.

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) …”

“…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)].…”

Mass‐spectrometry based oxidative lipidomics and lipid imaging: applications in traumatic brain injury

“…Endogenous polyunsaturated chains on sn‐2 position possess cis‐double bonds each separated by one CH 2 group. These hydrogens are very reactive to free‐radical attack, making polyunsaturated fatty acids subject to oxidative modification (Huvaere et al . 2010).…”

Light-induced protein and lipid oxidation in low-fat cheeses: whey proteins as antioxidants

“…Radical transport between lipid and proteins has, however, also been suggested to be important for the generation of dityrosine (Saeed et al 2006), and this mechanism seem to be of major importance in cheese (Dalsgaard et al 2010), but no dityrosine generation was found to occur in the cheese-WPI. These results indicate that whey proteins prevented interaction between triplet-excited riboflavin and the highly labile caseins (Huvaere et al 2010) in the cheese-WPI and also prevented interaction between caseins and lipids thus preventing radical transfer from the lipid phase to the casein fraction. The addition of WPI resulted in a lower accumulation of both dityrosine and DMDS indicating that scavenging was not the mechanism responsible for the antioxidative effect of WPI.…”