Light-Induced Oxidation of Unsaturated Lipids as Sensitized by Flavins

Author(s)

Huvaere, Kevin ^[1] ; Cardoso, Daniel R. ^[2] ; Homem-de-Mello, Paula ^[3] ; Westermann, Signe ^[1] ; Skibsted, Leif H. ^[1]

Affiliation

^[1] Univ Copenhagen, Fac Life Sci, Dept Food Sci, DK-1958 Frederiksberg C - Denmark

^[2] Univ Sao Paulo, Dept Quim & Fis Mol, Inst Quim Sao Carlos, BR-13560970 Sao Carlos, SP - Brazil

^[3] Univ Fed ABC, BR-09210170 Santo Andre, SP - Brazil

• Document type: Journal article
• Source: Journal of Physical Chemistry B; v. 114, n. 16, p. 5583-5593, APR 29 2010.
• Web of Science Citations: 33

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 similar to 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 Delta H double dagger = 22.6 kJ mol(-1) and Delta S 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 similar to 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 (Delta G(HAT)degrees = -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) similar to -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 Delta G(ET)degrees > 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 3RF{*} 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. (AU)

• FAPESP's process: 09/00858-0 - Mechanism and consequences of redox process in food: Chemical and Photochemical aspects of the oxidative deterioration and antioxidant activity of beer and dairy products
• Grantee: Daniel Rodrigues Cardoso
• Support Opportunities: Regular Research Grants