TY - JOUR
T1 - Light-induced oxidation of unsaturated lipids as sensitized by flavins
AU - Huvaere, Kevin André Jurgen
AU - R. Cardoso, Daniel
AU - Homem-de-Mello, Paula
AU - Westermann, Signe
AU - Skibsted, Leif Horsfelt
PY - 2010/4/29
Y1 - 2010/4/29
N2 - Triplet-excited riboflavin (3RF*) 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 ∼ 3.0 ×105 L mol-1 s-1 at 25 °C for methyl linoleate and 3.1 ×106 L mol-1 s-1, with δH‡ = 22.6 kJ mol-1 and δS ‡ = -62.3 J K-1 mol-1, for methyl linolenate in acetonitrile/water (8:2, v/v). For methyl oleate, k was <104 L mol-1 s-1. For comparison, β-casein was found to have a rate constant k ∼ 4.9 ×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 (δG°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 pKa ∼ -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 δG°ET > 0 for electron transfer. Interaction of methyl esters with 3RF* 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 104), 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.
AB - Triplet-excited riboflavin (3RF*) 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 ∼ 3.0 ×105 L mol-1 s-1 at 25 °C for methyl linoleate and 3.1 ×106 L mol-1 s-1, with δH‡ = 22.6 kJ mol-1 and δS ‡ = -62.3 J K-1 mol-1, for methyl linolenate in acetonitrile/water (8:2, v/v). For methyl oleate, k was <104 L mol-1 s-1. For comparison, β-casein was found to have a rate constant k ∼ 4.9 ×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 (δG°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 pKa ∼ -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 δG°ET > 0 for electron transfer. Interaction of methyl esters with 3RF* 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 104), 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.
U2 - 10.1021/jp9121744
DO - 10.1021/jp9121744
M3 - Journal article
C2 - 20377218
SN - 1520-6106
VL - 114
SP - 5583
EP - 5593
JO - Journal of Physical Chemistry Part B: Condensed Matter, Materials, Surfaces, Interfaces & Biophysical
JF - Journal of Physical Chemistry Part B: Condensed Matter, Materials, Surfaces, Interfaces & Biophysical
IS - 16
ER -