Light-Induced Oxidation of Unsaturated Lipids as Sensitized by Flavins

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 × 108 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 catio...