Flavin-sensitized photo-oxidation of unsaturated fatty acids

Photo-oxidation of unsaturated fatty acids or esters (LH = oleic acid, linoleic acid, methyl linoleate, and linolenic acid) by oxygen proceeds efficiently in the presence of a riboflavin derivative, riboflavin - 2′,3′,4′,5′-tetra-acetate (Fl), in acetonitrile by a type II (singlet oxygen) mechanism with a limiting quantum yield (φ = 0.65) which corresponds to the quantum yield of the intersystem crossing of the flavin. The addition of HClO₄ - by which Fl is protonated to FlH ⁺ - showed a remarkable inhibitory effect on the Fl-sensitized photo-oxidation of LH, since FlH⁺ is photo-reduced by LH to yield the dihydroflavin radical cation FlH₂^+. which has no ability to sensitize the energy transfer to oxygen in its ground state.