Electron-transfer oxidation properties of unsaturated fatty acids and mechanistic insight into lipoxygenases

Rate constants of photoinduced electron-transfer oxidation of unsaturated fatty acids with a series of singlet excited states of oxidants in acetonitrile at 298 K were examined and the resulting electron-transfer rate constants (k_et) were evaluated in light of the free energy relationship of electron transfer to determine the one-electron oxidation potentials (E _ox) of unsaturated fatty acids and the intrinsic barrier of electron transfer. The k_et values of linoleic acid with a series of oxidants are the same as the corresponding k_et values of methyl linoleate, linolenic acid, and arachidonic acid, leading to the same E_ox value of linoleic acid, methyl linoleate, linolenic acid, and arachidonic acid (1.76 V vs SCE), which is significantly lower than that of oleic acid (2.03 V vs SCE) as indicated by the smaller k_et values of oleic acid than those of other unsaturated fatty acids. The radical cation of linoleic acid produced in photoinduced electron transfer from linoleic acid to the singlet excited state of 10-methylacridinium ion as well as that of 9,10-dicyanoanthracene was detected by laser flash photolysis experiments. The apparent rate constant of deprotonation of the radical cation of linoleic acid was determined as 8.1 × 10³ s^-1. In the presence of oxygen, the addition of oxygen to the deprotonated radical produces the peroxyl radical, which has successfully been detected by ESR. No thermal electron transfer or proton-coupled electron transfer has occurred from linoleic acid to a strong one-electron oxidant, Ru(bpy)₃³⁺ (bpy = 2,2′-bipyridine) or Fe(bpy)₃³⁺. The present results on the electron-transfer and proton-transfer properties of unsaturated fatty acids provide valuable mechanistic insight into lipoxygenases to clarify the proton-coupled electron-transfer process in the catalytic function.