Analytical Chemiluminescence/Organic peroxides and lipid peroxidation

B6. Organic peroxides and lipid peroxidation

Metal ions such as iron decompose organic peroxides and hydroperoxides into free radicals; the rate of formation varies very much with different metal complexes and peroxides. The chemiluminescence intensity is directly proportional to the concentration of hydroperoxide. Cyclic organic peroxides include dioxetanes which have been disussed in connection with the peroxy-oxalate reaction. The mechanisms involved in the decomposition of 1,2-dioxetanes and analogous peroxides are: (i) unimolecular decomposition into excited state carbonyl compounds; (ii) intramolecular or intermolecular CIEEL (Chemically Initiated Electron Exchange Luminescence).

Lipid peroxidation is a process of great interest, especially in biochemical research, as it is associated with damage to biological cell membranes and has a putative role in pathological phenomena such as aging, cancer and other degenerative conditions. The process is a radical chain reaction that produces an ultraweak chemiluminescence signal. It has been proposed that in cells, the major excited species responsible for light emission are triplet carbonyls and singlet oxygen, which arise through the decomposition of hydroperoxides. Initiators such as hydroxyl radicals (^•OH) remove hydrogen from unsaturated fatty acids (LH) to produce lipid radicals (L^•):

LH + ^•OH → L^• + H₂O

which react with atmospheric oxygen to form lipid peroxyl radicals (LO₂^•):

L^• + O₂ → LO₂^•

that recombine to generate the excited products (P):

LO₂^• + LO₂^• → P* → P + Фhν

(h = Planck’s constant and ν = frequency of emitted light).

The emission intensity is determined by the quantum yield (Ф), which is low for lipid peroxidation, depending on the rate of processes competing with light emission for the deactivation of the lowest excited singlet state. Because the associated chemiluminescence is weak, it is useful to enhance the emission intensity using fluorescent dyes, as discussed in chapter 16.