Lipid peroxidation refers to the oxidative degradation of lipids. It is the process whereby free radicals `steal' electrons from the lipids in cell membranes, resulting in cell damage. This process is proceeded by a free radical chain reaction mechanism. It most often affects polyunsaturated fatty acids, because they contain multiple double bonds in between which lie methylene -CH2- groups that are especially reactive hydrogen. As with any radical reaction the reaction consists of three steps: initiation, propagation and termination.

Initiation

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Initiation is the step whereby a fatty acid radical is produced. The initiators in living are most notably reactive oxygen species (or ROS), namely OH° which abstracts a hydrogen to make water and a fatty acid radical.

Propagation

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The fatty acid radical is not a very stable molecule so it reacts readilly with molecular oxygen, thereby creating a peroxy-fatty acid radical. This too is an unstable species that reacts with another free fatty acid producing a different fatty acid radical and a hydrogen peroxide or a cyclic peroxide if it had reacted with itself. This cycle continues as the new fatty acid radical reacts in the same way.

Termination

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When a radical reacts it always produces another radical, which is why it is called the chain reaction mechanism. The only way to stop a radical reaction is for two radicals to react and produce a non-radical species. This is what happens when the concentration of radical species is high enough for there to be a high probability of two radicals actually colliding. But living organisms have evolved different molecules that catch free radicals and protect the cell membrane, one of which is alpha-tocopherol also known as vitamin E.