Lipid Peroxidation Simulator: Watch the Chain Reaction (and Break It)

Reading that lipid peroxidation is a “self-propagating chain reaction” is one thing. Watching a single free radical torch an entire cell membrane in a couple of seconds — and then watching a pinch of deuterium stop it dead — is another. So here is that mechanism, not described but played.

How to play with it

• Fire a radical. Click anywhere on the membrane (or press the button). One rogue oxygen radical lands, rips a hydrogen from a weak spot, and the fire spreads to the neighbours like falling dominoes.
• Now add deuterium. Drag the slider up. Green “D-PUFA” match-heads are reinforced at the weak spot — around 25–30% is enough to fragment the chain so it can no longer cross the membrane.
• Change the fatty acid. Oleic acid has no weak spots and will not burn at all; DHA has five and goes up like a torch. More double bonds means more places to catch.
• Try antioxidants. Switch them on and watch them get overwhelmed — the reason they cannot, on their own, stop the chain.

What you are actually watching

The membrane is packed with polyunsaturated fatty acids (PUFAs). Their weak points are the bis-allylic positions — the CH₂ groups sitting between two double bonds. A free radical yanks a hydrogen from one of them (the slow, rate-limiting step), the lipid ignites, and because the molecules are packed together the damage races outward. A single radical can wreck hundreds of them. This is the only non-enzymatic chain reaction in biology.

Deuterium is a heavy, stable isotope of hydrogen. The carbon–deuterium bond is roughly 13 times harder to break than carbon–hydrogen (reported values run from 6 to 80 depending on the system), so the radical stumbles precisely at that rate-limiting step. It is physics, not chemistry: the molecule is still the same fatty acid. And because you only need to protect one step, you do not have to deuterate everything — about 30% D-PUFA is enough to bring the whole chain to a halt.

Antioxidants, by contrast, work downstream. You would need to park one next to every single lipid to guard them all — and you cannot, which is why antioxidants fail to stop an exponential chain. Toggle them on in the simulator and you will see them run out.

Numbers behind the model: KIE k(H)/k(D) ≈ 12.8 (Hill, Shchepinov et al., Free Radical Biology & Medicine, 2012); protection threshold ~20–25% deuteration (Shchepinov et al., Trends in Molecular Medicine, 2020); bis-allylic sites LA 1 / ALA 2 / AA 3 / EPA 4 / DHA 5. Animation parameters are tuned for clarity, not literal reaction speed.