= Radical-trapping antioxidant
A \b[<radical-trapping antioxidant>] reacts with a chain-carrying <radical (chemistry)> in a way that reduces continued <radical chain propagation>. It is a mechanistic role in a particular reaction system, not a guarantee that every reaction product is harmless or that the substance improves health.
For a simplified oxidizing substrate $\mathrm{RH}$, propagation can involve a <peroxyl radical>:
$$
\mathrm{ROO^{\bullet}+RH\longrightarrow ROOH+R^{\bullet}},\qquad
\mathrm{R^{\bullet}+O_2\longrightarrow ROO^{\bullet}}.
$$
The first step consumes substrate while regenerating a radical that can continue the chain. A hydrogen-donating trap $\mathrm{AH}$ offers a competing route,
$$
\mathrm{ROO^{\bullet}+AH\longrightarrow ROOH+A^{\bullet}}.
$$
The newly formed $\mathrm{A^{\bullet}}$ must be less effective at continuing the unwanted chain or undergo <radical chain termination> for the trapping step to be protective overall. Producing a different radical does not alone establish termination.
Speed and capacity are separate properties. <Antioxidant reaction competition> depends on $k_A[\mathrm{AH}]$ relative to competing routes. Capacity instead counts how many radical events a finite amount of trap can intercept before it is consumed or transformed.
An original capacity model assumes that each antioxidant molecule traps $\nu$ radicals, all newly initiated radicals are trapped during an inhibited interval, and initiation supplies radicals at a constant rate $R_i$ per unit volume. Then
$$
t_{\mathrm{ind}}\simeq\frac{\nu[\mathrm{AH}]_0}{R_i}.
$$
With chosen $\nu=2$, $[\mathrm{AH}]_0=10\,\mathrm{\mu mol\,L^{-1}}$ and $R_i=0.20\,\mathrm{\mu mol\,L^{-1}\,s^{-1}}$, the predicted inhibited interval is $100\,\mathrm s$. The stipulated stoichiometry is not universal; incomplete trapping, regeneration or other reactive species change the relation.
A motivating primary experiment measured lipid-soluble chain-breaking activity by an <inhibited autoxidation assay> and compared it with independently measured <vitamin E> in human plasma and erythrocyte membrane preparations.<Is vitamin E the only lipid-soluble, chain-breaking antioxidant in human blood plasma and erythrocyte membranes? by G. W. Burton and others>[1983, Abstract, inhibited-autoxidation activity and HPLC composition]{ref} Activity under a defined chemical challenge and the amount of an identified substance are distinct observations. Their comparison can test an attribution in that system; the same conclusion cannot be read directly from unrelated colour-reduction or <cell viability> assays.
Back to article page
