Antioxidant reaction competition

Antioxidant reaction competition is competition between reactions that consume the same reactive intermediate, such as interception of a radical by an antioxidant and attack by that radical on a susceptible substrate. A large reaction rate constant alone does not establish which route dominates: the local concentrations of the reaction partners also enter the reaction rates.

For an original two-route model, let a radical react with donor $\mathrm{AH}$ or substrate $\mathrm{S}$. Hold both partner concentrations constant, with second-order rate constants $k_A,k_S$. The corresponding pseudo-first-order reaction rates per radical areThe probability of surviving without either event for time $t$ is $e^{-(a+b)t}$. Integrating the instantaneous interception probability giveswhere $\tau$ is the mean waiting time for the first event. This is a competing risks calculation for the stipulated constant-rate reaction model.

Take chosen $k_A=10^6\mathrm{L}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{s}}^{-1}$, $[\mathrm{AH}]=10\mu \mathrm{mol}{\mathrm{L}}^{-1}$, $k_S=10^2\mathrm{L}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{s}}^{-1}$ and $[\mathrm{S}]=0.10\mathrm{mol}{\mathrm{L}}^{-1}$. Both effective rates are $10{\mathrm{s}}^{-1}$, so interception has probability $1/2$ and the mean waiting time is $0.050\mathrm{s}$. Raising only the donor concentration tenfold gives $f_A=10/11$ and $\tau =1/110\mathrm{s}$.

This fraction describes the first reaction, not necessarily final protection. If substrate attack regenerates another reactive radical, radical chain propagation can continue. In a simplified chain where each event either propagates with probability $1-f_A$ or terminates by trapping with probability $f_A$, the expected number of propagation events satisfiesDepletion, additional termination and movement between phases alter this model.

An inhibited autoxidation assay measures a consequence of such chain chemistry under defined conditions. Primary work compared inhibited-oxidation measurements of lipid-soluble chain-breaking activity with independent chromatographic measurements of vitamin E in plasma and membrane preparations.^[ref] That independent comparison is stronger evidence than assigning protection from the chemical label antioxidant alone.