Question

Some chemicals, such as paraquat, produce toxicity via redox cycling. As you know, the final reactive oxygen species generated by paraquat that appears to be responsible for the ultimate toxicity appears to be hydroxyl radicals. BRIEFLY explain how paraquat can generate hydroxyl radicals.

Answer 1

Paraquat or N,N′-dimethyl-4,4′-bipyridinium dichloride is the organic compound. It is a family of redox-active heterocycles of similar structure.

Paraquat is an oxidant that interferes with electron transfer, a process that is common to all life. Addition of one electron gives the radical cation:

[MV]²⁺ + e⁻ ${\displaystyle {\ce {<=>>}}}$ [MV]⁺

The radical cation is also susceptible to further reduction to the neutral [MV]⁰

[MV]⁺ + e⁻ ${\displaystyle {\ce {<=>>}}}$ [MV]⁰

As an herbicide, paraquat acts by inhibiting photosynthesis. In light-exposed plants, it accepts electrons and transfers them to molecular oxygen. In this manner, destructive reactive oxygen species are produced.

Paraquat is more specifically, the superoxide free radical. Paraquat will undergo redox cycling being reduced by an electron donor such as NADPH, before being oxidized by an electron receptor such as dioxygen to produce superoxide. The single-electron reduced paraquat radical reacts with O2 to generate the superoxide radical, O2.-. Hydroxyl radicals (OH.) can also be detected in this assay system by their reaction with phenol to form diphenols, as assayed quantitatively by a highly specific and sensitive method employing gas-liquid chromatography. Formation of hydroxyl radicals can be virtually completely suppressed by catalase and partially suppressed by superoxide dismutase.