Solvent‐dependent mechanistic aspects for the redox reaction of paraquat in basic solution

Abstract

Detailed mechanisms for the redox cycling of paraquat in basic solutions have been revealed computationally. The reduction of paraquat dication (PQ2+) undergoes via the successive additions with two hydroxide (OH−) anions to form the neutralized intermediates, which can decompose to generate the cation radical (PQ+) by releasing either OH or the hydrated O− radical. PQ+ is neutralized by one OH−, converting molecular oxygen into superoxide (O2−) anion to regenerate PQ2+. The reduction of PQ2+ by OH− is an energy‐directive process whereas the oxidation of PQ+ prefers an entropy‐driving path in which OH− acts as a catalyst. It is found that the Gibbs free‐energy reaction paths are strongly solvent dependent. The redox cycle is energetically preferable in the solvents with low dielectric constants. The yellow‐blue‐transparent color‐changing sequence in the clock reaction of paraquat has been understood by means of the electronic absorption spectra of the cations and the neutral intermediates. Atomic radical anion O− is predicted besides the known OH and O2− radicals to stimulate experimental studies on the redox chemistry of paraquat.

Chlorogenic acid protective effects on paraquat‐induced pulmonary oxidative damage and fibrosis in rats

“…[55] Studies indicate that PQ +2 is reduced to PQ + by NADPH cytochrome P450 reductase in the presence of electron donors, [56] and then the unpaired PQ + electron is fixed by the conjugated double bond in the pyridine ring and quaternary nitrogen in the other ring. [56,57] The high oxygen pressure in the lung tissue causes the cationic form of PQ to transfer its unpaired electron to molecular O 2 and produce superoxide radicals, while PQ + becomes PQ 2+ . [56,58] Following the depletion of intracellular antioxidant reserves such as glutathione, the reactive oxygen radicals react with proteins and unsaturated fatty acids and destruct lung cell membranes by creating protein carbonyls and fatty acid hydroperoxides.…”