Is DPPH Stable Free Radical a Good Scavenger for Oxygen Active Species?

The reaction of 2,2-diphenyl-1-picrylhydrazyl free radical (DPPH) with active species of oxygen (hydrogen peroxide, tert-butyl peroxide, tert-butyl hydroperoxide, sodium peroxide, potassium superoxide, hydroxyl radical, tempo free radical, galvinoxyl free radical, Fremy’s salt, mchloroperbenzoic acid, potassium hydroxide, and sodium peroxynitrite) was reviewed and the new obtained results compared with previous literature data. It was shown that in all of these reactions the corresponding hydrazine DPPH-H (2,2-diphenyl-1-picrylhydrazine) is the major product, besides, in most of reactions, the nitro derivative of DPPH-H. The formation of the nitro derivative involves the previous formation of nitrogen dioxide, which can appear in the reaction mixture in two ways: via a radical attack on picryl moiety which releases the nitrogen dioxide or via a nucleophilic substitution of a nitro group, which releases the nitrite anion, which is subsequently oxidized to the nitrogen dioxide. Nitrogen dioxide reacts easily with DPPH to form the corresponding nitro derivative of DPPH-H. The scavenging activity of DPPH in terms of oxygen active species, which should yield oxy derivatives of DPPH, is very poor, only traces of these could be found.