Pulse radiolysis study on free radical scavenger edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one). 2: A comparative...

A comparative study using the pulse radiolysis technique was carried out to investigate transient absorption spectra and rate constants for the reactions of (•)OH and N(3)(•) with edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one) and its four analogue compounds, 1,3-dimethyl-2-pyrazolin-5-one, 3-methyl-1-(pyridin-2-yl)-2-pyrazolin-5-one, 1-phenyl-3-trifluoromethyl-2-pyrazolin-5-one and 1-(4-chlorophenyl)-3-methyl-2-pyrazolin-5-one. The results showed that, unlike reaction mechanisms previously proposed, the phenyl group of edaravone played an important role in the reaction with (•)OH and OH adducts to the phenyl group were formed. Quantum chemical calculations also strongly supported this attribution and suggested that the most favorable site for attacks by (•)OH is the ortho position of the phenyl group. Moreover, the rate constants for the reactions of edaravone and its analogues towards (•)OH and N(3)(•) were about 8.0 × 10(9), and 4.0 × 10(9) dm(3) mol(-1) s(-1), respectively. Edaravone displayed higher reactivity compared to the others, in contrast to a previous report in which 3-methyl-1-(pyridin-2-yl)-2-pyrazolin-5-one showed the highest reactivity towards (•)OH.