OH Radical Scavenging Activity of Edaravone: Mechanism and Kinetics

The reactions of OH radicals with the neutral and anionic forms of Edaravone (3-methyl-1-phenyl-2-pyrazolin-5-one, EDA) have been studied using Density Functional Theory. Different mechanisms and reaction sites have been considered. The overall rate constant was found to be diffusion-limited (1.35 x 10(10) s(-1), in aqueous solution), and in excellent agreement with the experimental results. Therefore, the present work supports previous evidence that EDA is an excellent (OH)-O-center dot scavenger. The anionic form of EDA is predicted to react 8.6 times faster than its neutral form. The preponderant mechanism was found to be different depending on the form of EDA reacting with the radical. For the anionic form, the single electron transfer mechanism was found to be the one contributing the most to the overall reactivity toward (OH)-O-center dot (similar to 44%), closely followed by radical adduct formation (similar to 40%). For the neutral form, the latter was found to be the main mechanism, with contributions larger than 98%, regardless of the polarity of the environment. For the global reactivity of EDA toward OH radicals, at physiological pH, the main mechanism of reaction is proposed to be the sequential electron proton transfer. A detailed analysis of the UV-vis spectra is also provided. The excellent agreement with the available experimental data supports the reliability of the new information provided in the present work.