Mechanism and kinetics of the hydroxyl and hydroperoxyl radical scavenging activity of N-acetylcysteine amide

The (OH)-O-center dot and (OOH)-O-center dot radical scavenging activity of N-acetylcysteine amide (NACA) has been studied using density functional theory, specifically the M05-2X functional. All possible reaction sites have been considered, and the branching ratios have been estimated. The efficiency of different mechanisms of reaction has been evaluated, and it has been concluded that NACA reacts exclusively by hydrogen atom transfer (HAT). The overall reactivity of NACA toward OH radicals is proposed to be diffusion-controlled in both non-polar and polar media. The values of the overall rate coefficients are 3.80 x 10(9) and 1.36 x 10(9) L mol(-1) s(-1) for benzene and aqueous solutions, respectively. The reactivity of NACA toward (OOH)-O-center dot, on the other hand, is much lower but still higher than those of melatonin and caffeine. HAT from the -SH site is proposed to be the channel accounting for most of the radical scavenging activity of NACA in aqueous solution. In non-polar environments, two channels of reaction were found to similarly contribute to the overall reactivity of NACA toward OH radicals. They are those corresponding to hydrogen atom transfer from -CH2 and -SH sites.