Theoretical Study of the Trapping of the OOH Radical by Coenzyme Q

• Published in: Journal of the American Chemical Society 2004-01, Vol.126 (3), p.920-927
• Main Author: Espinosa-García, Joaquín
• Format: Article
• Language: English
• Subjects: Biological and medical sciences; Free Radicals - chemistry; Fundamental and applied biological sciences. Psychology; Hydrogen Bonding; Mechanisms. Catalysis. Electron transfer. Models; Models, Chemical; Molecular biophysics; Peroxides - chemistry; Physical chemistry in biology; Thermodynamics; Ubiquinone - chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja037858j

The reactivity of the hydroperoxyl radical with coenzyme Q, as a prototypical chemical reaction involved in biological antioxidant actions, was studied theoretically. Two pathways were analyzed:  the hydrogen abstraction reaction from the phenolic hydrogen on the reduced form (ubiquinol), and OOH addition on the oxidized form (ubiquinone). Optimized geometries, harmonic vibrational frequencies, and energies of the stationary points (reactants, intermediate complexes, transition states, and products) for each pathway were calculated at the BHandHLYP/6-31G level of theory. The reaction paths for the two mechanisms were traced independently, and the respective thermal rate constants were calculated using variational transition-state theory with multidimensional small-curvature tunneling. We found that the reactivity of the OOH radical is dominated by the hydrogen abstraction mechanism on ubiquinol, with a rate constant of 5.32 × 105 M-1 s-1, at 298 K. This result strongly contrasts with that, also obtained by our group, for the more reactive OH radical, which attacks ubiquinone by an addition mechanism, with a diffusion-controlled rate of 6.25 × 1010 M-1 s-1, at 298 K.