Theoretical insights into the direct radical scavenging activities of 8-hydroxyquinoline: Mechanistic, thermodynamic and kinetic studies

A Density Functional Theory study of the mechanisms, thermodynamics and kinetics of some antiradical reactions of 8-hydroxyquinoline (8HQ) against •OH is reported. The radical adduct formation (RAF), hydrogen atom transfer (HAT) and single electron transfer (SET) mechanisms are investigated. The results indicate that 8HQ is an efficient RAF-dependent direct •OH radical deactivator, in accordance with recent experimental findings that 8HQ is a potent scavenger of reactive oxygen species (ROS). The ortho and para carbons to the hydroxyl group in 8HQ, not including the bridgehead carbon, are identified as highly favorable RAF sites in polar and nonpolar solvents. Interestingly, RAF via the para carbon is thermodynamically feasible, fast (with apparent rate constants 2.867 × 108 and 1.012 × 107 M-1s-1 in water and benzene, respectively), and contributes over 70% of 8HQ's antioxidant capacity. Our results support the viewpoint that direct ROS scavenging plays a key role in 8HQ's anti-neurodegenerative activities.