Is caffeine a good scavenger of oxygenated free radicals?

The reactions of caffeine (CAF) with different reactive oxygen species (ROS) have been studied using density functional theory. Five mechanisms of reaction have been considered, namely, radical adduct formation (RAF), hydrogen atom transfer (HAT), single electron transfer (SET), sequential electron proton transfer (SEPT), and proton coupled electron transfer (PCET). The SET, SEPT, and PCET mechanisms have been ruled out for the reactions of CAF with (•)OH, O(2)(•-), ROO(•), and RO(•) radicals. It was found that caffeine is inefficient for directly scavenging O(2)(•-) and (•)OOCH(3) radicals and most likely other alkyl peroxyl radicals. The overall reactivity of CAF toward (•)OH was found to be diffusion-controlled, regardless of the polarity of the environment, supporting the excellent (•)OH scavenging activity of CAF. On the other hand, it is predicted to be a modest scavenger of (•)OCH(3), and probably of other alkoxyl radicals, and a poor scavenger of HOO(•). RAF has been identified as the main mechanism involved in the direct ROS scavenging activity of CAF. The excellent agreement with the available experimental data supports the reliability of the present calculations.