A deeper insight into the oxidative mechanism of caffeine and related xanthines: may caffeine be considered as an antioxidant?

Caffeine (CAF) is present in many consumer products as coffee, tea, chocolate, soft and energy drinks, resulting the most widely consumed psychoactive substance in human dietary. Many of its physiological effects are well known. Recently, antioxidant properties have been suggested for CAF that seems showing protective effects against oxidative stress (OS).Inert to hydrogen (H)/electron (E) scavengers as ABTS·+ and DPPH, CAF has been proposed as an excellent HO· scavenger. The oxidative path of CAF in aqueous medium has been widely studied since 70s4, but until today the mechanism through which CAF and related xanthines might exert their antioxidant activity is not fully understood. The electrochemical oxidation of CAF, theobromine (TBr) and theophylline (TPh) was studied in aprotic medium by ciclyc voltammetry and controlled potential electrolysis carried out in UV cell modified for spectroelectrochemistry, to investigate on the radical intermediates involved in the primary oxidative process and their possible fate. The chemical oxidation of the same xanthines was carried out with PbO2, superoxide radical anion and galvinoxyl free radical and reactions were monitored via UV spectrophotometry, to investigate on the reactivity towards reactive oxygen species (ROS). Antioxidant activity based on electron transfer (ET) and/or H-atom transfer (HT) mechanisms was evaluated.