Health and environmental impacts of phenols: The role of radicals

The role of the most significant phenolic compounds such as phenol (PhOH), hydroquinone (HQ), catechol (CT) and radicals derived from these compounds as precursors for oxidative stress as well as their toxicological impact on human and environment is reviewed. The structure of phenolic compounds comprises an aromatic ring, bearing one or more hydroxyl and other substituents, and range from simple phenolic molecules to highly polymerized compounds. Phenol and its methylated derivatives o-, m-, p-cresols, HQ and CT, methoxy substituted phenols, monolignols (as precursors for lignin and derivatives of methoxy substituted phenols with propenoid units) here in referred to as phenolic compounds or phenols, are well-known xenobiotics and eco-toxicants. Phenols are easily absorbed by animals and humans resulting to serious damage to a variety of organs and tissues. The red-ox cycling of phenols - isolated or supported by transition metals and various matrixes (organic, inorganic) - generates a variety of the most aggressive oxygen-centered phenoxy type radicals, which induce oxidative stress by rapidly dividing human cells, and may cross biological barriers containing lipids, microsomal membranes and skin stratum causing cell aberations, subsequent oxidative stress and ultimately cancer. As a result, the cells produce more reactive oxygen species (ROS) which trigger the conversion of phenols to either free or stabilized on different matrixes containing toxic phenoxy type radicals. A detailed transformation mechanism of the biologically important catechol to ortho-semiquinone radical is elucidated based on the analysis of combined theoretical and experimental results.