ABSTRACT
This chapter describes radical reactions in organic chemistry and radical reactions in biological systems. Cells have developed enzymes to decompose peroxides, proteins to sequester transition metals and many antioxidants to "scavenge" free radicals, to prevent free radical formation, or to limit their deleterious effects. Hydroxyl radical (HO ) is highly reactive and it is considered to be one of the more important biological radicals. The reversible binding of molecular oxygen to form oxyhemoglobin is accompanied by side reactions in which methemoglobin is formed with the attendant formation of hydroxyl radicals. Such damage has been referred as "oxidative stress." Polyunsaturated fatty acid derivatives, for example, react with radicals to form conjugated allylic radicals that react with oxygen to give alkyl hydroperoxides via hydrogen abstraction from another lipid. The two major antioxidant mechanisms are primary antioxidants that react directly with the radicals (radical scavengers), or secondary antioxidants that trap chain propagating radicals.
