Nina Ruseva, Adriana Bakalova, Emiliya Cherneva
The antioxidant activity of bioactive compounds is fundamentally determined by their chemical structures and, more specifically, by the nature and positioning of key functional groups. Despite the structural diversity among natural and synthetic antioxidants, many share common molecular features that enable them to neutralize reactive oxygen and nitrogen species. The present review is an attempt to systematize the classification of antioxidants according to their functional groups. The role of redox-active moieties such as phenolic hydroxyl (-OH), thiol (-SH), amine (-NH2), and carbonyl (C=O) groups is also highlighted. The groups facilitate electron or hydrogen donation, stabilizing free radicals and interrupting oxidative chain reactions. Conjugated π-systems, such as those found in carotenoids and polyphenols, further enhance antioxidant capacity by allowing electron delocalization across the molecule. By examining the structural basis of antioxidant mechanisms, the review underscores the critical relationship between molecular structure and biological function in oxidative stress mitigation.