Antioxidant action of phenolic compounds is because of their high tendency to chelate metals with heavy metals like iron and copper. In human body as well as in plant system, free radicals are main component which can easily damage cell due to the presence of free electrons.
Proteins, green vegetables have releases some compounds which can neutralize free radicals and help to repair the damage occur in cell due to free radicals. Generally antioxidants can rapidly lose electron to a free radical and get paired. The pairing of electron with free radical makes it less harmful.