Polymerization of gallic acid enhances its antioxidant capacity: Implications for plant defence mechanisms

Abstract

Gallic acid (3, 4, 5-trihydroxybenzoic acid, GA) and its polymer, tannic acid (TA) are ubiquitous phytochemicals and are found to co-exist in plants. However, the rationale for the polymerisation of GA in plants is rather obscure. Hence, the present study compared the free radical scavenging ability, iron chelating potency, ferric reducing antioxidant power and inhibitory potential of both GA and TA against deoxyribose and lipid oxidative assaults in vitro. The results show that TA exhibited marked free radical scavenging ability as well as chelate Fe2+ and reduce Fe3+ more effectively than GA. Furthermore, this differential antioxidant capacity is also observed in the ability of both polyphenols to protect against hydroxyl radical induced deoxyribose degradation in the Fenton reaction. Similarly, in comparison with GA, TA also exerted marked inhibitory effect against mammalian cerebral and hepatic lipids assaulted with different prooxidants possessing diverse mechanisms of action. The present finding suggests that the polymerisation of GA to form a more efficient radical scavenger may have be associated with the need for increased antioxidant capacity required to cope with disturbed redox balance associated with pathological conditions in plants in which free radical induced oxidative stress may be implicated in their etiologies.

KEYWORDS: Gallic acid, tannic acid, antioxidant, oxidative stress, polymerisation