Iron binding characteristics of phenolic compounds: some tentative structure–activity relations

Two mechanisms are commonly proposed to explain the antioxidant role of phenolic compounds; these are metal chelation and/or free radical scavenging. However, the structural requirements for each may not be the same. This paper describes the determination of iron-binding efficiencies (in vitro) for a series of pure phenolics of known structure [gallic acid, tannic acid, catechin, epicatechin (EC), epicatechingallate (ECG), epigallocatechin (EGC), epigallocatechingallate (EGCG) and quercetin] and of tea beverages of known flavonoid composition. Iron-binding efficiency was measured as catechin equivalents or tannic acid equivalents using the two wavelength assay. High catechin equivalents require the presence of a 3′,4′ dihydroxy (catechol ) group on flavanoid ring B. In contrast, the presence of a 3′,4′,5′-trihydroxy (galloyl) group of ring B (epigallocatechin) or ring C (epicatechin gallate) was associated with reduced Fe-binding. The extent of Fe-binding was also found to be lower for quercetin than catechin, which is probably due to the presence of conjugation extending from the C4-keto group, via C2-3 to the 3′-OH group (rings B and C). For the polyphenols examined, catechin equivalents were inversely correlated with tannic acid equivalents and with antioxidant activity of flavonoids, as measured by the TEAC assay. Requirements for efficient iron-binding are discussed.