Structure–activity relationships of new 4-hydroxy bis-coumarins as radical scavengers and chain-breaking antioxidants

The main antioxidant properties of five new 4-hydroxy-bis-coumarins during bulk lipid autoxidation at 80 °C and 0.1 mM and 1.0 mM concentrations were studied and compared with 4-hydroxy-2H-chromen-2-one (1). These compounds are: 3,3′-((3,4-dihydroxy-phenyl) methylene) bis (4-hydroxy-2H-chromen-2-one) (2), 3,3′-((3,4-dimethoxyphenyl) methylene) bis (4-hydroxy-2H-chromen-2-one) (3), 3,3′-((4-hydroxy-3,5-dimethoxy-phenyl) methylene) bis(4-hydroxy-2H-chromen-2-one) (4) 3,3′-((3,4,5- trimethoxyphenyl) methylene) bis (4-hydroxy-2H-chromen-2-one) (5) 3,3′-((4-hydroxy-3-methoxy-5-nitrophenyl) methylene) bis (4-hydroxy-2H-chromen-2-one) (6), It was found that compound 2 with a catecholic structure in the aromatic nucleus showed the strongest antioxidant activity. Compound 4 showed a moderate antioxidant activity, and all the other compounds didn't show any capacity as chain-breaking antioxidants. Both 4-hydroxy-bis-coumarins (2 and 4) demonstrated also stronger radical scavenging activity towards DPPH radical by using TLC DPPH rapid test, than compound 1. The other compounds (3, 5, 6) didn't show any capacity as radical scavengers. The structure–activity relationship was discussed on the base of comparable kinetic analysis of studied 4-hydroxy-bis-coumarins with the known and standard antioxidants as α-tocopherol (TOH), caffeic acid (CA), sinapic acid (SA), ferulic acid (FA), and p-coumaric acid (p-CumA). In order to study the possible synergism between two phenolic antioxidants, the antioxidant efficiency and reactivity of two equimolar binary mixtures of coumarins and TOH (2 + TOH and 4 + TOH) and of corresponding cinnamic acid with TOH (CA + TOH and SA + TOH) were also tested and compared. The oxidation stability of the lipid substrate in presence of binary mixtures CA + TOH, SA + TOH and 2 + TOH appeared to be higher than that of the individual antioxidants. However, no synergism was obtained for all tested binary mixtures.