Structure-Antioxidant Activity Relationships of Luteolin and Catechin

Antioxidant activity of the natural flavonoids luteolin and catechin, which shows inconsistent comparative results in literature, was extensively evaluated as affected by the 1,4-pyrone moiety and 3-OH group. Catechin and luteolin were compared to the synthetic antioxidant-butylated hydroxytoluene for their 2,2-diphenyl-1-picrylhydrazyl (DPPH^● ) scavenging activity in polar protic and aprotic solvents and ferric reducing-antioxidant power (FRAP). Moreover, their effect on lipid oxidation kinetics of canola and olive oil triacylglycerols as well as their oil-in-water (O/W) emulsions, in addition to oil stability indices (OSIs), was evaluated. In the DPPH^● assay, catechin's 3-OH group led to lower IC₅₀ values than luteolin's 1,4-pyrone moiety in acetone (3.4 µM compared with 9.4 µM), while there was no significant difference in methanol (IC₅₀ = approximately 18.3 µM) or tetrahydrofuran (IC₅₀ = approximately 27.2 µM). The FRAP test indicated a higher reducing power for catechin than for luteolin (689.4 µM compared with 573.1 µM). The antioxidants showed various performances in the oil triacylglycerols and their O/W emulsions due predominantly to the interfacial phenomena. A better OSI was found for catechin than for luteolin (20.0 to 52.7 hr compared with 3.5 to 4.2 hr) in Rancimat test.