Relationship Between Chemical Structure and Antioxidant Activity of Luteolin and Its Glycosides Isolated from Thymus sipyleus subsp sipyleus var. sipyleus

One triterpenic acid (ursolic acid), one phenolic acid (rosmarinic acid), and four flavonoids (luteolin, luteolin 7-O-(6 ''-feruloyl)-beta-glucopyranoside, luteolin 5-O-beta-glucopyranoside, and luteolin 7-O-beta-glucuronide) were isolated from the aerial parts of Thymus sipyleus subsp. sipyleus var. sipyleus and identified by spectroscopic methods. In vitro lipid peroxidation inhibition effects of the compounds were determined using TBA test method in a bovine brain liposome system. All compounds inhibited lipid peroxidation in various degrees except for ursolic acid. The order of the lipid peroxidation activities of luteolin, its glycosides and rosmarinic acid were: Luteolin 7-O-beta-glucuronide> luteolin 5-O-beta-glucopyranoside> luteolin 7-O-(6 ''-feruloyl)-beta-glucopyranoside > rosmarinic acid > luteolin. However, the activity order of the compounds was completely different in DPPH radical-scavenging activity. None of the compounds shows Fe(2+) chelating activity. The results were discussed based on their chemical structures and polarities.