REDOX INTERMEDIATES OF FLAVONOIDS AND CAFFEIC ACID-ESTERS FROM PROPOLIS - AN EPR SPECTROSCOPY AND CYCLIC VOLTAMMETRY STUDY

The redox properties of flavonoids: chrysin (1), tectochrysin (2), gal angin (3), isalpinin (4), pinostrobin (5), pinobanksin (6), pinobanksi n-3-acetate (7), and of caffeic acid ester (8) and diacetylcaffeic aci d ester (9), all isolated from propolis, were investigated by cyclic v oltammetry in acetonitrile. The choice of aprotic solvent lowered the reactivity of the radical intermediates and made possible to identify redox steps and intermediates not detected so far. The oxidation poten tials (vs. saturated calomel electrode) of the investigated compounds were in the region of 1.5 V for 3 and 4; 1.9 V for 1, 2, and 5; 2.0 V for 6 and 7; 1.29 V for 8; and 2.3 V for 9. These oxidation potentials were mainly influenced by the presence of a double bond in 2,3-positi on and substituent R(1) in position 3. Comparison with our earlier dat a revealed that flavonoids, 1-4, and caffeic acid ester 8 with lower o xidation potentials showed the maximal lipid antioxidant activity, whe reas those with higher potentials (5, 6, 7, and 9) are less active. On reduction of 1-9 several one-electron-steps were typically observed i n the potential regions: -1.5 V, -1.8 V, and -2 V, where in simultaneo us EPR experiments anion radicals of 1 and 3 were observed with the ce nter of unpaired spin density on ring A. Upon oxidation of flavonoids 1-4 carbonyl carbon-centered radicals, . C(O)R, were identified as con secutive products using the EPR spin trapping technique.