Baicalin in radical scavenging and its synergistic effect with beta-carotene in antilipoxidation

The lipophilic flavonoid glycoside baicalin from the traditional oriental herb Scutellaria baicalensis Georgi (logP = 1.27, pK(a1) = 7.6, pK(a2) = 10.1 as determined at 25 degrees C in 0.1 M NaCl) is found to be as reducing (0.39 V vs NHE, reversible two-electron oxidation by CV at pH 7.4) as other catechol flavonoids but a poor radical scavenger (TEAC = 1.12, pH 7.4) and a poor antioxidant against free radical initiated lipid oxidation in liposomes. However, this compound is able to regenerate beta-carotene (beta-Car) from beta-Car(*+) with a second-order rate constant of (5.6 +/- 0.5) x 10(9) L mol(-1) s(-1) in the methanol/chloroform binary solvent (1:9, v/v) and, more importantly, to exhibit a prominent synergistic effect with beta-Car against the lipoxidation induced by AMVN-derived peroxyl radical in liposomal membrane. Thus, baicalin by itself is not an effective antioxidant, but it becomes one via interaction with beta-Car. The radical scavenging and antilipoxidation properties of baicalin are discussed in terms of its physicochemical properties and molecular structures.