Structural Origins of the Differential Antioxidative Activities between Baicalein and Baicalin
黄芩素与黄芩苷抗氧化活性差异的结构原因

Baicalein, a primary metabolite of baicalin, differs from its mother compound merely by the 7-substituent (i.e. it possesses a phenolic hydroxyl (7-OH) instead of a glucuronic acid). The present work is intended to compare, by means of steady-state optical absorption spectroscopy, cyclic voltammetry, ESR and quantum chemical calculation, their physicochemical properties, such as pKa, log10 partition coefficient, redox potential and dipole moment, as well as their capacities of scavenging ABTS·+ (TEAC) and autoxidation of baicalein. Experimental results showed that the 7-OH group of baicalein was rather acidic (pKa=5.4). In addition, baicalein showed a lower redox potential (0.32 V vs. NHE) and a 1.8-fold higher TEAC with reference to baicalin under physiological pH. Theoretical results showed that the difference in 7-substituent resulted in significantly different molecular structures, physicochemical properties and radical scavenging activities of the flavonoids in question, which are discussed in terms of their micro- and macro-molecular properties. It was concluded that the number of phenolic hydroxyls and the corresponding pKa values, as well as the molecular structures are all important for the antioxidative activity of flavonoids.