Structural Origins of the Differential Antioxidative Activities between Baicalein and Baicalin

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.