DFT study on the reactivity of OH groups in emodin: structural and electronic features of emodin radicals

Abstract

In the past years, the chemical behaviour of flavonoids and some anthraquinones as antioxidants has become the subject of intense experimental and theoretical research. In this paper, we used a quantum-chemical approach to shed light on the reactivity of the anthraquinone emodin. The structural and electronic features of emodin were investigated at the B3LYP/6-311++G** level of theory. We particularly focused on the 3-OH site and the role played by double bonds of benzene ring B in the reactivity of this site. The analysis of the theoretical bond dissociation enthalpy (BDE) values for all OH sites of emodin clearly shows the importance of the B ring and 3-OH group. The importance of keto-enol tautomerism has also been considered. Our analysis also includes the Mulliken spin density distribution for the radicals formed after H-removal on each OH site. The results clearly show that the BDE on the 1-OH and 8-OH sites are higher, because H-removal also implies the cleavage of the hydrogen bond.