Theoretical investigation of the effect of sugar substitution on the antioxidant properties of flavonoids

Natural flavonoids are secondary phenolic plant metabolites known for their bioactivity as antioxidants. The evaluation of this property is generally done by the estimation of their direct free radical-scavenging activity as hydrogen or electron donating compounds. This paper reviews experimental results available in the literature for a selection of flavonoids and compares them with calculated quantities characteristic of the hydrogen or electron donation. For that purpose, bond dissociation energies, ionization potentials and electron transfer enthalpies are computed by using DFT methods and the ONIOM procedure implemented in the ab initio program Gaussian. This process has been chosen because it can be extended to the study of large molecules. When acid dissociation and interaction with the solvent are taken into account, the results present very good concordance with experimental results, enlightening the complexity of the processes involved in the classical assays which measure the ability of compounds to scavenge the (2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonic acid) diammonium salt) radical cation (ABTS (+)) or the 2,2-diphenyl-1-picryl-hydrazyl radical (DPPH(·)). This study demonstrates the good accuracy of theoretical calculations in obtaining the relative energies involved in free radical scavenging abilities and its capacity for predictive behaviour. It also highlights the necessity to take into account the pK(a) of the compounds and the solvent interaction. The ability of the method to calculate the antioxidant properties of larger molecules are tested on glycosylated flavonoids and the effects of sugar substitution on the antioxidant properties of flavonoids are investigated, pointing out the importance of the charges on the oxygen atoms.