A DFT investigation on the structural and antioxidant properties of new isolated interglycosidic O-(1 → 3) linkage flavonols.

We present a computational study on two flavonols that were recently isolated from Loranthaceae family plant extracts: kaempferol 3-O-α-L-arabinofuranosyl-(1 → 3)-α-L-rhamnoside and quercetin 3-O-α-L-arabinofuranosyl-(1 → 3)-α-L-rhamnoside. Their structures and energetics have been investigated at the density functional level of theory, up to B3LYP/6-31+G(d,p), incorporating solvent effects with polarizable continuum models. In addition, their potential antioxidant activities were probed through the computation of the (i) bond dissociation enthalpies (BDEs), which are related to the hydrogen-atom transfer mechanism (HAT), and (ii) ionization potentials (IPs), which are related to the single-electron transfer mechanism (SET). The BDEs were determined in water to be 83.23 kcal/mol for kaempferol 3-O-α-L-arabinofuranosyl-(1 → 3)-α-L-rhamnoside and 77.49 kcal/mol for quercetin 3-O-α-L-arabinofuranosyl-(1 → 3)-α-L-rhamnoside. The corresponding IPs were obtained for both compounds as 133.38 and 130.99 kcal/mol, respectively. The BDEs and IPs are comparable to those probed for their parental molecules kaempferol and quercetin; this is in marked contrast to previous studies where glycosylation at the 3-position increases the corresponding BDEs, and, hence, decreases subsequent antioxidant activity. The BDEs and IPs obtained suggest both compounds are promising for antioxidant activity and thus further experimental tests are encouraged.