The influence of the carbohydrate anomeric linkage on the free radical scavenging activity of enzymatically-synthesized phenolic glycosides†
Phenolic glycosides, widely recognized for their nutraceutical properties exist in nature as α- and β-glycosides. Interestingly, the β-phenolic glycosides are the predominant form in natural products. In this work, the pure α- and β-glucosides and galactosides of homovanillyl alcohol (HVA) were enzymatically synthesized. Their free radical scavenger (FRS) activity was evaluated using the DPPH˙ endpoint assay. According to the EC50 values, all glycosides are better FRSs than the corresponding aglycone, with lower EC50 values found for the β-anomers. In contrast, a DPPH˙ kinetic analysis revealed that HVA-aglycone is more reactive than the glycosides. The analysis based on both EC50 and kinetic behaviour suggested that, although the HVA-aglycone is kinetically a better free radical scavenger, glycosidation renders HVA-aglycone more stable for long-term reactions. The molecular electrostatic potential and conformational analysis of α- and β-HVA glycosides demonstrated that β-HVA glycosides are better free radical scavengers than the corresponding α-anomers. In fact, the higher flexibility of β-HVA glycosides allow the exposure of advantageously negatively charged regions favouring their FRS capacity.