Free radical scavenging ability
The free radical scavenging abilities of PAs and PAGs in ethanol system were evaluated using DPPH assay. As shown in Fig. 7, the DPPH inhibition decreased with the reduction of sample concentration. The radical scavenging ability towards DPPH depends on structural features, such as the dissociation energy of hydrogen atoms from hydroxyl groups, resonance delocalization of the phenol radical (PheO·) and steric hindrance (Shahidi & Naczk, 1995). The amount of phenolic hydroxyl groups may have a dominant influence on the free radical scavenging ability (Sun et al., 2020), enabling CA (94.9 % at 1 mM) to have the highest radical scavenging ability because of its bis-hydroxyl structure. Although FA and p -HCA had the same number of phenolic hydroxyl groups, FA (80.5 % at 1 mM) exhibited higher antioxidant activity than p -HCA (13.1 % at 1 mM) because of the resonance delocalization of the phenol radical (PheO·) induced by methoxy group in the structure of FA. There was a similar variation rule in the radical scavenging ability of CG (91.5 % at 1 mM), FG (66.1 % at 1 mM) andp -HCG (5.74 % at 1 mM) with the parent PAs. The glycerol moiety of PAGs may have caused steric hindrance making antioxidants inaccessible to the DPPH radical, but also increased the hydrophilicity of PAGs causing antioxidants more available for the DPPH radicals in polar systems, so that the radical scavenging ability of PAGs was not significantly different to that of the parent PAs (Zhang & Akoh, 2020). Fig. 7 showed that the radical scavenging ability of PAGs was slightly lower than that of PAs, indicating that the accessibility of the molecule had greater effect on the scavenging activity in the DPPH assay. Therefore, the radical scavenging capacity order of PAs and PAGs was CA > CG > FA > FG > p -HCA > p -HCG.
Fig. 7 DPPH inhibition capacity of PAs and PAGs at various concentrations