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