To clarify the structure–activity relationship between phenolic
compounds and antioxidants, the biophenols present in OLE
and other common phenolic compounds such as flavones and
phenolic acid were evaluated based on their DPPH radical scavenging
capacity and FRAP.
The results for different phenolic compounds as well as Vc
and BHT as positive controls are summarized in Table 4. These
provide the number of hydroxyl groups, the presence of a catechol
moiety, the identity of the 7-position substituent in
flavones, the presence of an aromatic ring conjugated to
electron-donating or electron-withdrawing groups and synergistic
behaviors with OLE or the 10-standard mixture.
The twenty phenolic compounds shown in Table 4 were used
to evaluate what types of structural attributes strengthened
the antioxidant ability. The different isocratic gradients (n = 6)
of these standards revealed linear relationships for DPPH radical
scavenging (IC50) and FRAP (TEAC). The IC50 is the concentration
at which 50% of the DPPH radicals in the sample were
scavenged, and the samples (Table 4) were ordered as follows:
Vc > quercetin > hydroxytyrosol > oleuropein > luteolin
≈ caffeic acid > OLE > gallic acid > verbascoside > protocatechuic
acid > ten-standards mixture (equal proportion, the same
below) > Trolox > luteolin-7-glucoside > rutin > vanillic
acid > chlorogenic acid > p-coumaric acid > apigenin-7-β-Dglucose
≈ BPOL > BHT > tannic acid > BPOF > vanillin > OFE
> tyrosol. TEAC was computed by the following equation:
EC50(Trolox)/EC50(sample), where EC50 is the concentration at
which the reducing power was reduced by half of the initial
value. The relative FRAP (indicating total reducing power)
capacities were ordered as follows: Vc > gallic acid > quercetin
> hydroxytyrosol > caffeic acid > OLE > ten-standard mixture
(equal proportions) > luteolin > protocatechuic acid >
oleuropein > luteolin-7-β-D-glucose > verbascoside > Trolox > apigenin-
7-glucoside > rutin > BPOL > vanillic acid > chlorogenic
acid > p-coumaric acid ≈ BPOF > OFE > BHT ≈ tannic acid >
vanillin > tyrosol.
Differences were found between the orderings based on the
DPPH scavenging ability and FRAP, likely due to the mechanisms
of action of the different phenolic compounds. However,
the trend in antioxidant activity was similar with the exception
of a few compounds.
Hydroxytyrosol with a catechin structure possesses extremely
strong antioxidant activity compared with tyrosol (no
catechin structure), reflecting its H-donor ability and the formation
of a highly stable aromatic ring radical when losing a
hydrogen atom for conjugation. Oleuropein and verbascoside,
including the hydroxytyrosol profile, have high antioxidant