250
et al. 2005; Neudörffer et al. 2006; Giftson et al. 2010; Velmurugan et al. 2018). The
strong free radical scavenging activities of polyphenols are well-documented and
have been extensively reviewed. Two main mechanisms are generally considered,
based on the capability of phenol groups to donate hydrogen atom or electron
(Wright et al. 2001).
The hydrogen atom transfer between phenol group and radical is especially
involved in the deactivation of peroxyl radicals, which are key intermediates as
chain propagators, thereby terminating the chain reaction. Peroxyl radicals (LOO
•
)
are issued from the auto-oxidation of lipids (LH). A propagating radical chain reaction can occur according to LOO
•
+ LH ➛ LOOH + L
•
. After hydrogen atom transfer, the phenol group becomes a phenoxy radical (ArO
•
). The efficiency of the
reaction (LOO
•
+ ArOH ➛ LOOH + ArO
•
) depends on both the kinetics of the H
transfer to LOO
•
and on the stability of phenoxy radical. The phenoxy radical is
mainly stabilized through electron delocalization, becoming less reactive and hence
unable to react back neither with LOOH nor LH. A key point is the structural features of ArOH, more precisely the position and number of hydroxyl groups, allowing electron delocalization over the molecule along with the formation of
intramolecular hydrogen bonds (Quideau et al. 2011). This aspect can be quantified
using O-H bond dissociation enthalpy values as a parameter (de Heer et al. 1999).
The lower the bond dissociation enthalpies, the more efficient is the hydrogen atom
transfer reaction.
The second mechanism involves a single electron transfer from phenol group to
free radical, yielding a radical cation (Cren-Olivé et al. 2002). Similar to hydrogen
atom transfer process, the structure of phenols plays a key role, and the same determining factors apply. The quantitative physicochemical parameter is the redox
potentials or ionization potentials. The easier the oxidation of the polyphenols, i.e.,
lower redox potentials, the more efficient is the electronic transfer reaction.
Deciphering the nature of mechanisms is then strongly related to polyphenol
structures, and many predictive works based on structure-activity relationship have
been undertaken, mainly on plant polyphenols, while algae remain still poorly
investigated. Moreover, works for establishing a correlation between antioxidant
activity and phlorotannins have even given contradictory conclusions (Wang et al.
2014). However, many works combining density functional theory calculations
with experimental determinations of O-H bond dissociation enthalpies and redox
potentials/ionization potentials of plant polyphenols allow a fairly good rationalization of their antioxidant properties and underlying mechanism (Quideau et al. 2011).
A very interesting point with plant polyphenols is that the products formed during
ROS scavenging process are phenols themselves, which may keep a further ROS
scavenging activity. As a consequence, they are long-lasting antioxidant, then able
to potentially reduce much more ROS than the most efficient antioxidant, i.e.,
α-tocopherol (Roche et al. 2005).
In addition to direct radical scavenging, other modulation effects have been suggested to explain the antioxidant activities of polyphenols, for instance, synergistic
effect with other potent antioxidants like α-tocopherol (Zhou et al. 2005; Dai et al.
2008; Achat et al. 2016) or in cells through modulations of the protein kinase and
L. Percevault et al.
et al. 2005; Neudörffer et al. 2006; Giftson et al. 2010; Velmurugan et al. 2018). The
strong free radical scavenging activities of polyphenols are well-documented and
have been extensively reviewed. Two main mechanisms are generally considered,
based on the capability of phenol groups to donate hydrogen atom or electron
(Wright et al. 2001).
The hydrogen atom transfer between phenol group and radical is especially
involved in the deactivation of peroxyl radicals, which are key intermediates as
chain propagators, thereby terminating the chain reaction. Peroxyl radicals (LOO
•
)
are issued from the auto-oxidation of lipids (LH). A propagating radical chain reaction can occur according to LOO
•
+ LH ➛ LOOH + L
•
. After hydrogen atom transfer, the phenol group becomes a phenoxy radical (ArO
•
). The efficiency of the
reaction (LOO
•
+ ArOH ➛ LOOH + ArO
•
) depends on both the kinetics of the H
transfer to LOO
•
and on the stability of phenoxy radical. The phenoxy radical is
mainly stabilized through electron delocalization, becoming less reactive and hence
unable to react back neither with LOOH nor LH. A key point is the structural features of ArOH, more precisely the position and number of hydroxyl groups, allowing electron delocalization over the molecule along with the formation of
intramolecular hydrogen bonds (Quideau et al. 2011). This aspect can be quantified
using O-H bond dissociation enthalpy values as a parameter (de Heer et al. 1999).
The lower the bond dissociation enthalpies, the more efficient is the hydrogen atom
transfer reaction.
The second mechanism involves a single electron transfer from phenol group to
free radical, yielding a radical cation (Cren-Olivé et al. 2002). Similar to hydrogen
atom transfer process, the structure of phenols plays a key role, and the same determining factors apply. The quantitative physicochemical parameter is the redox
potentials or ionization potentials. The easier the oxidation of the polyphenols, i.e.,
lower redox potentials, the more efficient is the electronic transfer reaction.
Deciphering the nature of mechanisms is then strongly related to polyphenol
structures, and many predictive works based on structure-activity relationship have
been undertaken, mainly on plant polyphenols, while algae remain still poorly
investigated. Moreover, works for establishing a correlation between antioxidant
activity and phlorotannins have even given contradictory conclusions (Wang et al.
2014). However, many works combining density functional theory calculations
with experimental determinations of O-H bond dissociation enthalpies and redox
potentials/ionization potentials of plant polyphenols allow a fairly good rationalization of their antioxidant properties and underlying mechanism (Quideau et al. 2011).
A very interesting point with plant polyphenols is that the products formed during
ROS scavenging process are phenols themselves, which may keep a further ROS
scavenging activity. As a consequence, they are long-lasting antioxidant, then able
to potentially reduce much more ROS than the most efficient antioxidant, i.e.,
α-tocopherol (Roche et al. 2005).
In addition to direct radical scavenging, other modulation effects have been suggested to explain the antioxidant activities of polyphenols, for instance, synergistic
effect with other potent antioxidants like α-tocopherol (Zhou et al. 2005; Dai et al.
2008; Achat et al. 2016) or in cells through modulations of the protein kinase and
L. Percevault et al.