Biology Reference
In-Depth Information
By using these data, it is possible to draw
important conclusions about structure-
activity relationships and the mechanisms of
1
O
2
(Mukai
et al
., 2005; Nagai
et al
., 2005;
Yamaguchi
et al
., 2005). A quick glance at
Table 6.2 shows that carotenoids are clearly
the most efficient singlet oxygen suppressors,
the efficiency of which is similar to that of
DPBF, which is an efficient singlet oxygen
probe. All other molecules have non-bonding
electron pairs like those found in the azide
ion, polyphenols, DNA bases and proteins.
Polyphenols are also good
1
O
2
suppres-
sors. By comparing the value of k
Q
of myri-
cetin (k
Q
= 5.1 × 10
8
mol.l
−1
.s
−1
) with that of
flavone (k
Q
<0.003 × 10
8
mol.l
−1
.s
−1
) it is
clear that the number of available OH
groups is an important factor, suggesting
that physical quenching is taking place
(Mukai
et al
., 2005; Nagai
et al
., 2005).
It seems to be necessary for these molecules
to have aryllic O-R groups that carry non-
bonding electron pairs that favour com-
plexation with singlet oxygen by forming a
charge-transfer complex. It therefore seems
that the suppression mechanism of flavo-
noids is basically due to the process of the
reversible electron transfer reaction; how-
ever, before reaching that conclusion, one
should analyse further the data shown in
Table 6.2. We have thus presented data as
two figures, in which k
Q
is plotted as a func-
tion of the number of O-R groups (Fig. 6.17)
and HOMO energies (Fig. 6.18).
Note that there is a clear relationship
between the number of O-R available groups
and the value of k
Q
(Fig. 6.17). However, it is
not only the number of O-R groups that mat-
ters because sugars have lots of OH groups,
but are poor singlet oxygen suppressors
(Table 6.2). In fact, one can notice that the
energy of the HOMO orbitals is also impor-
tant (Fig. 6.18), in agreement with the mech-
anism of the electron transfer reaction.
Therefore, a higher HOMO energy allows
the formation of a charge transfer complex
and reversible electron transfer reaction.
Tannins are an exception to this rule, once
they have a large number of O-R groups,
and we could expect more efficient singlet
oxygen suppression than is observed. We
suspect that this low value of k
Q
of tannins is
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
2
4
6
8
10
Number of O-R ligands
Fig. 6.17.
Number of oxygens with a non-bonding
electron pair in linear relationship with Log K
Q
.
(Adapted from Mukai
et al.
, 2005 and Nagai
et al.
,
2005.) O-R are chemical groups in which O is
bound to an H or alkyl group.
due to the formation of aggregates and low
availability of the O-H groups that could
deactivate singlet oxygen. However, it is
important to mention that, besides their low
1
O
2
suppression constant measured
in vitro
,
tannins have shown expressive protection
against singlet oxygen induced damage in
DNA, indicating that other factors besides k
Q
should be considered in understanding pro-
tection against specific oxidative damages.
Another factor that should be consid-
ered in terms of the efficiency of singlet
oxygen suppression is the partition in the
aqueous and organic phases. Rutin and
myricetin are good suppressors of singlet
oxygen, but present a logP lower than zero
(Table 6.2). It means that they should work
well in solution but in compartmentalized
systems and membranes their protection
efficiency should be small. On the other
hand, quercetin is an efficient singlet oxy-
gen suppressor and has a logP value of 2.26,
indicating that it will partition well in mem-
branes and therefore have a better potential
to protect them from oxidative damage.
Although physical quenching is clearly
the most efficient mechanism of interaction
between
1
O
2
and flavonoids, chemical prod-
ucts have also been detected indicating that
chemical quenching also takes place.
1
O
2
cannot react by Diels-Alder with benzo-
furan; however, it can attack the 2-3 double
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