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OH
h
v
,
1
O
2
(a)
MeOH
Ph
Ph
Ph
h
v
,
1
O
2
H
2
O
-H
2
O
2
(b)
Ph
Ph
Ph
Fig. 6.10.
Mechanism of reaction of 2,5-dimethylfuran (a) and DPBF (b) with singlet oxygen.
1
O
2
LOO
.
metals
Peroxyl
Alkoxyl
LO
.
LOOH
PS + hydroperoxides
Fig. 6.11.
Main routes of membrane damage through lipid peroxidation.
Fatty acids quench
1
O
2
with rate con-
stants from 10
4
to 10
5
M
−1
s
−1
(Krasnovsky
et al
., 1983). The values of the rate constants
depend on how electron-rich the double
bound is and the solvent capacity to stabi-
lize the reaction intermediaries that control
the reaction velocity (Machado
et al
., 1995;
Girotti, 2001). Membrane destruction can
be microscopically observed by experi-
ments with giant vesicles (GUVs) (Riske
et al
., 2009). In the manuscript by Caetano
and coworkers, GUVs were destroyed after
a few minutes of exposure to
1
O
2
(Caetano
et al
., 2007). The mechanism of membrane
damage was attributed to lipid chain break
with formation of short-chain amphiphiles.
Although this general picture is well
accepted, it is important to understand the
effect that the progressive increase in lipid
hydroperoxide concentration has on the
membrane properties. Lipid hydroperoxides
have a more hydrophilic character than the
lipid itself because of the hydroperoxide
group incorporated into the acyl chain. The
peroxidized chain tends to migrate to the
bilayer surface (Riske
et al
., 2009). This change
causes an increase in area per lipid, disturb-
ing chain packing order and increasing mem-
brane fluctuations. Riske
et al.
(2009) observed
that peroxidation of as much as 60% of the
lipids was still compatible with intact mem-
branes. Using the rate of singlet oxygen pro-
duction of the photosensitive molecule, it was
estimated that the efficiency of the oxidative
process is 0.0037. This work suggested a pos-
sible protection role of the lipid structure in
keeping the membrane integrity even at high
levels of molecular damage.
6.5
Deactivation of Singlet Oxygen:
Kinetics and Mechanisms
Singlet oxygen can be suppressed (quenched)
by two main mechanisms: physical (there is
no formed product) and chemical (there is
an oxidized product), whose constants are
represented as k
q
and k
r
, respectively
(Wilkinson
et al
., 1995). The total or observ-
able quenching constant (k
Q
) is the sum of
three terms: k
d
, which is the pseudo-first-
order rate constant for solvent deactivation
of
1
O
2
and k
q
[S] and k
r
[S] that account for the
physical and chemical quenching, respec-
tively, of substrate S over
1
O
2
.
Several molecular interactions can lead
to the physical deactivation of
1
O
2
: energy
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