Biology Reference
In-Depth Information
to obtain singlet oxygen by direct excita-
tion of molecular oxygen using irradiation
with an intense light source in the 0-1 tran-
sition (1070 nm), but this is a spin-forbidden
process and is therefore inefficient. It
requires a pressure cell in which oxygen is
dissolved in a good solvent (such as hex-
afluorobenzene) under high pressure (140
atmospheres). It is also possible by micro-
wave discharge in a steam of oxygen at
1-10 nm, which generates a mixture of sin-
glet oxygen and atomic oxygen, the latter
being scrubbed out by passing the gas
stream over mercuric oxide (Baptista,
1998). Finally, it is possible to generate it
chemically by thermal decomposition
(Foote, 1968); however, the most common
method for producing singlet oxygen in the
laboratory is by photosensitization with a
strongly absorbing dye such as methylene
blue (Severino
et al
., 2003) or chlorophyll
(Krasnovskii, 1976).
Photosensitization is a process in which
a molecule absorbs light and gets excited
from the ground-state (PS) into a singlet, a
short-lived (
~
10
−9
s) excited state (
1
PS
*
) that
can be deactivated by chemical reactions, or
by radiative and non-radiative processes.
A good photosensitizer (PS) will undergo a
spin-forbidden intersystem crossing that
requires a spin inversion, converting the PS
to a triplet state (
3
PS
*
). The triplet states relax
back to ground states via a spin-forbidden
radiative pathway (phosphorescence), which
imposes relatively long lifetimes. The triplet
state can also be disabled by electron or
proton transfer, originating radicals, as in
mechanism type I (Fig. 6.4). In oxygenated
environments, PS can undergo a type II
photochemical process that involves energy
transfer between the excited triplet state of
photosensitizer (
3
PS*) and the triplet state
of molecular oxygen (
3
O
2
), producing short-
lived and highly reactive excited singlet
oxygen (
1
O
2
) (Wilkinson
et al
., 1993; Abdel-
Shafi and Wilkinson, 2002; Junqueira
et al
.,
2002; Schmidt, 2006). The competition
between type I and type II reactions is diffi-
cult to predict in the biological environment
because the presence of biomolecules or
interfaces can shift the relative rates of these
processes that are observed in anisotropic
solutions (Macpherson
et al
., 1993; Baptista
and Indig, 1998).
MECHANISMS OF PHOTOSENSITIZED
OXIDATIONS
SUBSTRATE
Reactions Type II
∗
1
O
2
PRODUCT
O
2
1
PS
∗
3
PS
∗
PS
BM
e
-
+2H
+
RADICALS
O
2
.
-
BM
H
2
O
2
O
2
O
2
Fe
2+
Fe
3+
OH
-
PRODUCT
PRODUCT
OH
Reactions Type I
Fig. 6.4.
Photosensitization mechanisms, where PS is a photosensitizer that absorbs light going to the
first singlet state (
1
PS*), converting into a triplet state (
3
PS*) by intersystem crossing. The excited species,
especially
3
PS*, can react by electron transfer forming radical species (Type I mechanism) and start
radical chain reactions or react with molecular oxygen by energy transfer forming singlet oxygen
(Type II mechanism). BM, biomolecules.
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