Chemistry Reference
In-Depth Information
unwanted by-products. To investigate O
2
(
1
∆
g
)-reactions separately, one may pro-
duce O
2
(
1
∆
g
) separately by, e.g., using Rose Bengale and visible light (or H
2
O
2
plus hypochloride; Muñoz et al. 2001). The triplet state of Rose Bengal lies too
low to undergo ET or H-abstraction reactions from the kind of substrates that we
are concerned with here.
The benzophenone ketyl radical reacts rapidly with O
2
yielding O
2
•
−
[reac-
tion (74)].
•
C(Ph)
2
−O
−
(
•
C(Ph)
2
−OH) + O
2
Ph
2
C=O + O
2
•
−
(HO
2
•
)
→
(74)
This type of reaction is also given by the menadione semiquinone radical (one-
electron reduced menadione). However, the reduction potential of the ribof lavin
semiquinone [p
K
a
= 8.3 (Land and Swallow 1969); E
7
=
0.31 V (Anderson 1983a)]
is so close to that of O
2
•
−
that ET to O
2
becomes very slow [
k
<3.8
−
10
4
dm
3
mol
−1
s
−1
for lumif lavin (Vaish and Tollin 1971), see also Faraggi et al. (1975)]. The
f flavin semiquinone radicals thus rather decay bimolecularly [their rate of decay
depends on the charge, notable at the reaction site (Anderson 1983b), e.g. 2
k
(FH
•
)
= 1.2
×
10
9
dm
3
mol
−1
s
−1
, FAD = f flavin-ad-
enine dinucleotide]. They also readily react with O
2
•
−
[
k
= (2-7)
10
9
dm
3
mol
−1
s
−1
, 2
k
(FAD
•
−
) = 1.2
×
×
10
8
dm
3
mol
−1
s
−1
(Anderson 1981, 1982)]. Thus, although the primary reaction may be the
same for two photosensitizers, e.g., benzophenone and ribof lavin, the products
or their yields may be noticeably different whenever O
2
•
−
plays a major role in
their formation. Whether this is one of the reasons for the differences observed
with these photosensitizers in their reactions with some DNA model compounds
(Delatour et al. 1999) has to await further investigation.
In contrast to the foregoing, porphyrins attached to DNA by, e.g., a positively
charged polyethylenimine linker are thought to cause DNA strand breakage only
by singlet O
2
reactions (Suenaga et al. 2000 and references cited therein).
Another widely investigated group of photosensitizers are ruthenium(II) and
osmium(II) complexes which become strongly oxidizing in their excited state
(for reviews see Kirsch-De Mesmaeker et al. 1996, 1998; Moucheron et al. 1997;
Ortmans et al. 1998). As with other good one-electron oxidants, their primary
site of attack is at G. Yet in contrast to the other systems discussed above, DNA-
sensitizer cross-linking via G moiety and the ligands of the transition-metal ions
(Jaquet et al. 1995) is often a major process (Kirsch-De Mesmaeker et al. 1998).
Photoexcitation of quinoxaline-carbohydrate hybrids cleave DNA selectively
at GG sites (Toshima et al. 2002). Mechanistic details have not been explored, but
the high GG specificity points to an ET in the excited state.
×
2.5
Transition-Metal Ions and Hydroperoxides
Cells contain low concentrations of transition-metal ions, notably iron and cop-
per. For example, the DNA scaffolding protein is reported to contain copper
(Lewis and Laemmli 1982). However, no intracellular free copper is detectable
(Rae et al. 1999). The intracellular labile iron pool is reported to be around mi-
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