Chemistry Reference
In-Depth Information
Figure 13.10
The HXTA ligand
such high reactivity is counterbalanced by a low affi nity for the substrate
(
K
a
= 1.0
10
3
M
− 1
) so that high concentrations of complex are required to obtain
high reaction rates.
A very effi cient dicerium complex of the polyaminocarboxylate ligand HXTA
(Figure 13.10) has been reported by Que and coworkers.
29
At a concentration of
10 mM, pH 8 and 37 °C, the Ce
2
-HXTA complex cleaves plasmid DNA with a rate
constant of 1.4
×
10
− 4
s
− 1
, which corresponds to a half-life of 1.4 hours for the nicking
process. Interestingly, double-strand scission is preferred to single-strand scission
and the complex also cleaves linear double-stranded DNA with high regioselectivity
(more than 90%) favouring the scission of the P-O3
×
bond.
Fe(III) is present in the active site of some phosphatases, but it has been rarely
employed in artifi cial nucleases. Interestingly, the few Fe(III) complexes reported
in the literature with hydrolytic activity in DNA cleavage are all binuclear. The fi rst
example, described by Schnaith, Que and coworkers in 1994,
30
is based on a tetra-
benzimidazolylmethyl derivative of 1,3 - diamino - 2 - hydroxypropane (Figure 13.11 ,
HPTB) and shows a surprising reactivity: the Fe(III)
2
- HPTB complex requires
' oxidative ' conditions, i.e. the presence of H
2
O
2
or O
2
and a reductant (dithiothreitol
or ascorbate), but yields DNA fragments that are consistent with hydrolytic cleav-
age. In fact, the linearized plasmid DNA can be enzymatically religated with a
quantitative conversion, indicating that only the products of P-O3
′
′
bond scissions
are formed. This high regioselectivity is also supported by 3
- end labelling
studies. The cleavage of plasmid DNA is so effi cient that, in the presence of 10 m M
complex, at 25 °C and pH 8.0, the supercoiled form is completely degraded imme-
diately after mixing. The extent of cleavage depends on the concentration of the
complex, but, surprisingly, it does not increase with extended reaction times. To
justify this unusual reactivity, the authors proposed a mechanism involving nucle-
ophilic attack of a metal-ion-bound peroxide to the phosphodiester bond.
31
Similar
behaviour has been recently reported for the dinuclear complex [CpFe(CO)
2
]
2
which hydrolytically cleaves DNA at very low concentrations in the presence of
H
2
O
2
or other peroxides.
32
Another, again very effi cient, diiron(III) complex, based on a benzimidazolyl-
methyl derivative of 1,4,7-triazaheptane (Figure 13.11, DTPB), has been investi-
gated by Liu and coworkers.
33
In the presence of 100 mM of Fe(III)
2
- DTPB complex,
at 37 °C and pH 7.0, the supercoiled DNA is degraded with a rate constant of
2.1
′
- and 5
′
10
− 3
s
− 1
, which corresponds to a half-life of 5 minutes. This is the current 'world
record' for the cleavage of plasmid DNA under 'hydrolytic conditions'. The extent
of cleavage does not depend on the presence of O
2
and the linearized DNA can be
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