Chemistry Reference
In-Depth Information
Figure 13.5
The [Zn
2
(pyrimol)
2
]
2+
complex: the fi rst Zn-based artifi cial oxidative nuclease
reaction. However, oxidative agents that cleave DNA through nondiffusible radicals
have been known for a long time and, more recently, several complexes able to
perform oxidative cleavage in the absence of coreactants and in anaerobic condi-
tions have been reported.
8
Remarkably, in a very recent example by Reedijk and
coworkers,
9
such behaviour has been described for the fi rst time, even in the case
of Zn(II) complexes, a metal ion so far considered only capable of promoting hydro-
lytic chemistry. This peculiar reactivity is probably related to the pirymol ligand
employed (Figure 13.5), but highlights the importance of a carefully performed
mechanistic investigation and indicates that any claim of hydrolytic reactivity, par-
ticularly in the case of Cu(II) or iron complexes, not supported by clear-cut evidence,
such as enzymatic religation or fragments identifi cation, should be considered with
caution.
13.3 Free Ions and Mononuclear Complexes
At the beginning of the last decade it was demonstrated that aqueous trivalent lan-
thanide ions accelerate the hydrolysis of DNA.
10
Subsequent papers by Schneider
and coworkers reported a systematic investigation of such ions aimed at determining
the relevant kinetic parameters for the nicking of plasmid DNA.
11
The different ions
show similar maximum reactivities under saturation conditions (
k
max
0.7 - 1.0
10
− 4
s
− 1
,
37 °C, pH 7.0), but different affi nities for DNA. The latter increase along the series
from La(III) (
K
a
= 5.0
×
10
4
M
− 1
). The combination of
moderate reactivity with low affi nity for the substrate is such that substantial deg-
radation of DNA can be obtained only at high metal ion concentrations. In contrast,
late lanthanides, such as Tm(III), Yb(III) and Lu(III), display a remarkable reactiv-
ity, even at low concentrations, due to their high affi nities for DNA. However, a
decrease of the cleavage rate is observed at high metal ion concentrations, probably
as a consequence of their aggregation, with formation of less active species.
Among lanthanides, cerium is peculiar in its ability to reach a stable tetravalent
oxidation state. Ce(IV) is much more effi cient in hydrolysing DNA than the
×
10
2
M
− 1
) to Er(III) (
K
a
= 1.7
×
