Chemistry Reference
In-Depth Information
Figure 13.15
The [Zn(phen)(dione)]
2+
complex
+
N
N
+
N
N
N
2+
OH
Zn
N
N
+
+
N
Figure 13.16
Proposed structure of the active form of the Zn(II) complex studied by Mao
DNA affi nity elements other than intercalators have also been used. Positively
charged ammonium or peralkylammonium groups have a good affi nity toward the
polyanionic DNA. Schneider and coworkers
43
studied the reactivity of the Co(III)
complexes of cyclen derivatives bearing side chains of different lengths and termi-
nating with a trimethyl ammonium group. At 37 °C and pH 7.0, they observed satu-
ration kinetics, both with the Co(III)-cyclen complex and with the complexes of the
alkylammonium conjugates. Interestingly, while the maximum reactivity remains
constant within the series (
k
cat
ca. 2
10
− 4
s
− 1
), the binding constant increases with
the length of the spacer (from 1.0
×
10
3
M
− 1
in the case of Co(III)-cyclen to
×
10
3
M
− 1
for the complex with a C
6
spacer). Further increments of activity could
be obtained by adding a second alkylammonium or Co(III)-cyclen groups.
Again, peralkylammium groups were employed as affi nity elements by Mao
and coworkers.
44
They synthesized two Zn(II) complexes of bipyridyl ligands bearing
ethylammonium or butylammonium groups in the positions 5 and 5
5.5
×
. The former
Zn(II) complex cleaves DNA at pH 7.2, 37 °C and 50 mM complex concentration
with a rate constant of 3.8
′
10
− 5
sec
− 1
. The pH reactivity profi le shows a maximum
at pH 7.5. On the basis of the crystal structure of the complex, the authors suggest
that the quaternary ammonium groups (Figure 13.16) are placed in the right posi-
tion to interact with the two negatively charged phosphate groups neighbouring the
one that is activated by the metal ion.
More sophisticated and biologically minded systems have been reported by the
groups of Franklin and Sugiura. The fi rst designed a 33-mer peptide incorporating
a DNA recognition sequence and a metal-ion binding site.
45
The peptide folds in a
×
