Chemistry Reference
In-Depth Information
500
Ag
+
400
*Cu
+
H
+
300
Cu
2+
200
Ni
2+
+ ethanol
Ni
2+
Zn
2+
100
Mn
2+
Co
2+
Ni(en)
2+
Mg
2+
Ni[(en)(NO
2
)(NCS
−
)]
0
0
0.0
0.4
0.8
1.2
1.6
2.0
1/
∆
G
a
Figure 2.5
Correlation between
G
a
refers
to the reaction of the hydrated electron with the ion under study and is related to the rate
constant by the expression
K
∆
ν
of ctDNA with different metal ions and 1/
∆
G
a
.
∆
(
)
∆
G
RT
10
11
exp
a
. The vertical factor for Cu
+
is
=
−
×
2
eaq
More details follow from Figure 2.5, which exhibits the correlation between the
shift of the absorption band (D
n) of DNA bound to the investigated ions and the
reaction rate constants (
K
e/aq
) of hydrated electrons with the same ions, particularly
with the 1/D
G
a
values (Table 2.1 ). D
G
a
is the hydrated electron activation energy in
the expression:
K
e/aq
= 10
11
exp(
D
G
a
/
RT
).
29,30
At
I
= 0.01 M, a linear relationship
between these values seems to exist only for Mn
2+
, Co
2+
, Ni
2+
and Zn
2+
while Mg
2+
,
Cu
2+
, H
+
, Ag
+
and Cu
+
deviate from the line. So do Ni(en)
2+
,
Ni
−
[
)
(
)(
)
]
0
(
en
NO
−
NCS
−
2
2
and Ni
2+
(40% ethanol). From the UV, visible and near IR spectroscopic studies it
may be concluded that Mn
2+
, Co
2+
, Ni
+2
and Zn
+2
form outer-sphere complexes with
DNA. The small infl uence of these divalent ions on the DNA absorption spectrum
and the correlation of D
n with 1/D
G
suggest that their interactions with the DNA
nucleobases in solution take place through hydrogen bonding involving their
hydrated coating. A
31
P NMR spectroscopy study of the interaction between DNA
and the paramagnetic ions Mn
2+
and Co
2+
showed that only 15
5% of the total
bound ions was directly coordinated to the phosphate groups (inner-sphere com-
plexes). The remaining Mn
2+
and Co
2+
ions were bound either as outer-sphere com-
plexes relative to the phosphate groups or elsewhere to the DNA, possibly to the
bases.
58
These results are in agreement with X-ray crystallographic data, which show
that transition metals may coordinate to mononucleotides by direct nucleobase N-
binding and/or phosphate binding.
59,60
The UV data indicate that Ag
+
, H
+
, Cu
+
and,
partially, Cu
2+
, interact directly with DNA without participation of water molecules.
Ions of Mg
2+
, Ni(en)
2+
and
Ni
±
[
)
(
)(
)
]
0
(
, which stay below the line
(Figure 2.5), have the lowest affi nity to DNA nucleobases.
In the case of direct binding of metal to nucleobases in DNA there is a certain
probability of electron tunnelling transitions from the nitrogen bases to the metal
en
NO
−
NCS
−
2
2
