Chemistry Reference
In-Depth Information
1.2
1.2
Pb(II)
1.0
1.0
a
1
A
0.8
B
0.8
0.6
0.6
0.4
0.4
0.2
a
4
0.2
a
3
a
2
0.0
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.0
0.1
0.2
0.3
0.4
0.5
r
r
Figure 2.1A:
The Scatchard plot for binding of Pb(II) with DNA (2 mM NaNO
3
, 20 °C) as
non-linear least square fi t
rm
2
=
05
.
Br
()
+
B r
()
+
4
C r
()
, where
B
(
r
) =
k
1
n
1
+
k
2
n
2
−
(
k
1
+
k
2
)
r
and
C
(
r
) =
k
1
k
2
r
(
n
1
+
n
2
r
).
Figure 2.1B:
The Klotz plot for binding of Pb(II) with DNA (2 mM NaNO
3
, 20 °C). Calcu-
lated values (
k
1
,
n
1
,
k
2
,
n
2
) were obtained from the Klotz equation system.
−
Table 2.2
Parameters of Pb
2+
binding with DNA
in 2 mM NaNO
3
, 20 °C
Pb
Analysis by the
Scatchard method
Analysis by the
Klotz method
Stability constant
K
×
10
4
, M
−
1
2.4
±
0.5
2.3
±
0.2
Free Gibbs energy
−
∆
G
0
, kcal mol
−
1
5.96
5.94
Number of binding sites at saturation
n
0.48
±
0.05
0.50
±
0.06
Microconstant
k
1
×
10
4
, M
−
1
21.1
±
0.7
24.7
±
1.2
Number of binding sites
n
1
0.043
±
0.002
0.034
±
0.001
Microconstant
k
2
×
10
4
, M
−
1
0.56
±
0.01
0.68
±
0.01
Number of binding sites
n
2
0.44
±
0.03
0.47
±
0.01
c
2
distribution
0.81
3.08
The stability constants (log
K
) for Mg
2+
, Cr
3+
, Mn
2+
, Co
2+
, Ni
2+
, Cu
2+
, Zn
2+
, Ag
+
,
Cd
2+
and Pb
2+
binding to DNA are in the range 4-6 (Table 2.3), and assuming a t
0
value of 10
− 11
s, the lifetimes
t fall in the range 10
− 7
- 10
− 5
s. For DNA this time corre-
sponds to the movements of phosphates, sugars and bases with large amplitudes
(see introduction). These movements are related to conformational changes and
untwisting of the double helix. As metal ions of the fi rst transition series form
chelates with DNA, the stabilities of the complexes may be estimated from the DNA
