Chemistry Reference
In-Depth Information
In addition, the dependence of the surface electrostatic potential (
) of a cylindrical
polyion (
r
= 1 nm) is plotted as a function of the ionic strength of the solution as
obtained from the Poisson- Boltzmann equation.
48
For Mg
2+
the p
K
dependence and
the surface potential are closely related, indicating a nearly exclusive binding of
Mg
2+
to the negatively charged phosphate groups of DNA.
One may notice that the binding constants of Ag
+
and Pb
2+
are only weakly
dependent on ionic strength. This is in agreement with the expected inner-sphere
complexation of Ag
+
ions involving nitrogen atoms of the nucleobases (soft binding).
Divalent lead is one of the post-transition metal elements that exhibits the so-called
' inert - pair effect '. This lone pair can cause a non-spherical charge distribution
around the Pb
2+
cation. 'Stereochemically active lone pairs' can take various forms
in Pb
2+
compounds.
49
Recent data show that Pb
2+
forms hemidirect coordination
compounds both in complexes with water and in complexes with O and N atoms of
ligands,
50
which, in turn, are reactive centres in DNA for other metal ions. The stere-
ochemistry of Pb
2+
may be the reason of such a low-effect action of ionic strength.
The log
K
dependencies for the transition metal ions Cr
2+
, Co
2+
, Ni
2+
, Cu
2+
and
Zn
2+
are in between the two extremes mentioned above. The slopes of their lines
allow determination of the ' softness ' , or ' transition degree ', of these ions assuming
that the slope angle of the lines is proportional to this property.
ϕ
Effect of DNA G - C Content on Metal Complexation
To quantify the effect of DNA heterogeneity upon metal ion binding, the values of p
K
for Ni
2+
complexes with DNA of various origins (T7 phage, calf thymus, bovine spleen,
salmon sperm,
Spirulina platensis, E.coli
) have been determined.
51,52
In Figure 2.3
5.4
Ni(II)-DNA
E.coli
T7
5.2
Spirulina
platensis
5.0
4.8
Bovine
spleen
Calf thymus
Salmon sperm
Mice liver
4.6
40
42
44
46
(GC) %
48
50
52
Figure 2.3
Dependence of Log
K
versus G-C content
