Chemistry Reference
In-Depth Information
between difference spectra of G - C + H
+
and DNA + M
2+
(M
2+
= Cu
2+
, Ni
2+
and Fe
2+
).
Our investigations, carried out on metal ion-DNA complexes by thermodynamic
and spectroscopic methods, in particular spectrophotometry in the UV, visible and
near IR ranges, allowed us to propose a scheme for the structure and energetics of
DNA interactions with metal ions.
31,75
2.3.2 Depurination
Interaction of H
+
and M
n
+
with N3 and N7 of guanine in DNA may induce DPT (1)
and/or hydrolysis of the glycosidic linkage C1- N9 (2) :
−
+
and
N
9
(1)
C
1
+
OH
−
−
H
+
+
C
1
OH
and
+
C
1
N
9
N
9
H
(2)
The process of depurination may take place only in the unwound state of
double-helical DNA. Crothers and collaborators
76
studied the energetics of the
opening of the central base pairs in different triplets of double-stranded RNA,
which was often used as a model for the DNA double helix since crystallographic
data for DNA was not known at the time. According to this study the energy of
opening of a G-C pair at 25 °C is at a maximum and equals 7.5 kcal mol
− 1
in the
sequence:
GGG
C
C
C
10
− 5
s, while the energy of opening
an A-U pair is at a minimum (4 kcal mol
− 1
) for the sequence:
corresponding to a minimum probability of 0.3
×
AAA
U
U
U
10
− 5
s. By inserting these data in
corresponding to a maximum probability of 120
×
10
− 6
s for G - C and
10
− 8
s for
Equations (2.1) and (2.2), we obtain the times
∼
3
×
∼
A - U.
The probability of metal-induced DPT in DNA double helix is equal to the
ratio of the wave shift caused by metal ion interactions with DNA G-C pairs to the
wave shift after total protonation of guanine (pH 1) and cytosine (pH 2), which
amounts to
700 cm
− 1
:
15
∼
−
1
P
=
∆∆ ∆
v
v
=
v
n
700
cm
DPT
n+
+
+
M
G+C+H
M
Considering the values of D
n from Table 2.4, we can assume that for the
metal-DNA complexes studied by us these probabilities are in the range from 10
− 1
