Chemistry Reference
In-Depth Information
(
(
N
O
O
H
O
O
N
N
Hg
II
NHg
II
N
N
O
H
O
H
N
O
O
Figure 16.1
Experimentally observed U-Hg
II
-U
13
and T-Hg
II
-T
14
base pairs in crystals. Solution
conditions for crystallization: (a) 9.72 mM uracil and 4.86 mM HgCl
2
at pH 4.2; (b) 20 mM
1-methylthymine and 10 mM HgO
1 - methylthymine - Hg
II
(2 : 1) complex
14
(Figure 16.1b). Interestingly, this structure
essentially refl ects Katz's model. This crystal structure demonstrated for the fi rst
time that N3 of T can also be an Hg
II
binding site. Although these two crystal struc-
tures do not directly indicate that the binding modes observed in the crystals cor-
respond to those observed in natural DNA, the determined structures were important
experimental data for considering an Hg
II
binding mode.
On the other hand, there are studies exploring other types of Hg
II
- U complexes
and Hg
II
complexes with other nucleobases
15 - 28
(Figure 16.2). Beauchamp and cow-
orkers reported crystal structures of methylmercury complexes with adenine and
cytidine (Figures 16.2 a - g).
15 - 19
In 1988, Sheldrick and Gross also reported methyl-
mercury complexes with 7-methylguanine (Figures 16.2h,i).
20
Lippert and coworkers
reported Hg
II
complexes with 1,3-dimethyluracil at the C5-position (Figures 16.2j-
l),
23 - 25
an Hg
II
complex with 9 - methyladenine at the N7 - position (Figure 16.2 m)
26
and bimetallic Hg
II
,Pt
II
complexes with 1-methylcytosine (Figures 16.2n,o).
21,27
These
data indicated the intrinsic potential of nucleobases to form various types of metal
complexes.
16.3 UV , UVCD and Vibrational ( IR/Raman) Spectral Studies
As mentioned above, UV absorption spectra were employed from the early stages
of studies on the Hg
II
- DNA interaction. UV spectral studies
12,29
were continued by
Gruenwedel's group, which developed their study into UVCD spectra.
30 - 32
They
presented comprehensive UVCD spectra of Hg
II
- DNA complexes under various
conditions (see references for details).
In the case of vibrational (IR/Raman) spectra, complexations between meth-
ylmercuric ions and nucleosides were studied (Figure 16.3).
33 - 44
In 1974, Mansy,
Tobias and coworkers reported Raman spectra of a CH
3
Hg
II
- uracil complex
with an N3-Hg
II
bond (Figure 16.3a),
33
a CH
3
Hg
II
- 5
-
monophosphate) complex with an N1-Hg
II
bond at neutral pH in H
2
O solution
(Figure 16.3 b)
34
and a CH
3
Hg
II
- 5
′
- GMP
(guanosine - 5
′
-GMP complex with an N7-Hg
II
bond at acidic pH
in H
2
O solution (Figure 16.3c).
34
Proton - Hg
II
exchange processes are required to
form the complexes (Figures 16.3a,b), and the derived Raman spectra provide refer-
ence data for the T-Hg
II
-T base pair. There are also Raman spectra of CH
3
Hg
II
- cyti-
dine and CH
3
Hg
II
- 5
′
- monophosphate) complexes in H
2
O
solution (pH 3, 5 and 7) (Figures 16.3d,e),
36
the Hg
II
- uracil complex with O4 - Hg
II
′
- AMP
(adenosine - 5
′
