Chemistry Reference
In-Depth Information
H
O
N
N
N
H
N
N
N
N
R
O
R
Scheme 16.3
coordination of N1 of A [N1(A)] to Hg
II
, a higher - fi eld shift is plausible for the
15
N
resonance of imine nitrogen (Table 16.2).
62 - 64,75 - 79
It was also proposed that the
bound Hg
II
would be ligated with carbonyl oxygen (O4) of T [O4(T)] in the opposite
strand.
69 - 72
As a result, A - Hg
II
-T base pairs were thought to be formed in the DNA
duplex (Scheme 16.3). This structure was supported by Hg
II
titration experiments
with
1
H NMR spectra of d(GCCGATATCGGC)
2
.
70,72
From the
1
H NMR data, imino
proton resonances of thymine residues in A-Hg
II
-T base pairs were observed after
Hg
II
binding was saturated.
70,72
This means that imino protons of thymine residues
were preserved after A-Hg
II
- T base pair formation (Scheme 16.3 ).
Thus, all NMR data are consistent with the proposed structure of the A-Hg
II
- T
base pair. Interestingly, this pairing arrangement is similar to the one proposed by
Davidson ' s group (Scheme 16.2 ),
11
but the correct structure of the A-Hg
II
- T base
pair might be the one by Sletten's group (Scheme 16.3). From these data, it was
found that in the A,G,C,T mixed sequence, Hg
II
can penetrate between a Watson-
Crick A-T base pair without strand slippage or imino proton-Hg
II
exchange
processes.
16.4.3 T - H g
II
- T Base Pair
In 1996, Kuklenyik and Marzilli reported a possible formation of T-Hg
II
- T base pairs
in DNA oligomers using
1
H NMR spectroscopy.
80
They performed Hg
II
titration
experiments with several DNA oligomers, namely, d(ATGGG
TT
CCCAT),
d(GCGC
TTT
GCGC) and d(GCGC
T
TT
T
GCGC). As the concentration of Hg
II
was increased, the imino proton resonances of underlined thymines decreased, and
then disappeared around 1 molar equivalent to the T-T mismatches in the following
context.
80
For d(ATGGG
TT
CCCAT) and d(GCGC
TTT
GCGC), they seemed to
form self-associated duplexes in the presence of Hg
II
.
80
In the case of
d(GCGC
T
TT
T
GCGC) a hairpin structure was formed in the presence of Hg
II
.
80
However, on the addition of excess Hg
II
, this hairpin molecule seemed to be trans-
formed into a self-associated duplex gradually.
80
However, the disappearance of the
imino proton resonances suggested T-Hg
II
-T base pair formation in the DNA
duplexes. Thus, this NMR study provides the fi rst NMR data for a possible forma-
tion of T - Hg
II
-T base pairs in DNA oligomers with defi ned chain lengths and
sequences.
Similar Hg
II
titration experiments were presented in 2006, and imino proton
resonances of thymidine residues were seen to disappear upon the addition of
approximately 1 molar equivalent of Hg
II
to a T-T mismatch in a DNA duplex:
