Chemistry Reference
In-Depth Information
Figure 1.6
400 MHz
1
H NMR spectra, the aromatic region, of the dodecamer
[d(GCCGATATCGGC)]
2
: (a) the spectrum of Mn
2+
-free solution, and (b) with added MnCl
2
at a Mn
2+
/duplex ratio of approximately 10
−
3
. The H8 and H6 proton resonances of purine
and pyrimidine residues are labelled according to their sequential assignment
8
distinct sequence - selective pattern: GA
GC. The adenine A - H8 resonances,
plotted as references, are not infl uenced by the paramagnetic ions.
Elmroth and coworkers have shown that metalation of single-stranded oligo-
nucleotides of the type d(T
n
GT
16−
n
) reach a maximum in the central part of the oli-
gomer indicating lack of sequence-selective infl uence on the reaction rate.
33,34
This
result is in accordance with NMR and photo-cleavage studies on metalation of
single - stranded
DNA oligomers.
18
Based on Monte Carlo descriptions of the oligo-
electrolyte properties of double-stranded DNA oligomers it has been postulated
that outer-sphere Coulombic interactions cause cations to be localized preferen-
tially in the interior rather than the terminal part of the DNA oligomers.
35
For
inner-sphere metalation other factors may dominate, as demonstrated for the pal-
indromic hexanucleotide d(G
1
G
2
C
3
G
4
C
5
C
6
) (seq. VI).
36
Addition of MnCl
2
induces
selective line-broadening on the terminal G
1
- H8 with no signifi cant effect on G
2
and
G
5
, as shown in the plot of line-broadening versus added MnCl
2
. In contrast, the
terminal G
1
-H8 in duplex II (see above) shows almost no line-broadening.
Labiuk
et al.
37
have reported X-ray determinations of Co
2+
, Ni
2+
and Zn
2+
com-
plexes of d(G
1
G
2
C
3
G
4
C
5
C
6
) (seq. VI), where the metal ions are coordinated only to
the terminal guanine G
1
-N7 position, with no metal ions binding to nonterminal
guanine positions. The authors concluded that in the regular B-DNA conformation
the internal binding sites are not accessible to Co
2+
, Ni
2+
and Zn
2+
, and that conse-
quently these metal ions bind exclusively to the terminal region of double-helical
B-DNA, irrespective of base sequence. This is in contrast to our studies, which
show a clear sequence-selective binding pattern for 3d metal ions with no special
>
GT
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