Chemistry Reference
In-Depth Information
N
NH
3
S
S
NH
Pt
N
H
3
N
H
HN
HN
HN
NH
N
N
N
N
Cl
NH
3
Pt
N
O
N
O
N
O
N
O
H
3
N
Cl
oligonucleotide
(A, C, G, T)
oligonucleotide
(A, C, G, T)
X
S
X
I
Figure 9.10
Modifi ed oligonucleotides containing the four nucleobases and two cytosine
analogues can be selectively platinated at the appending thioether and imidazole tethers.
(Adapted from
J. Biol. Inorg. Chem.
, 2007,
12
, 901-911.)
addition, platinum preferred the
N
7 of guanine rather than nitrogens from the imi-
dazole ring. However, thioether-platinum-imidazole adducts could form in the
presence of guanines, provided sulfur promotes binding to the imidazole ring rather
than the
N
7 of guanines.
78,80
On the basis of these studies, Marchán and Grandas
82
envisaged an alternative
method for preparing modifi ed oligonucleotides that included, in addition to all the
natural nucleobases, the thioether and imidazole functional groups present in the
methionine and histidine side chains. Sulfur was expected to direct the platination
process and prevent uncontrolled reaction with the nucleobases (Figure 9.10).
For this purpose, two 5-methylcytidine analogues with thioether or imidazole
groups attached to the 4-position (X
S
and X
I
, respectively; see Figure 9.10) were
synthesized by reacting protected 4-(1,2,4-triazole)cytidine with the corresponding
amines.
82
After phosphitylation, the phosphoramidite derivatives were successfully
introduced in oligonucleotide chains using the standard phosphite triester chemistry
with no protection on either the thioether or the imidazole ring. Several oligonu-
cleotide sequences containing either one or two analogues appended to the 5
- end
were synthesized and used in preliminary platination studies. The modifi ed nucleo-
sides were placed at the 5
′
-end to minimize the destabilization that modifi ed cytosines
cause in a DNA duplex, especially in the middle of the chain.
82,83
The platination studies were carried out using transplatin, because it is a simple,
mononuclear complex whose reactions with single-stranded oligonucleotides have
been thoroughly studied, and whose capacity to form crosslinked duplexes is clearly
established.
49 - 51
Unlike cisplatin, transplatin offers the advantage that the
trans
geometry prevents the sulfur ligand from labilizing the
trans
ammine group. Hence,
using transplatin, mixtures of di- and tricoordinate adducts, which are more diffi cult
to handle, are not formed.
80
The reaction between transplatin and oligonucleotides containing the thioether-
modifi ed nucleobase was fast, but generally afforded unstable adducts and complex
reaction mixtures. However, the imidazole-containing oligonucleotides reacted
with transplatin much more slowly, in particular in slightly basic solutions, and it
was found that imidazole-modifi ed cytosine was less reactive than the natural
nucleobases.
′
