Chemistry Reference
In-Depth Information
5'
dTCCGCCTTT
5'
dTCCGCCTTT
+
3'
dAGGCGG
5'
dTCCGCCTTT
Pt
Pt
Pt
Pt
pH 7.1
pH 5.0
Pt
Pt
3'
dAGGCGG
= binuclear platinum(II) complex
Pt
Pt
Figure 9.13
Regioselective reaction with a binuclear complex is followed by hybridization
to the complementary chain, which triggers formation of the crosslink
3'
3'
5'
S
Pt
5'
Figure 9.14
Oligonucleotides with one phosphorothioate group are regioselectively plati-
nated at sulfur. Then duplex formation induces formation of an interchain crosslink. (Adapted
from
Nucleic Acids Res.
, 1990,
18
, 5163-5171.)
experiments, the platinum(II) compound (cisplatin, K
2
PtCl
4
, transplatin or a binu-
clear platinum complex) was added once hybridization of both strands was achieved,
as demonstrated by nondenaturing gel electrophoresis. Then, crosslinking occurred
when the single-stranded tail of the template bent around the 5
- terminal phospho-
rothioate of the antisense oligonucleotide, which placed sulfur and residues 2-5 in
the target strand suffi ciently close to be crosslinked by the platinum(II) compound
(Figure 9.14). The different assays showed that guanine and adenine and, to a lesser
extent, cytosine were the preferred sites for crosslinking, but, in the absence of these
residues in positions 2-5, crosslinking occurred at thymine. The antisense oligonu-
cleotides contained few guanines and when a guanine was placed close to the
phosphorothioate group, an intramolecular 'suicide' reaction occurred. Among the
platinum(II) compounds, the crosslinking effi ciency of transplatin was comparable
to that of K
2
PtCl
4
, but cisplatin was less effi cient.
′
9.4.2 Crosslinking Experiments Using Platinated Natural Oligonucleotides and
PNA Analogues
While conducting research into the interactions between cis- and transplatin and
DNA, Leng
et al.
discovered a linkage isomerization reaction between transplatin-
modifi ed oligonucleotides and their complementary strands, promoted by the for-
mation of a DNA double helix.
52
Although oligonucleotides with
trans
- Pt(NH
3
)
2
1,3-intrastrand crosslinks at GNG triplets (where N is any nucleoside but G) were
kinetically inert as long as they remained single-stranded (except those containing
