Chemistry Reference
In-Depth Information
other chelates were detected again confi rmed that the high affi nity of sulfur for the
metal led to the formation of the intrachain thioether-Pt-imidazole adduct, pre-
venting other oligonucleotide sequences from being platinated, even when they
contained various guanine residues.
The ligand-exchange substitution process was highly regioselective, since, in
crosslinking experiments involving platinated
5′
d
X
I
X
S
ACGTTGAG and comple-
mentary oligonucleotides differing in the overhang sequence, migration from the
thioether to the guanine opposite X
S
was always preferred, even over migration to
the guanine opposite X
I
. Noncomplementary strands did not become crosslinked,
which confi rmed that crosslinking was the result of annealing-promoted rearrange-
ment. Hence, the crosslinking process was also sequence selective, since it only took
place if the two chains were complementary. Although reaction times were high,
particularly when the modifi ed oligonucleotide was not preplatinated, changes in
the modifi ed oligonucleotide and the complementary chain could accelerate the
crosslinking reaction, as shown by Giraud-Paris and Leng in their experiments with
oligonucleotides containing a platinated GNG triplet.
92,94
9.5 Use of Platinated Oligonucleotides for Triplex Crosslinking
As previously stated (Section 9.1), one of the problems of antigene strategy is the
low stability of triple helices in physiological conditions, as the third strand can be
displaced by the replication or the transcription machineries. Taking into account
the requirements of the parallel motif for triplex formation (polypyrimidine oligo-
nucleotides recognize polypurine-polypyrimidine tracks of DNA), in 1996 Leng and
coworkers attempted to covalently attach the third oligonucleotide strand to the
duplex by using transplatin as the crosslinking reagent.
98
Then, polypyrimidine oli-
gonucleotides containing a single guanine either in the middle or at the 5
- end of
the sequence were platinated at acidic pH, which enabled the regioselective forma-
tion of the monofunctional adduct at the
N
7 of guanine. Both platinated oligonu-
cleotides bound the 'complementary' duplex and formed interstrand crosslinks that
involved almost exclusively guanines within the purine-rich strand. The crosslinking
yield was higher when the monofunctional adduct was located at the 5
′
- end of the
third strand because the triplex was more stable than when the platinum adduct
was located in the middle of the sequence. In addition, several oligonucleotides
containing a monofunctional adduct at cytosine were evaluated in crosslinking
experiments and, although the triplex partially dissociated, the interstrand crosslinks
were also formed. The rate of the crosslinking reaction depended on the nature of
the adduct, but not strongly on its location.
99
The problem associated with this strat-
egy is, again, the low stability of monofunctional adducts, which may react with many
different compounds (glutathione, proteins) before reaching dsDNA. Moreover, as
reported by Leng
et al.
,
98
platinated oligonucleotides with one labile bond can be
deactivated by suicide reactions to form intrastrand crosslinks.
More recently, Miller
et al.
examined the effect of oligonucleotides platinated
at
N
4
- (2 - aminoethyl)cytosines
85
(with Pt(dien) or
cis
- Pt(NH
3
)
2
Cl moieties) in
′
