Chemistry Reference
In-Depth Information
A different approach to crosslink platinated oligonucleotides with their com-
plementary strands was developed by Lippert
et al.
97
It was based on generating a
transplatin monofunctional adduct in a single-stranded oligonucleotide, which, upon
hybridization with the complementary chain, evolved into an interstrand crosslink.
To minimize the formation of multiplatinated side products, the reaction was carried
out at low pH (3.6) and temperature (4 °C), and the oligonucleotide to be platinated
contained a single guanine at the 5
- end. The corresponding
N
7 - Pt(NH
3
)
2
Cl adduct
reacted with the opposite guanine in the complementary strand to form the fi nal
interstrand crosslink. Broad application of this strategy is severely limited by the
above-stated requirements and by the fact that monofunctional adducts may be
easily deactivated by intracellular nucleophiles before reaching the therapeutic
target. As also stated above (Sections 9.3.1 and 9.3.5), use of a platinum protecting
group might at least prevent undesired replacement of labile ligands in monofunc-
tional adducts.
46
As in the previous strategy, formation of an interstrand crosslink between a
PNA and the complementary DNA strand was achieved by introducing a transplatin
monofunctional adduct at a guanine of the PNA chain.
67
Unlike the previous
approaches, in this case there was no sequence restriction because the platinated
PNA was synthesized by standard solid-phase methodologies that used a platinated
monomer of guanine (see Figure 9.6B). However, the crosslinking reaction was too
slow, which was attributed to the low solubility of the platinated PNA in the reaction
media.
′
9.4.3 Crosslinking Experiments Using Oligonucleotides Platinated at
Base - Modifi ed Nucleosides
As oligonucleotides regioselectively platinated at thioether and imidazole groups
were made available,
82
studying whether hybridization with the complementary
chain would promote platinum migration and generate an interstrand crosslink was
the next step.
Annealing of platinated oligonucleotide
5′
d
X
I
X
S
ACGTTGAG (coordination
positions are highlighted in bold) to an equimolar amount of the complementary
chain (
5′
dCTCAACGTGTTTG) triggered a ligand-exchange substitution, affording
exclusively an interchain crosslink product, as detected by both anion exchange
HPLC and polyacrylamide gel electrophoresis (Figure 9.16).
84
The crosslinked
duplex was isolated and its structure was inferred from MALDI-TOF mass spec-
trometric analysis before and after chemical and enzymatic reactions and from
NMR. It was confi rmed that platinum remained coordinated to imidazole in the
modifi ed nucleoside X
I
, but linked to a new position, the
N
7 of the guanine opposite
X
S
. Similar results were obtained using platinated
5′
d
X
S
X
I
ACGTTGAG. Although
the crosslinking reactions were slow, under the hybridization-favouring reaction
conditions and in the presence of the complementary chain, the chelates
(
5′
d
X
I
X
S
ACGTTGAG or
5′
d
X
S
X
I
ACGTTGAG) remained stable. In other words,
under these conditions the intrachain rearrangement to a more stable chelate
(
5′
d
X
I
X
S
AC
G
TTGAG or
5′
dX
S
X
I
AC
G
TTGAG) was not observed.
