The results of these experiments demonstrate that photo-
cleavage induced by the oligonucleotide
- fullerene conjugate
occurred at well-defined sites in the duplex, triple helix, and
triple helix with hairpin. This is in sharp contrast with the lack
of site selectivity in the cleavage by the free C,, carboxylic
acid.[61 Figure
2 shows the results obtained with the hairpin
target. Cleavage was observed at guanines but also at thymine
residues (with a lower efficiency). Treatment with piperidine
slighthly increased the intensity of the cleavage bands. Compari-
son with bands generated by piperidine treatment after reaction
with dimethyl sulfate (DMS) suggested that fragments obtained
by photoinduced cleavage were terminated by a 3'-phosphate.
They had the same mobility in the gel as 3'-phosphorylated
products of DMS-induced piperidine cleavage. The cleavage
sites on the three target sequences are shown in Figure
1. The
most reactive guanines were those located in the loop region.
With the
26 base pair duplex (devoid of hairpin structure),
cleavage occurred at guanines of the purine-containing strand
positioned near the 3'-end of the third oligonucleotide strand
(Fig.
1). No cleavage was observed on the other strand of the
duplex. which did not contain any guanines
in the vicinity of the
3'-end
of the third strand (the two terminal Gs could not be
analyzed owing to their too close proximity to the
5' label). On
the single-stranded target photoinduced cleavage occurred at
guanines near the 3'-end of the complementary oligonucleo-
tide-fullerene conjugate (Fig.
1). In all three cases the sites of
cleavage were located in the region of the target expected from
the known orientation of the oligonucleotide carrying the
fullerene derivative
: antiparallel orientation with respect to the
complementary strand in the duplex and parallel to the oligo-
purine target sequence in the triplex structures. The enhanced
reactivity of guanines
in the loop as compared to the stem of the
hairpin duplex might be due to a higher accessibility to singlet
oxygen in the single-stranded loop, even though the reaction
might involve other mechanisms (e.g. electron transfer) result-
ing from folded structures.
These results indicate that the spherical fullerene terminus on
the oligonucleotide chain did not hamper duplex and triplex
formation. The cleavage nearly exclusively took place at gua-
nine bases; this is consistent with singlet oxygen mediation. The
same profile of site and base selectivity was observed with an
oligonucleotide-chlorin conjugate (with the same oligonucleo-
tide sequence). which was shown to generate singlet oxygen
upon irradiation and to cleave nucleic acids (results not shown).
In summary, we have shown that fullerene -oligodeoxynucle-
otide conjugates effectively modify
DNA with high site specifici-
ty by forming double and triple helices. The present results
clearly indicate that
fullerene-oligonucleotide conjugates are
promising photoprobes for the investigation of gene transcrip-
tion and mRNA translation. Design of the optimal linker moi-
ety and conditions needed to achieve high efficiency are now
under investigation. In addition, the hydrophobicity
of the
fullerene moiety can be exploited to enhance cell uptake for
biological applications.