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into a solution of dT20 (Fig. 3b and Fig. S18, ESI†). The shape of the
CD spectra is similar at a N/P ratio B0.5 for both dT40:GuaBiPy and
dT20:GuaBiPy systems, while differences are observed at N/P 4 2,
suggesting the existence of other self-assembly mechanisms at high
content of GuaBiPy (Fig. 3a and b and Fig. S21, ESI†). The plot of
E/M ratios indicates that dT20 also promotes excimer formation,
with a maximum E/M ratio at N/P B 1.25 (Fig. 3d). However, when
using a smaller template dT10, no ICD band was observed in the
320–400 nm region, and no shifts of absorption bands were seen in
UV-Vis spectroscopy (Fig. S18 and S22, ESI†). Though, changes in
fluorescence were observed, the maximum E/M ratio being reached
at N/P B0.3–0.5 (i.e. 3 to 5 GuaBiPy per dT10) (Fig. 3d). Overall,
UV-Vis titration at 349 nm shows that the template length affects
the self-assembly process showing a slower absorbance increase
with dT20 compared to dT40, suggesting a weaker binding (Fig. 3c).
The fluorescence titration also shows weaker changes with shorter
oligonucleotides compared to dT40 (Fig. S23, ESI†). Heating/
cooling cycles were carried out on dT20:GuaBiPy (N/P = 1.25) and
dT10:GuaBiPy (N/P = 0.5), showing minor changes (Fig. S24, ESI†).
These results indicate that shorter ssDNAs also act as a template and
generate a chiral arrangement of GuaBiPy onto the ssDNA, but that
the resulting complexes are less stable than with a dT40 template.
To further study the effects of the DNA sequence and topology, dR43
and duplex dR43ÁdRrev43, with mixed purine–pyrimidine, were used
as templates. The different spectroscopic techniques also confirm
the DNA-templated self-assembly through ‘‘phosphate clamp’’
binding (Fig. S25, ESI†).
Titration experiments of dT40:GuaBiNaph monitored by
UV-Vis, CD and fluorescence spectroscopies show spectral
changes compared to pure compounds (Fig. S26 and S27, ESI†).
For instance, the emission intensity of GuaBiNaph strongly
decreases upon addition of dT40. Although the changes are
much less pronounced than with GuaBiPy, this indicates that a
self-assembly process takes place. Heating/cooling cycles were
carried out on dT40:GuaBiNaph (N/P = 0.75) but showed, in
contrast with dT40:GuaBiPy, no significant variations of fluores-
cence signal compared to GuaBiNaph alone (Fig. S28, ESI†).
These results indicate that, compared to GuaBiPy, GuaBiNaph
forms weaker self-assemblies with dT40 as further evidenced by
mass spectrometry studies.
Fig. 4 Competition experiment analyzed by MALDI-ToF mass spectro-
metry: GuaBiPhe vs. GuaBiNaph vs. GuaBiPy. Template: ssDNA dT10
.
Complexes of dT10 with GuaBiNaph are marked with a lozenge (B) and
complexes of dT10 with GuaBiPy are marked with a square (’).
The combination of (chir)optical spectroscopy and mass
spectrometry techniques demonstrate that bisfunctionalized
guanidinium compounds possessing aromatic side groups
undergo a ssDNA-templated self-assembly in aqueous media
through non-covalent interactions with the phosphodiester
backbone of DNA. p-stacking interactions have been shown to be
critical in stabilizing these supramolecular assemblies. In our
forthcoming studies, we will attempt to reveal the supramolecular
organization of these DNA-templated self-assemblies, especially
towards longer DNA sequences.
We thank the CNRS, the ANR (ANR-11-PDOC-002-02) and the
LabEx CheMISyst (ANR-10-LABX-05-01) for funding. This work
was also supported by the Fonds de la Recherche Scientifique –
FNRS (Belgium) under Grant n1. 2.4615.11-BINDER. J.R.-M. and
M.S. are post-doctoral researcher and research associate of the
FNRS, respectively. Mass spectrometry analyses were performed at
´
the Laboratoire de Mesures Physiques of Universite Montpellier 2,
IBMM platform of instrumentation.
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oligonucleotides.15 In this case, we used a dT10 ssDNA and
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Chem. Commun., 2014, 50, 14257--14260 | 14259