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ether to a concentrated solution of
typically produces a stable gelly
material, which small-angle X-ray diffraction (SAXS) demonstrated
to be an unusual lyotropic liquid crystalline phase [24]. CD spectra,
recorded in chloroform at different temperatures and concentrations,
showed weak signals, without any exciton pattern, suggesting that
no supramolecular chirality was originated by self-assembly of
guanosines in these conditions.
1
1
show broad
signals if compared to the ones recorded in the strongly competing
solvent DMSO-d6, as expected for largely associated molecules.
The existence of oligomeric structures in CDCl
H-NMR spectra in CDCl
3
was supported by
ESI mass spectrometry [25]. Moreover, when increasing guanosine
concentration (or lowering temperature), progressive deshielding
of both the imino N
3
H protons takes place, indicating
that the H-bond donor groups of the guanine bases are progressively
involved in the self-assembly process. IR spectra lead to the same
conclusion.
Hence, the lack of added cations and all of the spectral data ruled
out the presence of stacked G-quartet aggregates: the structural
elements giving rise to the mesophase had to be the result of a
different form of LG self-assembly. The structure of these aggregates
was characterized in the solid state (by X-ray diffraction MAS-NMR
spectroscopy and AFM), in solution (by NMR spectroscopy and ESI-
MS) and at graphite-solution interface (by STM) (Fig. 4.11) [25].
1
H and amino N
2
Figure 4.11
Left: current STM survey image of self-assembled architecture
of LG recorded at the solid-liquid interface on HOPG in the
absence of added cations. Right: SFM on mica reveals a single
dry nanoribbon of LG on mica (up to 8 mm long).
Two different ribbon-like aggregates, with different patterns of
hydrogen bonds, were identified in the solid state and in chloroform
solution. The first species (ribbon A, Fig. 4.12A), which is stable in
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