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after the addition of acetonitrile, indicating the disassembly of
the G-quartet-based octameric structure, while an intense quasi-
conservative exciton splitting in the 300-450 nm spectral region
becomes predominant in the CD spectrum. This latter strong bisignate
optical activity can be ascribed to the helical packing of conjugated
terthiophene moieties stabilized by
interactions. NMR spectra
and photophysical investigations confirm the structures of the
guanine-directed and thiophene-directed assemblies.
π
-
π
Figure 4.21
CD spectra of (a)
6
in CHCl
(black line), (b) 6-KPic in CHCl
3
3
(blue line), and (c) 6-KPic in MeCN-CHCl
9: 1 (red line).
3
to
control the form of self-assembly: this inevitably leads to a progressive
degradation of the system. This problem can be circumvented by
using light as the external stimulus. Light is a very appealing trigger
because of its ready availability, easy manipulation, and noninvasive
character. By introducing a photoactive moiety at C
In the above examples, molecules were used as external
stimuli
in a lipophilic
guanosine derivative it is possible to operate photocontrol over the
self-assembly of the molecule, such that the existence of G-quartets
can be alternately switched on and of [38]. Compound (E)-
8
(Chart
4.5) in the presence of a measured amount of KI self-assembles
into a
7
-symmetric complex consisting of two stacked G-quartets.
Photoisomerization to the Z isomer determines the decomposition
of the octameric complex, which is re-formed when the molecule is
reverted to the E isomer by either thermal or photochemical back
isomerization (Fig. 4.22). Thus, the G-quartet-based octamer can be
cyclically assembled and disassembled by light, and the process is
perfectly reversible.
D 4
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