The Self-Assembly of Lipophilic Guanosine Derivatives - Molecular Self-Assembly: Advances and Applications - page 107

4.4 From LG's Supramolecular Complexes to LG-

Based Functional Materials

In the octamers and -polymers, the quartets are not stacked

in register, but rotated ca. 30° with respect to each other. As for DNA,
we have a helical structure, which, for our guanosines, resembles
a four-stranded helix: the sugar moieties transfer their chirality to
the supramolecular structure in a very efficient way, despite the
nucleosides are not covalently bonded. Given the apolar medium,
the anion is in close contact with the cation-templated chiral
assembled structure, and it was worth investigating whether chiral
anions could be discriminated during the self-assembly process.
As chiral anions were chosen,

pseudo

-(2,4-dinitrophenyl) derivatives
of α-aminoacids: aqueous solutions of the racemic form of these
derivatives (potassium salts) were used to induce self-assembly into
a chloroform solution of LG, with a ratio of aminoacid derivative/
guanosine corresponding to the formation of either the octamer or
the

N

-polymer. Discrimination was monitored by observing the
enantiomeric excess (e.e.%) of the residual aminoacid derivatives left
in water. The best results were obtained with aromatic aminoacids
and LG

pseudo

with the deoxyribose esterified with an aromatic acid
[26]. Although the e.e.ís were not high (being 29% the best result),
the procedure consists of a simple extraction and it was one of the
rare cases of chiral discrimination during self-assembly.
Most of the LGs studied so far are able to form lyomesophases in
apolar organic solvents above a critical concentration. This result,
which now may seem obvious, was instead surprising, considering
that in no case the molecular structure of LGs meets the requirements
for liquid crystalline behavior. In every example reported it has been
demonstrated that mesophases originated instead from anisotropic
objects, arising from LGs by self-assembly, as the cooperative effect
of solvophobic interactions and hydrogen bonding.
The elongation ratio of the supramolecular assembly is a crucial
factor for the appearance of lyotropic liquid crystalline phases
and only supramolecular polymeric assemblies, like (LG

1a

4 .

+

M ) and

n

LG , can self-correlate to produce this type of supramolecular self-

organization. Thus, in contrast to discrete assemblies, e.g., octamers,
columnar

n

-polymeric aggregates form lyomesophases of
the cholesteric and hexagonal types (Fig. 4.13a,b) [27], just as the

pseudo