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selects only one of the four possible shapes
found in the crystal of its parent cyclo-Gly
Since cyclo-(Gly//Gly)
3
we started a program to
look for ways to stabilize other isomorphs more favorable to form
the hexagonal tubes as originally planed. Attempts to crystallize
lactam (
6
) from other isotropic solvents led to the same rectangular
tubes. Alternatively, (
1
) was dissolved in a nematic liquid crystal
(LC), an anisotropic medium [67]. Self-assembly was triggered when
thehotsolution(0.5%peptide)wascooledveryslowly(0.1°C/
min) [68].
1
Belowthenematic-isotropictransition(115°C)wire-like
aggregates were formed as long as several millimetres and around
2
m thick. When washed from liquid crystal matrix and observed
by scanning electron microscopy (SEM), these long wires or threads
revealed their true nature, they were in fact hexagonal hollow
tubes.TheirstructurewasprobedbyRamanmicrospectroscopy
on one single tube. The polarization of the laser beam was set to
beparallelandorthogonaltothelongaxisofthetube.Recording
oftheRamanspectraintheregionoftheamidebands(1600-1700
cm
µ
-1
) showed two types of C=O stretching modes associated with
two types of hydrogen bonds. The 1664 cm
-1
band corresponds
to a stronger hydrogen bond than the one at 1634 cm
-1
, the latter
being a bifurcated bond [69]. Along the main axis of the tube, the two
bands are observed whereas only the weak hydrogen bond band is
observed perpendicularly to the tube. Calculations that incorporated
theRamandataandwereconsistentwiththehexagonalshapeof
the tubes showed the whole picture: two conformers, which present
many similarities with two cyclohexaglycine conformers, stack on
topofeachotherinanalternatewaytofulfilltheirhydrogenbonding
capabilities(Fig.2.8).
Fromthenonafractalself-assemblyprocessiscarriedon
untilmesoscopichexagonaltubesappear.Thus,theresultingfirst
generation tubes pack side by side in an antiparallel fashion, by
means of inter-stack hydrogen bonds to create a second generation
tube. This process is driven by aggregates of liquid crystals acting as
templatesforthelipophilicfacesofthefirstgenerationtubes(Fig.
2.9). The same hierarchical process is then repeated on and on until
micrometer-scaletubeswithlargeinteriorcavitiesfilledwithliquid
crystalareobtained(Fig.2.10).
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