E and Z Secondary Amides in Supramolecular Nanotubes and Nanospheres - Molecular Self-Assembly: Advances and Applications - page 41

Figure 2.2 Equilibrium between flat cyclopeptides and their

corresponding cylindrical supramolecular stack.

β β

Figure 2.3 Crystal structure of cyclo-Ser(OtBu)- -Asp(OMe)

showing, (a) the tubular assembly and (b) the amides
involved in the hydrogen bonds (CO···HN distances are all
around 1.7Å).

-Ala-Gly-

Afewyearslater,thefirsttubesmadefromcyclo-(

a

-amino acid)

n

=8)hadbeendesigned,preparedandcharacterized(Fig.2.4)
[12,51,52]. In general, supramolecular nanotubes of that kind proved
difficulttohandleduetotheirstrongpropensitytoaggregate(their
structurescouldonlybedefinedbyelectrondifraction,calculations,
andIRspectroscopyofmicrocrystals)[53-55].Theproblemwas
only partially alleviated by using appropriate side chains [13,56].
The same extreme insolubility was also the main characteristic
of tubes made of cyclo-(

( n

= 3,4), their structures
wereobtainedbyX-raydifractionanalysesofpowders,orinferred
moreindirectlybycalculationsorIRspectroscopy(Fig.2.5)[57-
60].Anotherwaytoprovethetubularstructureofthesematerials
consisted in stopping the self-assembly at the dimer stage through
N-alkylation to increase their solubility [61].

b

-amino acid) ( n n