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Formation of Large Polymeric Aggregates via Self-assembly

Self-assembly is however not limited to the aggregation of a small discrete number of oligomeric strands. Also much larger aggregates can be formed by the hierarchical interaction of suitable monomers. Again, the building blocks of these assemblies normally do not possess a well-defined conformation of their own as monomers but the assemblies can be relatively well defined. For example, Meijer's four H-bonded supramolecular polymers attracted much attention in this context (Fig. 4.24).

The resulting superstructures are not covalent but supramolecular polymers held together by specific noncovalent interactions between bifunctional building blocks. This linear supramolecular polymer then folds into a specific three di-

Fig. 4.24 Hierarchical stacking of chiral four H-bonded dimers (A) leads to large helical super structures (B) [69].

mensional structure [69]. Such a supramolecular foldamer can also further assemble into larger aggregates. For example, a helical supramolecular polymer based on the dipolar stacking of bis-merocyanine dyes forms intertwined linear rods via self-assembly of six such helices. These rods can then even further self-organize into hexagonal stacks (Fig. 4.25) [70]. However, exact structure information on a molecular level is often not available for such large aggregates and hence only reasonable models can be suggested at best.

Peptide amphiphiles (PAs) as introduced by Stupp [71] are another highly interesting class of self-assembling polymers forming large aggregates. They consist of a hydrophilic peptide segment with most often ionizable side chains cova-lently coupled to a lipid tail (e.g. palmitic acid). The structure of the oligopeptide segment is hence pH sensitive. For example, when ionized the charges prevent structure formation, whereas after protonation at low pH values the peptide adopts a helical secondary structure. This change in protonation state and structure ultimately drives self-assembly through hydrophobic collapse of the lipid tails into cylindrical nanostructures of considerable size (Fig. 4.26). Such aggre-

Fig. 4.25 Hierarchical transition of bis-merocyanin dyes (a) from the supramolecular polymer (b) over the rod (c) to a hexagonally ordered pillar (d) [70].
Fig. 4.26 Chemical structure of a peptide amphiphile (A) and the resulting aggregate caused by hydrophobic collapse of the lipid tail (B).

gates have interesting properties. For example, they can be used as scaffolds to direct the mineralization of hydroxyapatite to form composite materials similar to bone growth, where collagen fibrils serve the same function [72]. Also dendritic dipeptides have been shown to form large aggregates via self-assembly. In this case a Tyr-Ala dipeptide is functionalized via the tyrosine OH group with a Frechet-type dendron which long alkyl chains attached to its end. Depending on the stereochemistry of the dipeptide, allosteric self-assembly into helical porous structures occurs [73].

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