Enhanced Sheet Formation upon Adsorption

Other examples of sheet formation are known in which the comparison between structures in bulk solution and at interfaces is made. In Section 13.3.1 the landmark study of DeGrado and Lear on a-helix formation of leucine- and lysine-consisting peptides 1-2, both in solution and at hydrophobic-hydrophilic interfaces was discussed [12]. In this paper the authors also describe peptide 3 with a hydrophobic periodicity of 2, i.e. an alternating sequence of leucine and lysine residues. It appeared that this peptide adopts a b-sheet conformation upon adsorption at the air-water interface, whereas a-helices were formed from peptides with a hydrophobic periodicity of 3.5 (Fig. 13.13). As in the case of the a-helices, in solution there is a dynamic equilibrium between b-sheet and random coil conformation. However, at the interface the equilibrium is completely shifted to the b-sheet conformation. This nicely demonstrates the stabilization of the b-sheet structure by interfacial interactions.

Fig. 13.13 Schematic representation of the adsorption of an amphiphilic peptide with a periodicity of 3.5 into an a-helical structure (left) and of an amphiphilic peptide with a periodicity of 2.0 into a b-sheet structure (right) (Adapted from ref. [12]) [12].

Comparable results were found by Tirrell and coworkers for long peptides consisting of three pairs of alternating alanine and glycine units combined with single glutamic acid residues as turn elements, i.e. [(Ala-Gly)3-Glu-Gly]36 [28]. The authors studied the peptides by IR absorption spectroscopy in aqueous solution and by external IR reflectance spectroscopy at the air-water interface. In solution, the peptide adopts the random coil conformation over a wide pH range (5 a pH a 14). However, at the air-water interface the peptide adopts a b-sheet conformation that could further be attenuated when the pH of the solution was changed from basic (pH = 10) to acidic (pH = 5). From this study it can be concluded that sheet formation was stimulated by adsorption.

Fig. 13.14 Schematic representation of amphiphilic oligo(meta-phenylene ethynylene) 11 adopting a sheet structure at the air-water interface (left) [30, 31] and STM image of n-decyl-substituted hexa(meta-phenylene ethynylene) 12 on HOPG adopting a zig-zag structure at the solid-liquid interface (right) [32].

Fig. 13.14 Schematic representation of amphiphilic oligo(meta-phenylene ethynylene) 11 adopting a sheet structure at the air-water interface (left) [30, 31] and STM image of n-decyl-substituted hexa(meta-phenylene ethynylene) 12 on HOPG adopting a zig-zag structure at the solid-liquid interface (right) [32].

In analogy to the aforementioned amphiphilic peptides, other amphiphilic oligomers can form sheet structures at interfaces as well. For instance, amphiphilic oligo(meta-phenylene ethynylene) foldamers pioneered by the Moore group (see Chapter 3) adopt a helical conformation in solution in order to maximize sol-vophilic interactions of the side chains with the solvent and due to p,p-stacking interactions between the aromatic backbone repeat units [29]. Tew and coworkers found that this foldamer family, if appropriately substituted with alternating hy-drophobic and hydrophilic groups, i.e. 11, forms sheet-like structures at the air-water interface when the adsorbed monolayers are compressed (Fig. 13.14 left) [30, 31]. From surface-pressure isotherms and calculations it was concluded that polymer 11 adopted a zig-zag-type transoid conformation, with the aromatic rings perpendicular to the interface, i.e. in an edge-on conformation, thereby presumably stabilizing the sheets by p,p-stacking. In related work at the solid-

Helical Conformation

monomer tape ribbon fibril fibre concentration

Fig. 13.15 Model for the hierarchical self-assembly of peptide monomer P11-2, i.e. Ac-Gln-Gln-Arg-Phe-Gln-Trp-Gln-Phe-Glu-Gln-Gln-NH2, into tapes, ribbons, fibrils and fibers as a function of concentration and corresponding electron micrographs (scale bar 100 nm in each image) [33]. (Reproduced in part from ref. [33] with permission.)

liquid interface, Hecht, Rabe, and coworkers studied SAMs of alkyl-substituted hexa(meta-phenylene ethynylene) 12 by scanning tunneling microscopy (STM) and could show that the foldamers adopt face-on sheet structures on HOPG (Fig. 13.14 right) [32]. Interestingly, the corresponding ortho-linked hexamers show a markedly different behavior by preferentially forming lower dimensional aggregates.

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