Another important class of secondary structures is the sheet motif. b-Sheet structures are abundant peptide and protein secondary structures, and similar secondary sheet structures are also found in a variety of artificial backbones (see Chapter 4). In most sheet structures, the individual foldamer chains are stretched and intermolecular interactions between different strands lead to the formation of two-dimensional ribbons or sheets via dimerization and further aggregation.
A few examples of peptides that adsorb as b-sheets at the air-water interface have been described by Rapaport and coworkers [24-26]. They designed peptides of alternating hydrophobic and hydrophilic amino acids, which are able to assemble at the air-water interface. It was found that proline residues at the N and C termini induce two-dimensional order in the monolayer because prolines are preferentially located at the rim of a ribbon. Other amino acids do not display this preference (Fig. 13.10) .
In another study, peptides 8 and 9, incorporating alternating hydrophobic and hydrophilic amino acids as well as complementary electrostatic interactions between lysine and glutamic acid residues, were designed to favor parallel b-sheets at the air-water interface (Fig. 13.11) . Grazing incidence X-ray diffraction, IR
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