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Fig. 13.11 Self-assembly of an equimolar mixture of peptides 8 and 9 having complementary electrostatic interactions (residues shown in bold) to parallel b-sheet structures at the air-water interface [26]. (Reproduced in part from ref. [26] with permission.)

Fig. 13.11 Self-assembly of an equimolar mixture of peptides 8 and 9 having complementary electrostatic interactions (residues shown in bold) to parallel b-sheet structures at the air-water interface [26]. (Reproduced in part from ref. [26] with permission.)

spectroscopy, and mass spectrometry studies on films composed of an equimolar mixture of both peptides confirmed the formation of parallel b-sheets, whereas this is not the case in films solely composed of either one of the peptides alone. This emphasizes the importance of intermolecular electrostatic interactions between hydrophilic amino acid residues in the formation of b-sheets. Unfortunately, in these studies no comparison has been provided between the secondary structures of the peptides at the air-water interface and in solution.

Another example of b-sheet structures at interfaces can be found in the work of Ree, Magonov, and coworkers [27]. They prepared monolayers of short chain poly(benzyloxycarbonyl lysine) 10 on highly ordered pyrolytic graphite (HOPG)

Fig. 13.12 Self-assembled monolayers of short chain polypeptide 10 (left) on HOPG. AFM image of the monolayer formed upon spin-coating (center) and structural analysis (right) [27]. (Reproduced in part from ref. [27] with permission.)

by a solution spin-coating process (Fig. 13.12). The monolayers were characterized by AFM and IR spectroscopy. From inspection of the amide carbonyl stretching vibration in films it was concluded that 10 adopts a b-sheet conformation. However, the analysis of the AFM images suggests that the peptides lie flat on the HOPG surface to maximize favorable contacts between the aromatic side chains and the graphite substrate (Fig. 13.12). Intermolecular p-p interactions between neighboring aromatic side chains rather than hydrogen bonds between amide units lead to maximum surface coverage by subsequent formation of ribbons, which propagate along the direction perpendicular to the long axis of the peptide.

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