Fig. 13.5 Peptides 1 -3 with different periodicities, i.e. hydrophobic repeat unit distances, and their conformational preference in solution and at interfaces .
peptides and support the conclusion that folding into the a-helical conformation at interfaces is supported by the appropriate hydrophobic-hydrophilic periodicity.
The formation of helices at interfaces is not limited to peptides. Helical structures are also formed by the self-assembly of foldamer 4 consisting of 8 pyridine and 5 pyridazine units (Fig. 13.6) . This compound folds into crescent disclike structures, which stack to form filaments and fibrils. LB films of these compounds have been studied and from the surface pressure-area isotherm it was concluded that the filaments adsorb at the air-water interface in an edge-on arrangement, i.e. with the heteroaromatic repeat units perpendicular to the interface. Atomic force microscopy (AFM) studies on drop-cast films of these compounds on freshly cleaved mica revealed the presence of worm-like structures (Fig. 13.6). These studies show that the self-assembled superhelical structure of these foldamers is well preserved in solution as well as in monolayers at interfaces.
In the examples mentioned above, the respective foldamers form helices in solution as well as at the interface, and there is negligible influence of the interactions between the foldamer and the interface on secondary structure formation. However, this is not always the case. Lu et al. have presented a study on two 15-residue peptides 5 and 6 (Fig. 13.7), where the three tyrosine residues in peptide 5 have been substituted by three tryptophan residues in peptide 6 . In solution both peptides form a-helices. The adsorption of these peptides at the hydro-philic silicon oxide/water interface depends strongly on both pH and concentra-
5 Ty r-Va I-As n - Al a- Lys-G I n -Ty r-Ty r-Arg -1 le- Le u - Lys-Arg -Arg -Ty r
Fig. 13.7 Peptides 5 and 6 differing in three repeat units (shown in bold) show different adsorption conformations on silicon oxide depending on pH and concentration .
tion. In an acidic environment at pH = 5, peptide 5 is highly charged and hydro-philic and does not adsorb on the slightly negatively charged silicon oxide surface. Increasing the pH to pH = 7-9 leads to the formation of stable peptide layers at concentrations as low as 0.01 w/v%. Most interestingly, at these low concentrations the tryptophan-modified peptide 6 forms a loosely packed layer consisting of the peptide in the b-sheet conformation, but at higher concentrations (0.1 w/ v%) the peptide adopts the a-helical conformation. Most likely, this structural transition is caused by interchain contacts occurring at higher surface densities.
In addition, the monolayer preparation method and the employed solvent can have a pronounced effect on interfacial conformation. For instance, the backbone of a poly(phenylacetylene) derivative carrying i-valine pendant groups (see Chapter 11 for a more detailed account on related work by the Yashima group, i.e. Figures 11.4, 11.8, and 11.18) was shown by CD and UV-vis spectroscopy to adopt a helical conformation in solution . Films prepared by slow evaporation of a methanol solution resulted in the formation of globular micelles at freshly cleaved mica, whereas slow evaporation of THF solutions resulted in the formation of helical cables. At the air-water interface, on the other hand, the polymer self-assembles into extended fibers as concluded from structural investigations on LB films . This contrasting behavior can be explained by solvent influences. In polar solvents, the polymer prefers to adopt a helical structure since the hydro-phobic backbone is directed towards the inside of the helix whereas the more polar valine units are positioned at the outside, thereby increasing the solubility and stability of the helices. However, at the air-water interface, the polymer can adopt a nonhelical conformation in which the valine pendants are located in the water layer and the backbone of the polymer is exposed to the air at the interface.
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