Change in Sheet Structure upon Adsorption

In the group of Boden, the self-assembly of a peptide called P11-2, i.e. Ac-Gln-Gln-Arg-Phe-Gln-Trp-Gln-Phe-Glu-Gln-Gln-NH2, has been studied both in solution and adsorbed on mica substrates [33-35]. In solution, at very low concentrations (<0.05 mM) the peptide was present as random coil, as confirmed by CD spectroscopy. At slightly higher concentrations (@0.07 mM) b-sheet ribbons start to emerge and at even higher concentrations (>0.6 mM) rigid fibrils and fibers have been observed by electron microscopy (Fig. 13.15) [33].

The observed formation of helical structures from b-sheet assemblies of pep-tides is generally believed to be based on the chirality of the single peptide building blocks. However, the helical twisting can be overcome by adsorption on mica substrates when the peptide-surface binding energy outweighs the energy gained by twisting [35]. At specific conditions (5 mM P11-2 in 10 vol% water in n-propanol and pH = 5.5) tapes resembling antiparallel cross-b-sheet structures were found to grow, as observed by AFM on mica (Fig. 13.16). The formed tapes had a height

Fig. 13.16 Hierarchical self-assembly of peptide monomer P^-2, i.e. Ac-Gln-Gln-Arg-Phe-Gln-Trp-Gln-Phe-Glu-Gln-Gln-NH2, into tapes as shown by AFM images on mica (left, inset shows FT and hexagonal symmetry) and structural model derived from XPS data and modeling (right) [35]. (Reproduced from ref. [35] with permission.)

Fig. 13.16 Hierarchical self-assembly of peptide monomer P^-2, i.e. Ac-Gln-Gln-Arg-Phe-Gln-Trp-Gln-Phe-Glu-Gln-Gln-NH2, into tapes as shown by AFM images on mica (left, inset shows FT and hexagonal symmetry) and structural model derived from XPS data and modeling (right) [35]. (Reproduced from ref. [35] with permission.)

corresponding to the height of one molecule and a width corresponding to the length of one molecule. Therefore, the peptides are most likely bound via the replacement of potassium ions of the upper layer of the mica substrate by guanidi-nium groups of arginine residues while the glutamates' carboxylate groups are expected to bind the potassium ions. Obviously, in this example electrostatic interactions are strong enough to drive the adsorption of flat b-sheets on mica substrates. Furthermore, it is important to note that the critical concentration for the growth of these tapes at a mica surface is more than ten times lower than the critical concentration required for their self-assembly in solution.

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