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  • Title: Influence of end-capping on the self-assembly of model amyloid peptide fragments.
    Author: Castelletto V, Hamley IW, Cenker Ç, Olsson U, Adamcik J, Mezzenga R, Miravet JF, Escuder B, Rodríguez-Llansola F.
    Journal: J Phys Chem B; 2011 Mar 10; 115(9):2107-16. PubMed ID: 21309578.
    Abstract:
    The influence of charge and aromatic stacking interactions on the self-assembly of a series of four model amyloid peptides has been examined. The four model peptides are based on the KLVFF motif from the amyloid β peptide, Aβ(16-20) extended at the N terminus with two β-alanine residues. We have studied NH(2)-βAβAKLVFF-COOH (FF), NH(2)-βAβAKLVF-COOH (F), CH(3)CONH-βAβAKLVFF-CONH(2) (CapF), and CH(3)CONH-βAβAKLVFF-CONH(2) (CapFF). The former two are uncapped (net charge +2) and differ by one hydrophobic phenylalanine residue; the latter two are the analogous capped peptides (net charge +1). The self-assembly characteristics of these peptides are remarkably different and strongly dependent on concentration. NMR shows a shift from carboxylate to carboxylic acid forms upon increasing concentration. Saturation transfer measurements of solvent molecules indicate selective involvement of phenylalanine residues in driving the self-assembly process of CapFF due presumably to the effect of aromatic stacking interactions. FTIR spectroscopy reveals β-sheet features for the two peptides containing two phenylalanine residues but not the single phenylalanine residue, pointing again to the driving force for self-assembly. Circular dichroism (CD) in dilute solution reveals the polyproline II conformation, except for F which is disordered. We discuss the relationship of this observation to the significant pH shift observed for this peptide when compared the calculated value. Atomic force microscopy and cryogenic-TEM reveals the formation of twisted fibrils for CapFF, as previously also observed for FF. The influence of salt on the self-assembly of the model β-sheet forming capped peptide CapFF was investigated by FTIR. Cryo-TEM reveals that the extent of twisting decreases with increased salt concentration, leading to the formation of flat ribbon structures. These results highlight the important role of aggregation-induced pK(a) shifts in the self-assembly of model β-sheet peptides.
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