Peptide Self-Assembly Studied by Small and Wide angle X-ray Scattering

Peptides can self-assemble in solution into different aggregate structures, generally involving extended intermolecular b-sheets. In this talk we will discuss the self-assembly of peptides, from short synthetic model peptide aggregates to amyloid fibrils and plaque that are hallmarks of a number of neurodegenerative diseases like Alzheimer’s and Parkinson’s. Using a combination of small and wide angle X-ray scattering, SAXS/WAXS, covering three orders of magnitude in q-space, we obtain information on both aggregate size and shape, and the local molecular packing. The model peptides AnK, where A is alanine, K is lysine and n is the number of alanine residues in the peptide chain, allows for analyzing the effects of molecular size. Interestingly, A6K self-assemble into hollow nanotubes, while A8K and A10K form twisted ribbon-like aggregates. Both cases involve a peptide monolayer of laminated b-sheets, with the molecules arranged with a 2D crystalline packing, as seen from observed diffraction patterns in the WAXS regime. As an example, Figure 1A shows the SAXS/WAXS pattern from A6K nanotube, with the WAXS regime highlighted in the inset. For longer peptides, like Amyloid-b (Alzheimer’s - 42 amino acids) and a-Synuclein (Parkinson’s – 140 amino acids), the self-assembly is somewhat different. These molecules fold in 2D, and then stack in the third dimension with intermolecular b-sheets, forming the characteristic amyloid fibrils. While the 2D fold can be characterized in detail by for example solid state NMR methods, SAXS provides complementary information on the fibril cross section structure.

Figure: (A) SAXS/WAXS patterns obtained from aqueous A6K solutions at two different peptide volume fractions, f. The lowest concentration, f=0.07, is below the critical aggregation concentration, and the SAXS pattern show only the scattering from peptide monomers. The f=0.12 sample has a scattering pattern consistent with hollow nanotubes. The inset highlights the WAXS regime. (B) Anisotropic WAXS pattern due to partial alignment of the nanotubes in the X-ray capillary. (C) Model of the peptide b-sheet arrangement in the nanotube.