Two Dimensional Oblique Molecular Packing within a Model Peptide Ribbon Aggregate

A₁₀K (A=alanine, K=lysine) model peptides self-assemble into ribbon-like β-sheet aggregates. Here, we report an X-ray diffraction investigation on a flow-aligned dispersion of these self-assembly structures. The two-dimensional wide-angle X-ray scattering pattern suggests that peptide pack in a two-dimensional oblique lattice, essentially identical to the crystalline packing of polyalanine, A_n (for n>4). One side of the oblique unit cell, corresponding to the anti-parallel β-sheet, is oriented along the ribbon's axis. Together with recently published small angle X-ray scattering data of the same system, this work thus yields a detailed description of the self-assembled ribbon aggregates, down to the molecular length scale. Notably, our results highlight the importance of the crystalline peptide packing within its self-assembly aggregates, which is often neglected.