Comparative IR and X-ray studies of natural and model amyloid peptides at the air/water interface

Monolayers of an amyloid β peptide (Aβ40) and a much smaller model peptide (LSFD) at the air/water interface have been investigated by isotherm, IRRAS and GIXD measurements. Additionally, the LSFD monolayer has been transferred onto solid support and investigated by ATR–FTIR to test the influence of the transfer on the secondary structure of the peptide. Both peptides are surface active and form stable films of ordered β-sheet domains on the surface. The same absorption bands characteristic of an anti-parallel β-sheet conformation can be seen in the transferred LSFD film indicating that the transfer does not change the secondary structure. On the water surface, the β-sheets are oriented mostly parallel to the surface. GIXD experiments show a Bragg peak at characteristic repeat distances of 4.75–4.8 Å for both peptides. The full-width at half maximum (fwhm) of this peak shows that the smaller LSFD peptide forms a monolayer film with high degree of order perpendicular to the β-strands, whereas Aβ40 exhibits a drastically reduced crystallinity.