Adsorption of Amyloid β (1-40) Peptide at Liquid Interfaces

The folding of amyloid β (1-40) peptide into β-sheet containing fibrils, which are the main components of deposits and plaques in some neurodegenerative diseases, is thought to play a causative role in Alzheimer's disease. The peptide is amphiphilic and therefore surface active. Interactions with surfaces can play an important role in the secondary structure changes. Langmuir monolayers of zwitterionic (DMPC, DPPC, DMPE, DPPE) as well ionic (DMPG, DPPG, DPTAP, DSTAP) phospholipids have been used to study the influence of the peptide on the lipid packing and, vice versa, the influence of phospholipid monolayers on the peptide secondary structure by infrared reflection absorption spectroscopy and grazing incidence X-ray diffraction. The peptide adsorbs at the air/water (buffer) interface and penetrates into uncompressed phospholipid monolayers. After a special pre-treatment, the peptide forms predominantly a random-coil secondary structure in solution but adopts a β-sheet conformation, which is almost parallel oriented to the surface in the adsorbed state. The peptide does not influence the condensed phospholipid monolayer structure. In contrast to Aβ insertion into zwitterionic or anionic phospholipid monolayers, a non β-sheet structure was detected during the first stage of Aβ insertion into positively charged monolayers. At high lateral pressure, the peptide is squeezed out of the monolayer. It desorbs completely from zwitterionic monolayers and charged monolayers on buffer, but remains adsorbed in β-sheet conformation at charged monolayers on water. In contrast, Aβ remains in a fluid (disordered) and charged monolayer on buffer even above 30 mN m^-1. This observation shows that a combination of hydrophobic and electrostatic interactions is necessary to keep the peptide in the adsorbed state at high pressure on buffer.