Assembly by amyloid-beta (Aβ) peptides is vital
for various
neurodegenerative diseases. The process can be accelerated by hydrophobic
interfaces such as the cell membrane interface and the air–water
interface. Elucidating the assembly mechanism for Aβ peptides
at hydrophobic interface requires knowledge of the microscopic structure
of interfacial peptides. Here we combine scanning force microscopy,
sum-frequency generation spectroscopy, and metadynamics simulations
to probe the structure of the central fragment of Aβ peptides
at the air–water interface. We find that the structure of interfacial
peptides depends on pH: at neutral pH, the peptides adopt a less folded,
bending motif by forming intra-hydrogen bonds; at acidic pH, the peptides
refold into extended β-strand fibril conformation, which further
promotes their macroscopic assembly. The conformational transition
of interfacial peptides is driven by the reduced hydrogen bonds, both
with water and within peptides, resulting from the protonation of
acidic glutamic acid side chains.
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