Acidic pH Promotes Refolding and Macroscopic Assembly of Amyloid β (16-22) Peptides at the Air-Water Interface

Abstract

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.

Scheme 1. Molecular Structure of the Aβ_16–22 Peptide

At neutral pH: LYS and the N-terminal group are protonated and positively charged, while the GLU and C-terminal are deprotonated and negatively charged. Under acidic conditions the carboxyl group of GLU and that of the C-terminus are neutral as they are both protonated.

Figure 1

(a, b) SFM images of the transferred films of Aβ_16–22 peptides at air–water interface with a bulk solution pH of 7 (a) and 3 (b).

Figure 2

(a) Homodyne SSP SFG spectra in the frequency region 1440–1800 cm^–1 for Aβ_16–22 peptides at the air–water interface with bulk solution pH of 3 (red) and 7 (black). Characteristic bands from backbone and side chains are marked. (b) Homodyne PSP SFG spectra in amide I region for interfacial peptides at pH 3 (red) and 7 (black). (c) Homodyne SSP SFG spectra in CH/OH region for Aβ_16–22 peptides at air–water interface with solution pH of 3 (red), 7 (black), and 10.5 (cyan). The spectrum for pure water (dark yellow) is included as reference, and is indistinguishable from the pH 10.5 spectrum. (d) Heterodyne SSP SFG spectra in CH/OH region for interfacial peptides at pH 3. In parts a–c, the spectra fits (gray) were superimposed into experimental homodyne spectra.

Figure 3

(a, b) Typical snapshots of zwitterionic (pH 7, a) and positively charged (pH 3, b) Aβ_16–22 peptides at the water–vacuum interface: peptides differ in the protonation states of the carboxylic groups, at pH 7 being deprotonated and negatively charged while at pH 3 being protonated and neutral. (c) Free energy profiles according to the dihedral angles, as defined by the two phenyl rings within the positively charged (pH 3) and zwitterionic (pH 7) peptides. (d) Distribution of root-mean-square deviation (RMSD) with respect to the stable β-sheet polymorph (PDB code: 2y2a) for positively charged (pH 3) and zwitterionic (pH 7) peptides at the water–vacuum interface. (e) Orientation distribution of lysine (LYS) and glutamic acid (GLU) side chains for positively charged (pH 3) and zwitterionic (pH 7) peptides at the air–vacuum interface. (f) Average numbers of hydrogen bonds for side chain groups in positively charged (pH 3) and zwitterionic (pH 7) peptides at the air–vacuum interface. The LYS and GLU charge groups can form: (i) inter-hydrogen bonds with water molecules and (ii) intra- hydrogen bonds within Aβ_16–22 peptides.