Tip‐Enhanced Raman Spectroscopy to Distinguish Toxic Oligomers from Aβ_1–42 Fibrils at the Nanometer Scale

Abstract

For the first time, natural Aβ fibrils (WT) implicated in Alzheimer's disease, as well as two synthetic mutants forming less toxic amyloid fibrils (L34T) and highly toxic oligomers (oG37C), are chemically characterized at the scale of a single structure using tip-enhanced Raman spectroscopy (TERS). While the proportion of TERS features associated with amino acid residues is similar for the three peptides, a careful examination of amide I and amide III bands allows us to clearly distinguish WT and L34T fibers organized in parallel β-sheets from the small and more toxic oG37C oligomers organized in anti-parallel β-sheets.

Nanoscale Discrimination between Toxic and Nontoxic Protein Misfolded Oligomers with Tip‐Enhanced Raman Spectroscopy

“…Figure a,b reports a collection of selected TERS spectra acquired from type A and type B oligomer samples. In spite of the local signal variations that are consequential to the chemical complexity produced by adjacent amino acid residues, a tentative assignment of the main TERS bands was made based on the Raman spectra (see Table ). In particular, aromatic ring vibrations are visible at 1003 and 1027 cm −1 (Phe) and within the 1550–1620 cm −1 range (Trp, Tyr, Phe) .…”

“…In spite of the local signal variations that are consequential to the chemical complexity produced by adjacent amino acid residues, a tentative assignment of the main TERS bands was made based on the Raman spectra (see Table ). In particular, aromatic ring vibrations are visible at 1003 and 1027 cm −1 (Phe) and within the 1550–1620 cm −1 range (Trp, Tyr, Phe) . His ring deformations look like isolated (1500 cm −1 ) or overlapped with the amide II modes (1538 cm −1 ) .…”

“…In particular, aromatic ring vibrations are visible at 1003 and 1027 cm −1 (Phe) and within the 1550-1620 cm −1 range (Trp, Tyr, Phe). [32,44] His ring deformations look like isolated (1500 cm −1 ) or overlapped with the amide II modes (1538 cm −1 ). [47] The amide I modes appear at 1640-1680 cm −1 [48] in both oligomer spectra.…”

“…The authors evidenced the conformational variety along the fibril by mapping alternating hydrophobic and hydrophilic domains and providing valuable information on the sites active for biological interactions. Recently, Bonhommeau et al showed via TERS the differentiation between the wild type of Aβ fibrils and two synthetic mutants, the latter obtained by substitution of a specific amino acid in the polypeptide sequence and notably forming toxic oligomers and nontoxic fibrils …”

“…Recently, Bonhommeau et al showed via TERS the differentiation between the wild type of Aβ fibrils and two synthetic mutants, the latter obtained by substitution of a specific amino acid in the polypeptide sequence and notably forming toxic oligomers and nontoxic fibrils. [32] In this work, we take inspiration from the above-mentioned achievements to demonstrate the use of TERS in discriminating between two forms of soluble misfolded oligomers formed by the 91-residues N-terminal domain of the E. coli protein HypF (HypF-N, Figure S1, Supporting Information), denoted as type A and type B, which show the same polypeptide sequence, similar morphology but different abilities to cause cellular dysfunction, with type A oligomers being toxic compared to nontoxic type B species. [6] HypF-N represents a valuable model system as it forms spherical oligomers and amyloid-like fibrils in vitro similar to those associated with neurodegenerative diseases.…”