Directly Probing Intermolecular Structural Change of a Core Fragment of β-Microglobulin Amyloid Fibrils with Low-Frequency Raman Spectroscopy.

Amyloid fibrils, which are ordered aggregates of proteins or peptides, have attracted keen interest because their deposition causes serious human diseases. Despite many studies utilizing X-ray crystallography, solid-state NMR, and other methods, intermolecular interactions governing the fibril formation remain largely unclear. Here, we used low-frequency Raman (LFR) spectroscopy to investigate the intermolecular β-sheet structure of a core fragment of β-microglobulin amyloid fibrils, βm, in aqueous buffer solutions. The LFR spectra (approximately 10-200 cm) of βm amyloid fibrils measured at different pH values (ranging from 6.8 to 8.0) revealed a broad-spectral pattern with a maximum at ∼80 cm below pH 7.2 and at ∼110 cm above pH 7.4. This observation is attributed to a pH-dependent structural change from an antiparallel to a parallel intermolecular β-sheet structure. Multivariate curve resolution-alternating least-squares (MCR-ALS) analysis enabled us to decompose the apparently monotonous LFR spectra into three distinctly different contributions: intermolecular vibrations of the parallel and antiparallel β-sheets and intramolecular vibrations of the peptide backbone. Peak positions of the obtained LFR bands not only exhibit a much more pronounced difference between the two β-sheets than the conventional amide I band, but they also suggest stronger intermolecular interaction, due presumably to the hydrophobic effect, in the parallel β-sheet than in the antiparallel β-sheet. The present results show that LFR spectroscopy in combination with the MCR-ALS analysis holds promise for real-time tracking of the intermolecular dynamics of amyloid fibril formation under physiological conditions.