Different Individual Amyloid Fibrils Exhibit Different Beta Sheet Secondary Structures via Near-field Infrared Spectroscopy

Abstract

Morphology and secondary structure of individual amyloid fibrils synthesized from the #21-31 peptide fragment of B2-microglobulin were obtained using apertureless near-field scanning infrared microscopy (ANSIM). The sample exhibits a heterogeneous population of fibrils: some fibrils exhibit parallel and some with anti-parallel beta sheet structures. ANSIM is an optical technique which uses a carbon monoxide IR laser coupled with a tapping mode atomic force microscope using homodyne detection demodulated at the tip frequency. The experimental near-field spectra correlate strongly to the calculated near-field spectrum as well as the far-field spectrum. Near-field images exhibit high attenuation of the amyloid fibrils at approximately 1630 cm−1. Strong attenuation of the incident IR radiation at 1691 cm−1 corresponding to the presence of anti-parallel beta sheet structure is observed for a fraction of the fibrils. The observation of the anti-parallel beta sheet conformation is linked to the synthesis method used to produce these amyloid fibrils. It is shown that the addition of trimethylamine N-oxide (TMAO) can accelerate the formation of amyloid fibrils and induce anti-parallel beta sheet structure.