Nanoscale infrared spectroscopy identifies structural evolution of Aβ 16-22 fibrils from parallel to antiparallel beta sheets.

Abstract

Spontaneous aggregation of amyloid beta (Aβ) proteins into fibrillar plaque is a key pathological signature of Alzheimer's disease. Structure of Aβ fibrils have been studied in depth; however, the structure of early-stage aggregates, such as oligomers and protofibrils, is less known. This lack of knowledge stems in part from the limits of experimental approaches used to study amyloid structure. Due to lack of spatial resolution, it is not always possible to isolate specific early-stage aggregates and identify their structural features using conventional biophysical techniques. This is particularly relevant for transient intermediate species. As a result, the structural evolution of amyloid aggregates from its early oligomers to mature fibril is still not fully understood. Atomic Force Microscopy (AFM), coupled with Infrared (IR) spectroscopy enables morphological and spectral characterization of individual nanoscale aggregates, and is thus ideally suited for this challenge. Here we have applied AFM-IR nano-spectroscopy to investigate the aggregation of Aβ 16-22, which spans the amyloidogenic core of the Aβ peptide. We demonstrate that Aβ 16-22 involves a structural transition from oligomers with parallel beta sheets to antiparallel fibrils through disordered intermediate fibril structures. This is exactly opposite to the known aggregation mechanism of the full-length Aβ. Furthermore, the transient fibrillar structures also exhibit signatures of alpha helices. Our results indicate that observing fibrils does not necessarily imply structural order, and that fibrils can undergo spontaneous reorganization to form more stable secondary structures.