Nanoscale infrared spectroscopy identifies parallel to antiparallel beta sheet transformation of Aβ fibrils

Spontaneous aggregation of amyloid beta (A{beta}) proteins leading to the formation of oligomers and eventually into fibrils has been identified as a key pathological signature of Alzheimers disease. Structure of late stage aggregates have been studied in depth by conventional structural biology techniques including Nuclear Magnetic Resonance, X-ray crystallography and Infrared Spectroscopy; however the structure of early-stage aggregates is less known due to their transient nature. As a result, the structural evolution of amyloid aggregates from its early oligomers to mature fibril is still not fully understood. Here we have applied AFM-IR nanospectroscopy to investigate the aggregation of A{beta} 16-22, which spans the amyloidogenic core of the amyloid beta peptide. Our results demonstrate that A{beta} 16-22 involves a structural transition from oligomers with parallel beta sheets to antiparallel fibrils through disordered and possibly helical intermediate fibril structures, contrary to the known aggregation pathway of full-length A{beta}.