Nucleation dynamics in amyloid-beta dimerization revealed by single-molecule fingerprinting.

The aggregation of amyloid-beta ( A β ) peptides ( A β 1 - 40 or A β 1 - 42 ) is closely related to the pathology of Alzheimer's disease (AD). Soluble oligomers that appear during A β aggregation are primary neurotoxic species; however, their misfolding kinetics have yet to be determined. Here, we report a bottom-up construction of parallel and antiparallel A β 1 - 40 dimers, the first oligomers formed during A β aggregation. We apply single-molecule mechanical unfolding in optical tweezers to investigate the dynamic structural evolution of these dimers at the single-amino-acid resolution. We observe three intermediates during the association and dissociation of individual A β 1 - 40 dimers, with the diphenylalanine A β 19 - 20 dimer having the highest formation probability. Our single-molecule fingerprinting method reveals that a known A β aggregation inhibitor, rosmarinic acid, can reduce A β 1 - 40 dimerization by binding to the A β 19 - 20 site. We anticipate that the molecular tool innovated in our study is extensible to investigating other amyloid aggregations responsible for a myriad of neurodegenerative diseases.