Misfolding and aggregation of amyloid β-40 (Aβ-40) peptide play key roles in the development of Alzheimer's disease (AD). However, very little is known about the molecular mechanisms underlying these molecular processes. We developed a novel experimental approach that can directly probe aggregation-prone states of proteins and their interactions. In this approach, the proteins are anchored to the surface of the atomic force microscopy substrate (mica) and the probe, and the interaction between anchored molecules is measured in the approach-retraction cycles. We used dynamic force spectroscopy (DFS) to measure the stability of transiently formed dimers. One of the major findings from DFS analysis of α-synuclein (α-Syn) is that dimeric complexes formed by misfolded α-Syn protein are very stable and dissociate over a range of seconds. This differs markedly from the dynamics of monomers, which occurs on a microsecond to nanosecond time scale. Here we applied the same approach to quantitatively characterize interactions of Aβ-40 peptides over a broad range of pH values. These studies showed that misfolded dimers are characterized by lifetimes in the range of seconds. This value depends on pH and varies between 2.7 s for pH 2.7 and 0.1 s for pH 7, indicating that the aggregation properties of Aβ-40 are modulated by the environmental conditions. The analysis of the contour lengths revealed the existence of various pathways for dimer dissociation, suggesting that dimers with different conformations are formed. These structural variations result in different aggregation pathways, leading to different types of oligomers and higher-order aggregates, including fibrils.
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