Polymorphic structures of rapidly twisting 40-residue amyloid-β fibrils.

Fibrils formed by 40- and 42-residue amyloid-β peptides (Aβ40 and Aβ42) are polymorphic, containing molecular structures that vary with growth conditions in ways that are not fully understood. Here, we use cryoelectron microscopy to characterize the structure of rapidly twisting Aβ40 fibrils, for which the distance between apparent width minima in electron microscope images ("cross-over distances") is approximately 25 nm. From samples grown under a single set of growth conditions, we obtain high-resolution structures for three different rapidly twisting polymorphs. Although their cross-over distances are similar, the three rapidly twisting polymorphs differ in twist handedness, symmetry, molecular conformations, and intermolecular contacts. Two of the rapidly twisting polymorphs resemble Aβ40 polymorphs with longer cross-over distances that have been described previously, including polymorphs extracted from brain tissue of patients with Alzheimer's disease or created by seeded growth from amyloid in brain tissue, but have shorter conformationally ordered segments and other specific conformational differences. These results contribute to our understanding of amyloid polymorphism, connections between morphology and molecular structure, and relationships between brain-derived and in-vitro-grown fibrils.