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ABSTRACT
We report investigations of the morphology and molecular structure of amyloid fibrils comprised of residues 10-40 of the Alzheimer's β-amyloid peptide (Aβ^sub 10-40^), prepared under various solution conditions and degrees of agitation. Omission of residues 1-9 from the full-length Alzheimer's β-amyloid peptide (Aβ^sub 1-40^) did not prevent the peptide from forming amyloid fibrils or eliminate fibril polymorphism. These results are consistent with residues 1-9 being disordered in Aβ^sub 1-40^ fibrils, and show that fibril polymorphism is not a consequence of disorder in residues 1-9. Fibril morphology was analyzed by atomic force and electron microscopy, and secondary structure and inter-side-chain proximity were probed using solid-state NMR. Aβ^sub 1-40^ fibrils were found to be structurally compatible with Aβ^sub 10-40^:Aβ^sub 1-40^ fibril fragments were used to seed the growth of Aβ^sub 10-40^ fibrils, with propagation of fibril morphology and molecular structure. In addition, comparison of lyophilized and hydrated fibril samples revealed no effect of hydration on molecular structure, indicating that Aβ^sub 10-40^ fibrils are unlikely to contain bulk water.
INTRODUCTION
Amyloid fibrils are filamentous protein aggregates that can be formed by a large and diverse class of polypeptides (1-3). Deposition of amyloid fibrils occurs in >20 diseases (4,5), including several that are major public health problems, such as Alzheimer's disease, type 2 diabetes, Parkinson's disease, and transmissible spongiform encephalopathies. Amyloid fibrils are distinguished from other types of protein fibrils by the presence of a cross-β structural motif, established by x-ray fiber diffraction (4). Other aspects of the molecular structures of amyloid fibrils have been largely mysterious until recent studies using solid-state NMR (6-12), electron paramagnetic resonance (13-15), hydrogen exchange (16-18), and biochemical methods (19-21). Data from these techniques have been used to develop experimentally based structural models for amyloid fibrils (8,10,18,20,22-24), including fibrils formed by the 40-residue β-amyloid peptide associated with Alzheimer's disease (Aβ^sub 1-40^) (8,20).
It has been demonstrated by electron microscopy (EM) and atomic force microscopy (AFM) that a single peptide or protein can form amyloid fibrils with several distinct morphologies (25-27). Recent solid-state NMR and EM studies of Aβ^sub 1-40^ fibrils have shown that different morphologies have somewhat different underlying molecular structures, that the morphology and molecular structure can be controlled by subtle variations in fibril growth conditions (at fixed temperature, buffer conditions,...