Fundamental questions about the relative arrangement of the beta-sheet arrays within amyloid fibrils remain central to both its structure and the mechanism of self-assembly. Recent computational analyses suggested that sheet-to-sheet lamination was limited by the length of the strand. On the basis of this hypothesis, a short seven-residue segment of the Alzheimer's disease-related Abeta peptide, Abeta(16-22), was allowed to self-assemble under conditions that maintained the basic amphiphilic character of Abeta. Indeed, the number increased over 20-fold to 130 laminates, giving homogeneous bilayer structures that supercoil into long robust nanotubes. Small-angle neutron scattering and X-ray scattering defined the outer and inner radii of the nanotubes in solution to contain a 44-nm inner cavity with 4-nm-thick walls. Atomic force microscopy and transmission electron microscopy images further confirmed these homogeneous arrays of solvent-filled nanotubes arising from a flat rectangular bilayer, 130 nm wide x 4 nm thick, with each bilayer leaflet composed of laminated beta-sheets. The corresponding backbone H-bonds are along the long axis, and beta-sheet lamination defines the 130-nm bilayer width. This bilayer coils to give the final nanotube. Such robust and persistent self-assembling nanotubes with positively charged surfaces of very different inner and outer curvature now offer a unique, robust, and easily accessible scaffold for nanotechnology.

Direct electron detectors have made it possible to generate electron density maps at near atomic resolution using cryo-electron microscopy single particle reconstructions. Critical current questions include how best to build models into these maps, how high quality a map is required to generate an accurate model, and how to cross-validate models in a system independent way. We describe a modeling approach that integrates Monte Carlo optimization with local density guided moves, Rosetta all-atom refinement, and real space B-factor fitting, yielding accurate models from experimental maps for three different systems with resolutions 4.5 Å or higher. We characterize model accuracy as a function of data quality, and present a model validation statistic that correlates with model accuracy over the three test systems.

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