Fig. 1.

Schematic illustration of the cross-β structure of amyloid fibrils. a Experimental setup for X-ray fiber diffraction. The collimators filter the stream of X-rays so that only those traveling parallel to the specified direction are allowed through. When the fibril sample is exposed to the X-ray beam with the long axis of the fibrils more or less perpendicular to the direction of the beam, the meridional reflections are defined as those lying parallel to the fibril axis, and the equatorial reflections are those positioned at right angles to the fibril axis. b The X-ray fiber diffraction pattern is often interpreted as shown in the schematic illustration of stacked β-sheets with an interstrand distance of 4.8 Å and an intersheet distance of ~11 Å. c, d The strongest repeating feature of the fibril structure is β-strands running perpendicular to the fibril axis with an interstrand distance of 4.8 Å in the fibril direction [96, 97]. The adjacent chains forming the β-sheet have been proposed to be aligned as either parallel (c) or antiparallel (d) protein strands [97, 164–168]. The secondary structures are primarily stabilized by hydrogen bonding between the amino- and carbonyl groups of the main chain(s) (···) [169]. In β-sheets, the amino acid side groups (R) are directed orthogonal to the sheet plane and systematically decorate both surfaces [165]