Figure 3. PolyQ peptide fibril structure.

(A) Amino acid sequence of D₂Q₁₅K₂ peptide (top), 2D ¹³C–¹³C ssNMR spectrum (middle) of D₂Q₁₅K₂ fibrils with single ¹³C-labeled Gln (Q6), and negatively-stained TEM of the peptide aggregates (bottom). The ssNMR data adapted from Ref. 8. Signals from type ”a” and ”b” Gln conformers are shown in red and blue boxes, respectively. (B) 3D cartoon representation of the structural model of the D₂Q₁₅K₂ fibril. The alternation of β-strands of ”a” (red) and ”b” (blue) Gln conformers within a single sheet is shown on the right. (C) The χ₁, χ₂, and ψ dihedral angle distributions of “a” (left) and “b” (right) conformers in the context of the D₂Q₁₅K₂ fibril for the M1 (orange) and M2 (green) models; dashed vertical lines represent mean values. The data were obtained from 1-μs MD simulations (using the OPLSAA/M⁴⁶ force field; for Amber14SB⁴⁵ see Supplementary Fig. S10). Gray shading depicts the ssNMR constraints for the dihedral angles. The structures at the center show representative “a” (top) and “b” (bottom) conformers of the M1 and M2 models.