To further validate the models, we examined whether the simulated fibrils were structurally stable. We tested this by removing the structural constraints on the fibril structures and then running additional simulations. The unconstrained structures held together during the simulation production runs and did not dissociate. The C--O bond distributions (Figure ) of the unconstrained structures are bimodal, exhibiting peaks around ±10-15°from the fibril axes, in good agreement with our experimental data. Similarly, the median values for the ψangle distributions (Figure ) are close to our experimental measurements. The distribution of amylin 20-29 polymorph 1 is unimodal and peaked near 147°. In contrast, the distributions for amylin 20-29 polymorph 2 and Aβ 25-35 appear bimodal, exhibiting a main peak around 139°(close to experimental measurements) and a smaller peak that is downshifted to ca. 120°. This indicates that the fibril structures corresponding to amylin 20-29 polymorph 2 and Aβ 25-35 have both slightly evolved compared to the original constrained structures. The reason for these discrepancies is likely due to the limitations of our simulations since we could only model a single protofilament segment. We hypothesize that the structures we simulated are more conformationally flexible (thus allowing the ψ-angle distributions to evolve) since they are not subjected to lattice packing forces of multiple protofilaments that stabilize real amyloid fibrils.