Finally, we applied our newly developed methodology to study how the solution conditions dictate the stability of WT αSyn fibrils (Table ). First, we selected two well characterized αSyn fibril polymorphs -fibrils (polymorph S) assembled under physiological conditions (50 mM Tris-HCl, pH 7.4, 150 mM KCl; i.e., salt condition), and ribbons (polymorph NS) assembled in the absence of salt (5 mM Tris-HCl pH 7.4, i.e., no salt condition) . We prepared the two fibril polymorphs using the established protocols and confirmed their morphology using AFM (Figure ). Fibrils of polymorph S are composed of two protofilaments (8.23 ± 1.17 nm height) that twist with an average pitch length of 204 ± 63 nm. In contrast, the ribbons were thinner (6.59 ± 0.87 nm height), had no detectable twist, and were often found in bundles (Figure ). Both polymorphs were sonicated and depolymerized using increasing concentrations of urea in (i) their original buffer (native conditions) and in (ii) buffer with salt to allow direct comparison of their stability. Interestingly, although the two polymorphs showed similar stability in their native buffers, polymorph NS was 6 kJ.mol -1 more stable than polymorph S when measured in the presence of salt (Figure ). Using ssNMR, it has been shown that polymorph NS is formed by regular long β-strands whereas polymorph S has an irregular pattern of shorter β-strands . The higher stability of polymorph NS observed here can thus arguably reflect the stronger network of hydrogen bonds within its cross-β-sheet architecture and/or higher number of residues forming the amyloid core.