The structure was protonated in line with the acidic solution conditions used in the experiments. The structure was solvated, neutralized by chloride ions and energy minimized using steepest descent. Initially, the temperature was incrementally increased from 0 K to 300 K using the Berendsen thermostat . The fibril structure is not expected to be stable with only four subunits. To mimic the increased stability and structural order of long fibrils, we restrained all backbone atom positions with a potential of 100 kJ mol -1 nm -2 during equilibration. In the production simulations, bonds involving hydrogens were constrained with the LINCS algorithm , electrostatics were treated with the particle mesh Ewald scheme 71 with a short-range cutoff of 0.9 nm, and the van der Waals interaction was set to 0.9 nm. We ran production simulations at 300 K, maintained with the velocityrescaling thermostat and pressure of 1 atm, maintained with the Parrinello-Rahman barrostat .