Fibril samples equilibrated in 72 hours at different concentrations of urea were ultracentrifuged (180,000 x g, 1 h, 25 °C) to pellet the aggregated fraction. The supernatant was carefully collected to an Eppendorf tube and the concentration of the residual monomer determined by UV absorption using a NanoDrop instrument (ThermoFisher, USA) and the extinction coefficients of the respective variants (WT or F94W).