We found that the signal for the fibrils changed slightly with urea, however this change could not be directly modelled by the empirical fit using the asymmetric distribution (Eq. 7). Also, the amount of fibrils present in the signal was stochastically changing in each sample due to sticking, sampling-issues due to heterogeneous samples and sedimentation, etc. To limit the over-parameterization and correlation between parameters in the fit, which made the estimation of monomer unreliable, the global analysis therefore used a shared asymmetric distribution curve to account for the fibril signal in all samples, and only the total area of the fibril signal was an independent fit parameter for each individual measurement.