Peptide concentration alters intermediate species in amyloid β fibrillation kinetics

► Aβ(1-40) aggregation in vitro has been monitored at different concentrations. ► Aβ(1-40) fibrillation does not always follow conventional kinetic mechanisms. ► We demonstrate non-linear features in the kinetics of Aβ(1-40) fibril formation. ► At high Aβ(1-40) concentrations secondary processes dictate fibrillation speed. ► Intermediate species may play significant roles on final amyloid fibril development.