The highly negatively charged C-terminal and slightly positively charged N-terminal of α-synuclein fibrils are suspected to decorate the fibril surface as a polymer brush . The grafting of charged polymer brushes to colloids increases their colloidal stability against association . It has been shown that the higher order association of α-synuclein amyloid fibrils strongly depends on pH and ionic strength . Furthermore, electrostatic and steric repulsion between the fibril surface and monomer at neutral pH are suspected to largely prevent monomer-dependent secondary nucleation on the fibril surface . Upon shaving of the fibrils, repulsion between fibrils decreases, which results in a decrease in colloidal stability as demonstrated here. Furthermore, the electrostatic and steric repulsion between the fibril surface and monomer should decrease. However, we find that the primary signal related to mass addition at pH 7.4 originates from fibril elongation, independently of the presence of the fuzzy coat. Monomers remain highly charged and binding to the fibril surface may still be limited by electrostatic repulsion between the monomers. In accordance, we find that at pH 5.5, close to the isoelectric point and where secondary nucleation has been demonstrated to play an important role , removal of the fuzzy coat indeed leads to increased monomer binding. We therefore find no clear evidence that removing the fuzzy coat of the fibril at pH 7.4 enables significant monomer binding to the fibril surface. Furthermore, we do not observe continued acceleration of mass addition rates to the sensor surface during long incubation periods with monomer solutions post-shaving. If the increase in the number of growing fibril ends were caused by monomerdependent secondary nucleation, new fibrils ends should constantly be formed during incubation with monomer and therefore an acceleration, rather than a slowing down of mass deposition would be expected. Instead, it appears that new fibril ends are only created during the incubation with pK. Thus, the experimental results combined with the kinetic models suggest that the proteolytic modification of fibrils can act as a source of secondary growth sites. Monomer-dependent secondary nucleation of α-synuclein is strongly enhanced at mildly acidic pH , restraining efficient amplification of fibrils by secondary pathways to environments such as endosomes and lysosomes . However, our findings, that secondary reactions may occur at the surface of proteolytically modified α-synuclein fibrils even at neutral pH, suggests an alternative pathway for amyloid amplification in the cellular environment. Since the QCM-D instrument is a surfacebased biosensor, the detected accelerated rate of mass addition can only originate from material anchored to the surface. Hence, any secondary fibrils must adhere to the surface of the sensor in order to be detected. As the sensor surface itself has been passivated with a self-assembled monolayer of PEG, the newly formed fibril ends are likely to adhere to the surfaces of the initial population of seed fibrils. It follows that secondary fibrils detected here must effectively be anchored to the hydrophobic core of the shaven fibrils by non-covalent forces without detaching. This observation is similar to findings in Aβ 42, where daughter fibrils formed through secondary nucleation have been observed to adhere to the fibril surface for extended periods of time .