Evaluating Effect of Metallic Ions On Aggregation Behavior of Amyloid Aβ42 By AFM Imaging and SERS

Abstract

Background: Excessive aggregation of β-amyloid peptides (Aβ) is regarded as the hallmark of Alzheimer’s disease. Exploring the underlying mechanism regulating Aβ aggregation remains challenging and investigating aggregation events of Aβ in the presence and absence of metal ions at molecular level would be meaningful in elucidating the role of metal cations on interactions between Aβ molecules. In this study, chemical self-assembled monolayer (SAM) method was employed to fabricate monolayer of β-amyloid peptides Aβ42 on gold substrate with a bolaamphiphile named 16-Mercaptohexadecanoic acid (MHA). Firstly, the samples of gold substrate (blank control), the MHA-modified substrate and the Aβ42-modified substrate were detected by X-ray photoelectron spectroscopy (XPS) to track the self-assembly process. Aggregation behaviors of Aβ42 before and after metallic ions (Zn2+、Ca2+、Al3+) treated were monitored by atomic force microscopy (AFM) and the interaction between Aβ42 and metallic ions (Zn2+、Ca2+、Al3+) was investigated by surface-enhanced Raman Scattering (SERS), respectively.Results: The XPS spectra of binding energy of gold substrate (blank control), the MHA-modified substrate and the Aβ42-modified substrate are well fitted with the corresponding monolayer’s composition, which indicates that Aβ42 monolayer is well formed. The recorded surface morphology of different experimental groups obtained by AFM showed markedly different nanostructures, indicating occurrence of aggregation events between Aβ42 molecules after adding metal ions to the solution. Compared to the control group, the presence of metal ions resulted in the increased size of surface structures on the observed 3D topography. Further study by SERS showed that the Raman strength of Aβ42 changes significantly after the metal cation treatment. A considerable part of the amide bonds interacts with metal cations, leading to a structural change, which is characterized by the weakened β-fold Raman peak.Conclusion: The AFM imaging results suggest that aggregation events occurred between Aβ42 molecules with the addition of metal cations. Furthermore, the effect of metallic cations on the conformational change of Aβ42 studied by SERS supported the results obtained by AFM imaging. Taken together, the results showed that the presence of substoichiometric metal cations promotes aggregation behavior between Aβ42 molecules on the substrate at pH 7.4.