Abstract
Assembled amyloid beta (Aβ) peptides have been considered pathological assemblies involved in human brain diseases, and the electron transfer or electron transport characteristics of Aβ are important for the formation of structured assemblies. Here, we report the electrical characteristics of surface-assembled Aβ peptides similar to those observed in Alzheimer’s patients. These characteristics correlate to their electron transfer characteristics. Electrical current–voltage plots of Aβ vertical junction devices show the Aβ sequence dependence of the current densities at both Aβ monomers (mono-Aβs) and Aβ oligomers (oli-Aβs), while Aβ sequence dependence is not clearly observed in the electrical characteristics of Aβ planar field effect transistors (FETs). In particular, surface oligomerization of Aβ peptides drastically decreases the activity of electron transfer, which presents a change in the electron transport pathway in the Aβ vertical junctions. Electron transport at oli-Aβ junctions is symmetric (tunneling/tunneling) due to the weak and voltage-independent coupling of the less redox-reactive oli-Aβ to the contacts, while that at mono-Aβ junctions is asymmetric (hopping/tunneling) due to redox levels of mono-Aβ voltage-dependently coupled with contact electrodes. Consequently, through vertical junctions, the sequence- and conformation-dependent electrical characteristics of Aβs can reveal their electron transfer activities.
Highlights
Electron transfer and electron transport processes of proteins are important for understanding and gaining insight into their biological functions such as enzymatic reactions[1,2]
A self-assembled molecular linker monolayer of 1-mercaptopropionic acid (MPA) was used to immobilize Aβ onto a gold electrode, which was activated by coupling reactions with 1-ethyl-3-(3-dimethylamino) propyl carbodiimide/N-hydroxysuccinimide (EDC/NHS) (Scheme 1)
The electron transfer of Aβ hardly proceeded after surface oligomerization; this observation was strongly related to a change in the electron transport pathway through Aβ
Summary
Electron transfer and electron transport processes of proteins are important for understanding and gaining insight into their biological functions such as enzymatic reactions[1,2]. It is difficult to predict the mechanism of electron transport in proteins due to the size of the protein containing a number of amino acid residues. In biomolecular junctions composed of target biomolecules incorporated between two electrodes, longrange electron transport processes through biomolecules were found to be strongly influenced by molecular size and coupling effects between molecules and electrodes[3,4]. Scheme 1 a The formation of an Aβ1–40 molecular layer via a 1-mercaptopropionic acid (MPA) linker monolayer. Terminal carboxylic acid (–COOH) groups of an MPA SAM were activated by EDC/NHS and turned to NHS-ester groups (denoted by a blue circle). The amino group of a lysine residue (Lys 16-NH2 denoted by a red circle) was coupled with the NHS-ester group to form an amide bond (denoted by a purple circle).
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