Construction of Protein Probe with a His‐tag and an Electron‐transfer Peptide for a Target Protein Sensing

A protein probe with an electron‐transfer peptide and a His‐tag was designed to electrochemically sense a target protein. We selected tyrosine‐rich (Y 4 C) and tryptophan‐rich (W 4 C) peptides for use as electron‐transfer agents. The peak for oxidation was based on the oxidations of the phenolic hydroxy groups in Y 4 C and on the indole rings in W 4 C. Asialofetuin (ASF) with galactose residues was the protein probe, and a galactose recognition protein, soybean agglutinin (SBA), was the target protein. A protein probe composed of an amino acid and carbohydrate residue was expected to be biocompatible. When voltammetric measurements were performed using a glassy carbon electrode, the oxidation peaks of H 6 Y 4 C and ASF‐H 6 Y 4 C appeared at the same potential. The peak current of ASF‐H 6 Y 4 C was 4‐fold that of H 6 Y 4 C because of the stronger adsorption of ASF‐H 6 Y 4 C onto the electrode. The electrode response of ASF‐H 6 Y 4 C with SBA was half that of ASF‐H 6 Y 4 C alone. By contrast, the peak current of ASF‐Y 4 CH 6 was higher than that of ASF‐H 6 Y 4 C, which was the result of a greater degree of contact between the Y 4 C moieties and an electrode. On the other hand, the voltammetric behaviors of ASF with W 4 C and a His‐tag were similar to those with Y 4 C and a His‐tag. The sensitivity of SBA using ASF‐Y 4 CH 6 was at the 10 −13  M level. To confirm the function of the sensing system, measurements were performed in human serum with SBA and ASF‐Y 4 CH 6 . When SBA was added, the serum had a concentration that ranged between 5.0×10 −13 and 4.0×10 −12  M, and the amount of SBA that could be recovered ranged from 97 to 101%. Consequently, this system could be applied to the detection of SBA in serum.