Mechanically Controlled Electron Transfer in a Single-Polypeptide Transistor

Abstract

Proteins are of interest in nano-bio electronic devices due to their versatile structures, exquisite functionality and specificity. However, quantum transport measurements produce conflicting results due to technical limitations whereby it is difficult to precisely determine molecular orientation, the nature of the moieties, the presence of the surroundings and the temperature; in such circumstances a better understanding of the protein electron transfer (ET) pathway and the mechanism remains a considerable challenge. Here, we report an approach to mechanically drive polypeptide flip-flop motion to achieve a logic gate with ON and OFF states during protein ET. We have calculated the transmission spectra of the peptide-based molecular junctions and observed the hallmarks of electrical current and conductance. The results indicate that peptide ET follows an NC asymmetric process and depends on the amino acid chirality and α-helical handedness. Electron transmission decreases as the number of water molecules increases, and the ET efficiency and its pathway depend on the type of water-bridged H-bonds. Our results provide a rational mechanism for peptide ET and new perspectives on polypeptides as potential candidates in logic nano devices.

Figure 1. Chemical structures of the peptides.

Figure 2. I-V curve and conductance of the peptides.

(a) I-V curve of the peptide [Cys-Cysteamine]. A single molecular junction: peptide (O: red, N: blue, C: gray and H: light gray) was wired to the Au electrodes (yellow) through the interfacial S atom (brown). The source, scatter and drain components are the left electrode, the peptide and the right electrode, respectively. The curve of ^N[Cys-Cysteamine]^C (N ← C type, black square) and the experimental data (red square) are shown. In the positive (negative) V_bias region, the electron flow direction is denoted as . (b) The conductance G of the peptides [Cys-Cysteamine] and [Cys-Gly-Cysteamine]: our result (black circle) and the experimental data (red square). The structure was optimized at d_O–O = 5.0 Å because the peptide was stretched in the experiments.

Figure 3. Transmission analysis of R-L(Ala)₃.

(a) A single molecular junction. The notations are identical to those in the legend of Fig. 2. (b) TS at d_O–O: 1.70, 1.80, 1.92, 2.03, 2.42 and 2.88 Å. (c) Energy gap Δ versus d_O–O.

Figure 4. Transmission analysis of proline-based peptides.

(a) and (b) TS(D_e) intensity versus d_O–O for the peptides PPP, PKP, KPP, PPK, PRP, RPP and PPR. (c) and (d) Shift of the TS(D_e) peak versus d_O–O. (e) NBO charge of the ^CPPP^N versus d_O–O in the five regions of A, B, C, D and E. The notations of the molecular junction structure are the same as those in the legend of Fig. 2. The ΔQ versus the site of the peptide with (f) C → N type and (g) N ← C type.

Figure 5. Transmission analysis of the peptide SGS with water molecules.

(a) Plots of molecular junctions and water-bridged H-bond types, B (NHO), C (OHO), D (COHO) and E(OHO). The notations are identical to those in the legend of Fig. 2. A water molecule is shown in a ball-and-stick representation and color-coded by atom type. (b) TS(D_e) intensity versus the number of H₂O molecules. The system contains NHO-type water-bridges (colored in red), and the water-bridge types are indicated in parentheses. The number of water molecules dependence of ΔN is (c) SGS + H₂O, (d) SGS + 2 H₂O and (e) SGS + 3 H₂O.

Figure 6. Schematic three-state logic gate.

The tri-state buffer (left) is equivalent to a switch (right). There are ON and OFF states for the three-state logic gate. A and C are the junctions, and B is the gate.