Abstract:

The small size (nanoscale) of proteins and their favorable electron transport (ETp) properties make them suitable for various types of bioelectronic devices and offer a solution for miniaturizing these devices to nanoscale dimensions. The performance of protein-based devices is predominantly affected by the ETp property of the proteins, which is largely determined by the band gaps of the proteins, i.e., the energy difference between the conduction band (CB) and valence band (VB). Regulating the protein ETp band gaps to appropriate values is experimentally demanding and hence remains a significant challenge. This study reports a facile method for modulating the ETp band gaps of bovine serum albumin (BSA), via its binding with a foreign molecule, hemin. The formation of the hemin-BSA complex was initially confirmed by theoretical simulation (molecular docking) and experimental characterization (fluorescence and absorption spectra), which indicated that the hemin is positioned inside a hydrophobic cavity formed by hydrophobic amino acid residues and near Trp213, at subdomain IIA of BSA, with no significant effects on the structure of BSA. Circular dichroism (CD) spectra indicated that the BSA conformation remains essentially unaltered following the formation of the hemin-BSA complex, as the helicities of the free BSA (non-binding) and the hemin-BSA complex were estimated to be 66% and 65%, respectively. Moreover, this structural conformation remains preserved after the hemin-BSA complex is immobilized on the Au substrate surface. The hemin-BSA complex is immobilized onto the Au substrate surface along a single orientation, via the ―SH group of Cys34 on the protein surface. Atomic force microscopy (AFM) images indicate that hemin-BSA forms a dense layer on the surface of the Au substrate with a lateral size of ~3.2‒3.7 nm, which is equivalent to the actual size of BSA, ~4.0 nm × 4.0 nm × 14.0 nm. The current-voltage (I-V) responses were measured using eutectic gallium-indium (EGaIn) as the top electrode and an Au film as the substrate electrode, revealing that the ETp processes of BSA and hemin-BSA on the Au surface have distinct semiconducting characteristics. The CB and VB were estimated by analysis of the differential conductance spectra, and for the free BSA, they were ~0.75 ± 0.04 and ~ −0.75 ± 0.08 eV, respectively, being equally distributed around the Fermi level (0 eV), with a band gap of ~1.50 ± 0.05 eV. Following hemin binding, the CB (~0.64 ± 0.06 eV) and VB (~ −0.29 ± 0.07 eV) of the protein were closer to the Fermi level, resulting in a band gap of ~0.93 ± 0.05 eV. These results demonstrated that hemin molecules can effectively regulate ETp characteristics and the transport band gap of BSA. This methodology may provide a general approach for tuning protein ETp band gaps, enabling broad variability by the preselection of binding molecules. The protein and foreign molecule complex may further serve as a suitable material for configuring nanoscale solid-state bioelectronic devices.

Key words: Protein electron transport, Protein-based electronic device, Bovine serum albumin, Hemin