氯化血红素分子对牛血清白蛋白电子传输能级的调控

Regulating Electron Transport Band Gaps of Bovine Serum Albumin by Binding Hemin

蛋白质分子的电子传输(ETp)性能,即导带(CB)和价带(VB)的能量差(带隙)是影响蛋白质电子器件性能的主要因素之一。因此,调控蛋白质ETp带隙是提高这些电子器件性能并扩展其应用领域的重要途径。本文报道一种通过外部分子结合调控蛋白质ETp带隙的方法。以氯化血红素(hemin)与牛血清白蛋白(BSA)结合为例,首先运用分子对接方法从理论上确定hemin分子能结合到BSA分子IIA域的疏水口袋中,位于Tpr213附近;然后实验(荧光光谱和吸收光谱)证实hemin与BSA结合后,能形成hemin-BSA复合物,并且没有改变BSA的原有结构;最后将hemin-BSA通过BSA分子表面Cys34的―SH固定在金电极表面,形成有序的分子层,研究其ETp性能;I–V结果表明,BSA表现出半导体的ETp特征,并且hemin的结合能使BSA的带隙由原来的~1.50±0.05e V降低到~0.93±0.05e V。本文的结果为调控蛋白质分子的ETp带隙提供了一种简单有效的方法,通过选择不同的结合分子能使蛋白质分子的带隙调控至所需要的范围,并且形成的蛋白质复合物还能用于各种电子器件的制作。

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(EGa In) 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 e V, respectively, being equally distributed around the Fermi level(0 e V), with a band gap of ~1.50 ± 0.05 e V. Following hemin binding, the CB(~0.64 ± 0.06 e V) and VB(~-0.29 ± 0.07 e V) of the protein were closer to the Fermi level, resulting in a band gap of ~0.93 ± 0.05 e V. 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.