Evaluation of the photosynthetic reaction center protein for potential use as a bioelectronic circuit element

The characterization of a bioelectronic composite prepared by molecular wiring of a bacterial photosynthetic reaction center (RC) to a metal (Au) electrode is described. Two unique attachment sites on the protein surface were studied as sites for electrical connections--a polyhistidine tag introduced by site-directed mutagenesis and a native cysteine amino acid residue. These two attachment sites were evaluated independently and found to serve effectively in coupling the protein to the electrode surface asymmetrically. Cyclic voltammetry (CV) was used to monitor protein integrity and confirm protein chemisorption and orientation to the organofunctionalized gold electrode. Single-protein transport measurements made with conductive atomic force microscopy (C-AFM) were used to study the electrical transport. Current-voltage (I-V) curves obtained by wiring the protein at the polyhistidine tag showed diodelike behavior. The cysteine attachment site does not serve as an efficient means to address the protein electrically. Scanning tunneling spectroscopy (STS) performed on RCs coupled at the donor side under both dark- and white-light-illuminated conditions confirmed the C-AFM studies.