Photocurrent and Electronic Activities of Oriented-His-Tagged Photosynthetic Light-Harvesting/Reaction Center Core Complexes Assembled onto a Gold Electrode
2012
Kondo, Masaharu | Iida, Kouji | Dewa, Takehisa | Tanaka, Hirofumi | Ogawa, Takuji | Nagashima, Sakiko | Nagashima, Kenji V. P. | Shimada, Keizō | Hashimoto, Hideki | Gardiner, Alastair T. | Cogdell, Richard J. | Nango, Mamoru
A polyhistidine (His) tag was fused to the C- or N-terminus of the light-harvesting (LH1)-α chain of the photosynthetic antenna core complex (LH1-RC) from Rhodobacter sphaeroides to allow immobilization of the complex on a solid substrate with defined orientation. His-tagged LH1-RCs were adsorbed onto a gold electrode modified with Ni-NTA. The LH1-RC with the C-terminal His-tag (C-His LH1-RC) on the modified electrode produced a photovoltaic response upon illumination. Electron transfer is unidirectional within the RC and starts when the bacteriochlorophyll a dimer in the RC is activated by light absorbed by LH1. The LH1-RC with the N-terminal His-tag (N-His LH1-RC) produced very little or no photocurrent upon illumination at any wavelength. The conductivity of the His-tagged LH1-RC was measured with point-contact current imaging atomic force microscopy, indicating that 60% of the C-His LH1-RC are correctly oriented (N-His 63%). The oriented C-His LH1-RC or N-His LH1-RC showed semiconductive behavior, that is, had the opposite orientation. These results indicate that the His-tag successfully controlled the orientation of the RC on the solid substrate, and that the RC produced photocurrent depending upon the orientation on the electrode.
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