Photocurrent and Electronic
Activities of Oriented-His-Tagged
Photosynthetic Light-Harvesting/Reaction Center Core Complexes Assembled
onto a Gold Electrode
- Publication date
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Abstract
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 <i>a</i> 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