6 research outputs found
Electron Conduction and Photocurrent Generation of a Light-Harvesting/Reaction Center Core Complex in Lipid Membrane Environments
To reveal the structure–function
relationship of membrane
proteins, a membrane environment is often used to establish a suitable
platform for assembly, functioning, and measurements. The control
of the orientation of membrane proteins is the main challenge. In
this study, the electron conductivity and photocurrent of a light-harvesting/reaction
center core complex (LH1-RC) embedded in a lipid membrane were measured
using conductive atomic force microscopy (C-AFM) and photoelectrochemical
analysis. AFM topographs showed that LH1-RC molecules were well-orientated,
with their H-subunits toward the membrane surface. Rectified conductivity
was observed in LH1-RC under precise control of the applied force
on the probe electrode (<600 pN). LH1-RC embedded in a membrane
generated photocurrent upon irradiation when assembled on an electrode.
The observed action spectrum was consistent with the absorption spectrum
of LH1-RC. The control of the orientation of LH1-RC by lipid membranes
provided well-defined conductivity and photocurrent
Lipid-Controlled Stabilization of Charge-Separated States (P<sup>+</sup>Q<sub>B</sub><sup>–</sup>) and Photocurrent Generation Activity of a Light-Harvesting–Reaction Center Core Complex (LH1-RC) from <i>Rhodopseudomonas palustris</i>
The
photosynthetic light-harvesting–reaction center core
complex (LH1-RC) is a natural excitonic and photovoltaic device embedded
in a lipid membrane. In order to apply LH1-RCs as a biohybrid energy-producing
material, some important issues must be addressed, including how to
make LH1-RCs function as efficiently as possible. In addition, they
should be characterized to evaluate how many active LH1-RCs efficiently
work in artificial systems. We report here that an anionic phospholipid,
phosphatidylglycerol (PG), stabilizes the charge-separated state (a
photooxidized electron donor and reduced quinone pair, P<sup>+</sup>Q<sub>B</sub><sup>–</sup>) of LH1-RC (from <i>Rhodopseudomonas
palustris</i>) and enhances its activity in photocurrent generation.
Steady-state fluorometric analysis demonstrated that PG enhances the
formation of the P<sup>+</sup>Q<sub>B</sub><sup>–</sup> state
at lower irradiances. The photocurrent generation activity was analyzed
via Michaelis–Menten kinetics, revealing that 38% of LH1-RCs
reconstituted into the PG membrane generated photocurrent at a turnover
frequency of 46 s<sup>–1</sup>. PG molecules, which interact
with LH1-RC in vivo, play the role of an active effector component
for LH1-RC to enhance its function in the biohybrid system
Light-Driven Hydrogen Production by Hydrogenases and a Ru-Complex inside a Nanoporous Glass Plate under Aerobic External Conditions
Hydrogenases
are powerful catalysts for light-driven H<sub>2</sub> production using
a combination of photosensitizers. However, except
oxygen-tolerant hydrogenases, they are immediately deactivated under
aerobic conditions. We report a light-driven H<sub>2</sub> evolution
system that works stably even under aerobic conditions. A [NiFe]-hydrogenase
from <i>Desulfovibrio vulgaris</i> Miyazaki F was immobilized
inside nanoporous glass plates (PGPs) with a pore diameter of 50 nm
together with a ruthenium complex and methyl viologen as a photosensitizer
and an electron mediator, respectively. After immersion of PGP into
the medium containing the catalytic components, an anaerobic environment
automatically established inside the nanopores even under aerobic
external conditions upon irradiation with solar-simulated light; this
system constantly evolved H<sub>2</sub> with an efficiency of 3.7
μmol H<sub>2</sub> m<sup>–2</sup> s<sup>–1</sup>. The PGP system proposed in this work represents a promising first
step toward the development of an O<sub>2</sub>-tolerant solar energy
conversion system
Molecular Assembly of Zinc Chlorophyll Derivatives by Using Recombinant Light-Harvesting Polypeptides with His-tag and Immobilization on a Gold Electrode
LH1-α
and -β polypeptides, which make up the light-harvesting
1 (LH1) complex of purple photosynthetic bacteria, along with bacteriochlorophylls,
have unique binding properties even for various porphyrin analogs.
Herein, we used the porphyrin analogs, Zn-Chlorin and the Zn-Chlorin
dimer, and examined their binding behaviors to the LH1-α variant,
which has a His-tag at the C-terminus (MBP-rubα-YH). Zn-Chlorin
and the Zn-Chlorin dimer could bind to MBP-rubα-YH and form
a subunit-type assembly, similar to that from the native LH1 complex.
These complexes could be immobilized onto Ni-nitrilotriacetic acid-modified
Au electrodes, and the cathodic photocurrent was successfully observed
by photoirradiation. Since Zn-Chlorins in this complex are too far
for direct electron transfer from the electrode, a contribution of
polypeptide backbone for efficient electron transfer was implied.
These findings not only show interesting properties of LH1-α
polypeptides but also suggest a clue to construct artificial photosynthesis
systems using these peptide materials
Photocurrent and Electronic Activities of Oriented-His-Tagged Photosynthetic Light-Harvesting/Reaction Center Core Complexes Assembled onto a Gold Electrode
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
Immobilization and Photocurrent Activity of a Light-Harvesting Antenna Complex II, LHCII, Isolated from a Plant on Electrodes
A light-harvesting (LH) antenna complex II, LHCII, isolated
from
spinach was immobilized onto an indium tin oxide (ITO) electrode with
dot patterning of 3-aminopropyltriethoxysilane (APS) by utilizing
electrostatic interactions between the cationic surface of the electrode
and the anionic surface of stromal side of the LHCII polypeptide.
Interestingly, the illumination of LHCII assembled onto the ITO electrode
produced a photocurrent response that depends on the wavelength of
the excitation light. Further, LHCII was immobilized onto a TiO<sub>2</sub> nanostructured film to extend for the development of a dye-sensitized
biosolar cell system. The photocurrent measured in the iodide/tri-iodide
redox system of an ionic liquid based electrolyte on the TiO<sub>2</sub> system showed remarkable enhancement of the conversion efficiency,
as compared to that on the ITO electrode