Introducing appropriate artificial components into natural biological systems could enrich the original functionality. To expand the available wavelength range of photosynthetic bacterial light-harvesting complex 2 (LH2 from Rhodopseudomonas acidophila 10050), artificial fluorescent dye (Alexa Fluor 647: A647) was covalently attached to N- and C-terminal Lys residues in LH2 α-polypeptides with a molar ratio of A647/LH2 ≃ 9/1. Fluorescence and transient absorption spectroscopies revealed that intracomplex energy transfer from A647 to intrinsic chromophores of LH2 (B850) occurs in a multiexponential manner, with time constants varying from 440 fs to 23 ps through direct and B800-mediated indirect pathways. Kinetic analyses suggested that B800 chromophores mediate faster energy transfer, and the mechanism was interpretable in terms of Förster theory. This study demonstrates that a simple attachment of external chromophores with a flexible linkage can enhance the light harvesting activity of LH2 without affecting inherent functions of energy transfer, and can achieve energy transfer in the subpicosecond range. Addition of external chromophores, thus, represents a useful methodology for construction of advanced hybrid light-harvesting systems that afford solar energy in the broad spectrum.

An oxygen-evolving photosynthetic reaction center complex (PSII) was adsorbed into nanopores in SBA, a mesoporous silica compound. We purified the dimer of PSII complex from a thermophilic cyanobacterium, Thermosynechococcus vulcanus, which grows optimally at 57 °C. The thermally stable PSII dimeric complex has a diameter of 20 nm and a molecular mass of 756 kDa and binds more than 60 chlorophylls. The SBA particles, with average internal pore diameters of 15 nm (SBA(15)) and 23 nm (SBA(23)), adsorbed 4.7 and 15 mg of PSII/g SBA, respectively. Measurement with a confocal laser-scanning microscope indicated the adsorption of PSII to the surface and the inner space of the SBA(23) particles, indicating the adsorption of PSII into the 23 nm silica nanopores. PSII did not bind to the inner pores of SBA(15). PSII bound to SBA(23) showed the high and stable activity of a photosynthetic oxygen-evolving reaction, indicating the light-driven electron transport from water to the quinone molecules added in the outer medium. The PSII-SBA conjugate can be a new material for photosensors and artificial photosynthetic systems.

Photosystem II (PSII) is an enzyme that performs efficient light-driven water oxidation to provide electrons necessary for CO 2 fixation in photosynthesis. In this study, we have for the first time generated PSIIÀgold nanoparticle (GNP) conjugates dispersed in a solution aiming at applications in artificial photosynthesis. PSII core complexes from the thermophilic cyanobacterium Thermosynechococcus elongatus, in which a His-tag was introduced into the C-terminus of CP47, were immobilized on GNPs with a 20 nm diameter via nickel-nitrilotriacetic acid, orienting the electron acceptor side to the gold surface. Optical analysis showed that four to five PSII dimers are bound to a single GNP, which was confirmed by transmission electron microscopy. The PSII immobilized on GNP retained O 2 evolution activity comparable to that of free PSII. The PSIIÀGNP conjugate will be a useful nanodevice for the development of artificial systems for light-driven water splitting into O 2 and H 2 .

Time-resolved fluorescence spectra of photosystem I (PS-I) trimeric complex isolated from a thermophilic cyanobacterium, Thermosynechococcus (T.) elongatus, were observed at 15 K over the time range from 100 fs to a few nanoseconds under P700-oxidized condition and 10 ps to a few nanoseconds under P700-reduced condition. Global-fitting analysis of the data of P700-oxidized condition revealed the existence of three kinetically different red chlorophylls (Chls) having the energy-transfer times to P700(+) of 6.1 ps (C(6.1 ps)), 140 ps (C(140 ps)), and 360 ps (C(360 ps)). According to the spectral shape of DAS, C(6.1 ps), C(140 ps), and C(360 ps) were assigned to the previously reported red Chls with the absorption maxima at 715 nm (C715), 710 nm (C710), and 719 nm (C719), respectively. In PS-I containing P700(+), ca. 60 Chls funnel the excitation energy into C(6.1 ps) in a subpicosecond time region at 15 K. The analysis of the present data together with the conclusions of the previous reports revealed that in PS-I containing a neutral P700 the direct energy transfer from the bulk Chls to P700 seems to dominate the energy-flow process. Simulation of the energy-transfer time to P700(+) based on Forster theory suggested the dimeric Chls A32-B7 and A33-A34 as the most probable candidates for C(140 ps) (C710) and C(360 ps) (C719), respectively. C(6.1 ps) (C715) was tentatively assigned to the dimeric Chl B24-B25 or A26-A27, for which the fastest energy transfer to P700(+) was predicted from the simulation. However, the estimated energy-transfer times to P700(+) for these dimeric Chls were 44-46 ps, which were still much slower than the observed value of 6.1 ps. A theoretical framework beyond the standard Forster theory might be required in order to account for the severe deviation.

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, PQ) of LH1-RC (from Rhodopseudomonas palustris) and enhances its activity in photocurrent generation. Steady-state fluorometric analysis demonstrated that PG enhances the formation of the PQ 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. 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.

Hydrogenases are powerful catalysts for light-driven H2 production using a combination of photosensitizers. However, except oxygen-tolerant hydrogenases, they are immediately deactivated under aerobic conditions. We report a light-driven H2 evolution system that works stably even under aerobic conditions. A [NiFe]-hydrogenase from Desulfovibrio vulgaris 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 H2 with an efficiency of 3.7 μmol H2 m(-2) s(-1). The PGP system proposed in this work represents a promising first step toward the development of an O2-tolerant solar energy conversion system.

We designed novel peptide gemini surfactants (PG-surfactants), DKDKC12K and DKDKC12D, which can solubilize Photosystem I (PSI) of Thermosynecoccus elongatus and Photosystem II (PSII) of Thermosynecoccus vulcanus in an aqueous buffer solution. To assess the detailed effects of PG-surfactants on the original supramolecular membrane protein complexes and functions of PSI and PSII, we applied the surfactant exchange method to the isolated PSI and PSII. Spectroscopic properties, light-induced electron transfer activity, and dynamic light scattering measurements showed that PSI and PSII could be solubilized not only with retention of the original supramolecular protein complexes and functions but also without forming aggregates. Furthermore, measurement of the lifetime of light-induced charge-separation state in PSI revealed that both surfactants, especially DKDKC12D, displayed slight improvement against thermal denaturation below 60 °C compared with that using β-DDM. This degree of improvement in thermal resistance still seems low, implying that the peptide moieties did not interact directly with membrane protein surfaces. By conjugating an electron mediator such as methyl viologen (MV(2+)) to DKDKC12K (denoted MV-DKDKC12K), we obtained derivatives that can trap the generated reductive electrons from the light-irradiated PSI. After immobilization onto an indium tin oxide electrode, a cathodic photocurrent from the electrode to the PSI/MV-DKDKC12K conjugate was observed in response to the interval of light irradiation. These findings indicate that the PG-surfactants DKDKC12K and DKDKC12D provide not only a new class of solubilization surfactants but also insights into designing other derivatives that confer new functions on PSI and PSII.

Rubrobacter xylanophilus rhodopsin (RxR) is a phylogenetically distinct and thermally stable seventransmembrane protein that functions as a light-driven proton (H +) pump with the chromophore retinal. to characterize its vectorial proton transport mechanism, mutational and theoretical investigations were performed for carboxylates in the transmembrane region of RxR and the sequential proton transport steps were revealed as follows: (i) a proton of the retinylidene Schiff base (Lys209) is transferred to the counterion Asp74 upon formation of the blue-shifted M-intermediate in collaboration with Asp205, and simultaneously, a respective proton is released from the proton releasing group (Glu187/Glu197) to the extracellular side, (ii) a proton of Asp85 is transferred to the Schiff base during M-decay, (iii) a proton is taken up from the intracellular side to Asp85 during decay of the red-shifted o-intermediate. this ion transport mechanism of RxR provides valuable information to understand other ion transporters since carboxylates are generally essential for their functions.