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.

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.

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 .