Shigeichi Kumazaki

Pump—probe spectroscopy and exciton dynamics of J aggregates at high pump intensities

Abstract We have measured the picosecond pump—probe spectra of BIC J aggregates using a spectral continuum for the probe. The excitation pulses were intense (10 15 −2×10 16 photons/cm 2 ) and created many excitons per aggregate. The observed dynamics are intensity dependent and reflect exciton—exciton annihilation processes. In addition, the zero-time spectrum is also intensity dependent, probably due to a reduction of k space at high exciton concentrations.

Energy equilibration and primary charge separation in chlorophyll d-based photosystem I reaction center isolated from Acaryochloris marina

Primary photochemistry in photosystem I (PS I) reaction center complex from Acaryochloris marina that uses chlorophyll d instead of chlorophyll a has been studied with a femtosecond spectroscopy. Upon excitation at 630 nm, almost full excitation equilibration among antenna chlorophylls and 40% of the excitation quenching by the reaction center are completed with time constants of 0.6(±0.1) and 4.9(±0.6) ps, respectively. The rise and decay of the primary charge‐separated state proceed with apparent time constants of 7.2(±0.9) and 50(±10) ps, suggesting the reduction of the primary electron acceptor chlorophyll (A0) and its reoxidation by phylloquinone (A1), respectively.

Changes in Antenna Sizes of Photosystems during State Transitions in Granal and Stroma-Exposed Thylakoid Membrane of Intact Chloroplasts in Arabidopsis Mesophyll Protoplasts

In chloroplasts of plants and algae, state transition is an important regulatory mechanism to maintain the excitation balance between PSI and PSII in the thylakoid membrane. Light-harvesting complex II (LHCII) plays a key role as the regulated energy distributor between PSI and PSII. It is widely accepted that LHCII, which is bound to PSII localized mainly in the granal thylakoid, migrates to bind with PSI localized mainly in the stroma-exposed thylakoid under preferential excitation of PSII. The phenomena have been extensively characterized by many methods. However, the exchange of LHCII between PSII and PSI has not been directly observed in vivo at physiological temperatures. Herein we applied fluorescence spectromicroscopy to Arabidopsis mesophyll protoplasts in order to observe in vivo changes in fluorescence spectra of granal and stromal thylakoid regions during the state transition. The microscopic fluorescence spectra obtained from a few sections with different depths were decomposed into PSI and PSII spectra and self-absorption effects were removed. We were able to determine amplitude changes of PSI and PSII in fluorescence spectra solely due to state transition. Subdomain analysis of granal and stromal thylakoid regions clarified variant behaviors in the different regions.

Actively and passively mode-locked Nd:YAP(YAlO3) laser with negative feedback using CdSe and GaAs

The feedback-controlled mode-locking operation of a flash-lamp-pumped Nd: YAP(YAlO3) laser using a CdSe passive element is reported for the first time. Pulse trains with a duration of 1000 ns, single pulse width of 5 ps and pulse energy of 10 µJ are generated. The performances of CdSe and GaAs negative feedback elements are compared.

Characterization of thylakoid membrane in a heterocystous cyanobacterium and green alga with dual-detector fluorescence lifetime imaging microscopy with a systematic change of incident laser power.

Fluorescence Lifetime Imaging Microscopy (FLIM) has been applied to plants, algae and cyanobacteria, in which excitation laser conditions affect the chlorophyll fluorescence lifetime due to several mechanisms. However, the dependence of FLIM data on input laser power has not been quantitatively explained by absolute excitation probabilities under actual imaging conditions. In an effort to distinguish between photosystem I and photosystem II (PSI and PSII) in microscopic images, we have obtained dependence of FLIM data on input laser power from a filamentous cyanobacterium Anabaena variabilis and single cellular green alga Parachlorella kessleri. Nitrogen-fixing cells in A. variabilis, heterocysts, are mostly visualized as cells in which short-lived fluorescence (≤0.1 ns) characteristic of PSI is predominant. The other cells in A. variabilis (vegetative cells) and P. kessleri cells show a transition in the status of PSII from an open state with the maximal charge separation rate at a weak excitation limit to a closed state in which charge separation is temporarily prohibited by previous excitation(s) at a relatively high laser power. This transition is successfully reproduced by a computer simulation with a high fidelity to the actual imaging conditions. More details in the fluorescence from heterocysts were examined to assess possible functions of PSII in the anaerobic environment inside the heterocysts for the nitrogen-fixing enzyme, nitrogenase. Photochemically active PSII:PSI ratio in heterocysts is tentatively estimated to be typically below our detection limit or at most about 5% in limited heterocysts in comparison with that in vegetative cells.

Primary processes in plant photosynthesis: photosystem I reaction center

The photosystem I (PSI) pigment-protein complex of plants converts light energy into a transmembrane charge separation, which ultimately leads to the reduction of carbon dioxide. Recent studies on the dynamics of primary energy transfer, charge separation, and following electron transfer of the reaction center (RC) of the PSI prepared from spinach are reviewed. The main results of femtosecond transient absorption and fluorescence spectroscopies as applied to the P700-enchied PSI RC are summarized. This specially prepared material contains only 12–14 chlorophylls per P700, which is a special pair of chlorophyll a and has a significant role in primary charge separation. The P700-enriched particles are useful to study dynamics of cofactors, since about 100 light-harvesting chlorophylls are associated with wild PSI RC and prevent one from observing the elementary steps of the charge separation. In PSI RC energy and electron transfer were found to be strongly coupled and an ultrafast up-hill energy equilibration and charge separation were observed upon preferential excitation of P700. The secondary electron-transfer dynamics from the reduced primary electron acceptor chlorophyll a to quinone are described. With creating free energy differences (ΔG0) for the reaction by reconstituting various artificial quinones and quinoids, the rate of electron transfer was measured. Analysis of rates versus ΔG0 according to the quantum theory of electron transfer gave the reorganization energy, electronic coupling energy and other factors. It was shown that the natural quinones are optimized in the photosynthetic protein complexes. The above results were compared with those of photosynthetic purple bacteria, of which the structure and functions have been studied most.

Excitation Wavelength Dependence of the Excitation Transfer in Photosystem I Reaction Center With Reduced Number of Antenna Chlorophylls

The most recent X-ray analysis of the crystal structure of Photosystem I (PS I) has shown six chlorophyll a molecules (Chl) in the electron transfer system [1]. There seem to be two pathways of the electron transfer: P700→A→A0 and P700→A’→A0’. It is not yet clarified which pathway is used nor how A and/or A’ are involved in the electron transfer. This situation is analogous to those of purple bacterial reaction centers (RC). One unique feature of PS I RC is that there are two “connecting” Chls which structurally link core antenna Chls to the electron transfer system [1]. Main excitation transfer pathway might be like: core antenna→ “connecting” Chl→A0→A→P700 and core antenna→ “connecting” Chl’→A0’→A’→P700. It is thus important to characterize the spectral properties and the excitation transfer processes associated with the “connecting” Chls, A, A’, A0 and A0’. This is, however, an extremely difficult task, because there are ≈ 100 core antenna Chls which mask the excitation transfer steps.

Unidirectional Electron Transfer in Chlorophyll d -Containing Photosystem I Reaction Center Complex of Acaryochloris marina

The purified photosystem I (PS I) reaction center complex of a cyanobacterium Acaryochloris marina contained 88 Chl d: 1.1 Chl a: 19 carotenoids: 2.0 phylloquinone. Amino acid sequences of PsaA and PsaB polypeptides were almost homologous to those in the other cyanobacteria. The ligands for A0 was Met698A and that for A0′ was Leu688B but not Met that is conserved in all the other PS I. Laser excitation induced the 10-ps bleach and the 40- ps recovery at 680 nm of Chl a-type pigment in parallel with the 49-ps bleach of the Chl d-dimer P740 at 740 nm. The results indicate that A0 is Chl a-680 ligated by Met698A and that the PsaB branch with Leu688B is inactive for the electron transfer. The PS I of A. marina, thus, is the unique asymmetric type I reaction center with the unidirectional electron transfer pathway only through PsaA branch.

Self-Defocusing of Mode-Locked Nd:YAG Laser Radiation in GaAs, CdSe and InP

Defocusing in GaAs, CdSe and InP of 25 ps pulse trains from an active-passive mode-locked flashlamp pumped Nd:YAG laser at 1.064 µm is investigated. The distortion of the train envelope and of the spatial beam profile induced by charge carriers created by two-photon absorption are observed at light intensities of 50-100 MW/cm2 for GaAs and CdSe and 25 MW/cm2 for InP. This different behavior is indicative of the different response of InP which is found to be not useful for passive negative feedback of mode-locked Nd:YAG lasers.

Relaxation processes of photoexcited carriers in GaAs/AlAs multiple quantum well structures grown by molecular beam epitaxy at low temperatures

The relaxation processes of photoexcited carriers in GaAs/AlAs multiple quantum well structures grown at low temperatures by molecular beam epitaxy were studied by a tunable single-beam femtosecond pump–probe method. Concentrations of singularly ionized antisite arsenic ions, AsGa+, in the quantum wells, which were considered as traps of photoexcited carriers, were estimated from flux conditions and substrate temperatures in the growth. Transient transmittivity of the structures were measured by varying the pump–probe photon energy. The trapping rate of photoexcited carriers, which corresponded to the reciprocal of the carrier lifetime, was derived from the relaxation profile at the pump–probe photon energy close to the exciton resonant excitation energy for each structure. The trapping rate was found to increase linearly with AsGa+ in a lower concentration range and superlinearly in a higher concentration range. Photoluminescence and absorption spectra were observed at room temperature and their correlat...