Search

Comparison of the steady-state FTIR absorption spectra of coumarin-102 (C-102) in tetrachloroethylene with added aniline of various concentrations, in neat aniline and in neat N,N-dimethylaniline (...

Excitation-wavelength dependence of the transient absorbance changes of photosystem I reaction center particles with a reduced number of antenna chlorophylls (16 chlorophylls/primary electron donor chlorophyll (P700)) hasbeen studied in an effort to understand the energy equilibration among the chlorophylls in the electron-transfer system. The photobleaching and stimulated-emission signals in the Q y band region of the chlorophylls upon the preferential excitation of chlorophyll spectral forms at the blue edge (662 nm) and red edge (697 nm) of the Q y band are analyzed. In the case of the red-edge excitation, spectral equilibration proceeds with a time constant of 0.34 (′0.07) ps, which is attributable to the energy equilibration between P700 and neighboring chlorophylls absorbing around 686 nm in the electron-transfer system. This equilibration seems to precede the fastest phase of the primary charge separation (apparent time constant of 0.8 ps) reported previously (Kumazaki et al. J. Phys. Chem. B 2001, 105, 1093). A slow decay of the excited states because of the slow phase of the primary charge separation proceeds with a time constant of 7.2 (′0.6) ps. In the case of the blue-edge excitation, vibrational relaxation and downhill energy transfer proceed with a time constant of 0.38 (′0.08) ps, which are followed by a slow downhill energy transfer from residual antenna chlorophylls to the electron-transfer system. Even with the slow energy transfer from the residual antenna chlorophylls to the electron-transfer system, the overall primary charge separation is completed with a time constant of 10 (′0.7) ps. These interpretations are in part supported by control experiments on chlorophyll a in ethanol under equivalent optical conditions. Implications of these results for understanding the primary processes in more intact photosystem I are discussed.

Spectral sensitization and supersensitization of the silver bromide nanocrystals were studied by the femtosecond fluorescence up-conversion technique. An annihilation-free fluorescence from J-aggregates of the Dye 1 (3,3‘-disulfopropyl-5,5‘-dichloro-9-ethylthiacarbocyanine) adsorbed on the silver bromide nanocrystals (edge-to-edge distance from 40 to 900 nm) of different shapes (cubic and octahedral) was measured. Very fast nonexponential fluorescence decays were observed with a fast component from 0.4 to 2.5 ps, and an average decay time from 1.1 to 5.5 ps, depending on the type and size of AgBr grains. The average fluorescence decay time is several times longer on the cubic grains than that on the octahedral grains. Upon addition of a supersensitizer (SS) (3,3‘-disulfopropyl-9-ethyl-4,5,4‘,5‘-dibenzothiacarbocyanine), which is coadsorbed on the surface of silver bromide grains, the fluorescence decay became several times faster with a mean decay time of 0.60−1.3 ps. Different models of supersensitizatio...

Radiationless deactivation of 2-piperidinoanthraquinone (2PAQ) in its excited singlet state induced by an intermolecular hydrogen bond interaction was studied by picosecond and femtosecond transien...

CooA from Rhodospirillum rubrum is a transcriptional activator in which a heme prosthetic group acts as a CO sensor and regulates the activity of the protein. In this study, the electronic relaxation of the heme, and the concurrent recombination between ligands and the heme at approximately 280 K were examined in an effort to understand the environment around the heme and the dynamics of the ligands. Upon photoexcitation of the reduced CooA at 400 nm, electronic relaxation of the heme occurred with time constants of 0.8 and 1.7 ps. The ligand rebinding was substantially completed with a time constant of 6.5 ps, followed by a slow relaxation process with a time constant of 173 ps. In the case of CO-bound CooA, relaxation of the excited heme occurred with two time constants, 1.1 and 2.4 ps, which were largely similar to those with reduced CooA. The subsequent CO recombination process was remarkably fast compared with that of other CO-bound heme proteins. It was well described as a biphasic geminate recombination process with time constants of 78 ps (60%) and 386 ps (30%). About 10% of the excited heme remained unligated at 1.9 ns. The dynamics of rebinding of CO thus will help us to understand how the physiologically relevant diatomic molecule approaches the heme binding site in CooA with picosecond resolution.

The rate of the electron transfer reaction from the reduced primary electron acceptor chlorophyll a (A0-) to the secondary acceptor quinone (Q) was measured by picosecond−nanosecond laser spectroscopy at 280 K in the photosynthetic reaction center (RC) complex of plant photosystem I (PS I). The free energy change (ΔG0) of the reaction was varied between −1.1 and +0.2 eV by the reconstitution of 13 different quinone/quinonoid compounds after the extraction of the intrinsic phylloquinone. Phylloquinone and its natural analog menaquinone, both of which show a ΔG0 value of −0.34 eV, gave the highest rate constant (k) of (23 ps)-1. Analysis of log k versus ΔG0 plot according to the quantum mechanical electron transfer theory gave the total reorganization energy (λtotal) of 0.30 eV and the electronic coupling (V) of 14 cm-1. The natural system is shown to be highly optimized to give a ΔG0= −λtotal condition. The λtotal value is smaller and the V value is larger than those estimated in the corresponding reaction...

Rates of sequential electron transfer reactions from the primary electron donor chlorophyll dimer (P700) to the electron acceptor chlorophyll a-686 (A[sub 0]) and to the secondary acceptor quinone (Q[sub [phi]]) are measured by picosecond absorption spectroscopy in spinach photosystem I (PS I) particles. In the particles, 85-90% of antenna chlorophylls are extracted and the intrinsic phylloquinone (Q[sub [phi]]) is removed and replaced by different quinones. (1) The excited single state of P700, monitored by the absorbance change at 695 nm, appeared with a time constant shorter than 1 ps after excitation with a 605 nm, 1 ps pulse at an intensity of 0.8 or 1.5 photons/reaction center. Indication for the absorbance change of the chlorophylls, which may function as the intermediate electron acceptor between P700 and A[sub 0] or between A[sub 0] and Q[sub [phi]], was not obtained. 45 refs., 5 figs., 1 tab.

We have investigated the ultrafast intermolecular electron transfer (ET) from an electron-donating solvent (aniline (AN) or N, N -dimethylaniline (DMA)) to an excited dye molecule (oxazines (Nile blue and oxazine 1) or coumarins). A non-exponential time dependence was observed in AN and can be explained by solvent reorientation and nuclear motion of the reactants. However, in DMA, a single exponential process was observed for Nile blue (160 fs) and oxazine 1 (280 fs), which can be explained by assuming that the rate of ET is limited mainly by ultrafast nuclear motion. A clear substituent effect on intermolecular ET was observed for the 7-aminocoumarins. When the alkyl chain on the 7-amino group is extended and a hexagonal ring with the benzene moiety is formed, the rate of ET is reduced by three orders of magnitude. This effect can be explained by a change in the free energy difference of the reaction and by the vibrational motion of the amino group.

In this study, we produced selective images of photosystems in plant chloroplasts in situ. We used a spectroimaging microscope, equipped with a near-infrared (NIR) laser that provided light at wavelengths mainly between 800 and 830 nm, to analyze chlorophyll autofluorescence spectra and images from chloroplasts in leaves of Zea mays at room temperature. Femtosecond laser excitation of chloroplasts in mesophyll cells revealed a spectral shape that was attributable to PSII and its antenna in the centers of grana spots. We found that a continuous wave emitted by the NIR laser at a wavelength as long as 820 nm induced chlorophyll autofluorescence with a high contribution from PSI through a one-photon absorption mechanism. A spectral shape attributable to PSI and its antenna was thus obtained using continuous wave laser excitation of chloroplasts in bundle sheath cells. These highly pure spectra of photosystems were utilized for spectral decomposition at every intrachloroplast space to show distributions of PSI and PSII and their associated antenna. A new methodology using an NIR laser to detect the PSI/PSII ratio in single chloroplasts in leaves at room temperature is described.

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.

Spectral sensitization and supersensitization of silver bromide nanocrystals are studied by the femtosecond fluorescence up-conversion technique. Fluorescence from J-aggregates of cyanine dyes adsorbed on AgBr nanocrystals (40-900 nm) with different shapes (cubic or octahedral) is measured. Fast non-exponential fluorescence decays are observed with a fast component ranging from 400 fs to 2.5 ps depending on the type and size of the crystals. The rates of electron injection from J-aggregate dye to the conduction band of AgBr at various conditions are determined. The increase of the electron injection rate with the increase of the grain size is observed and explained by the space charge theory. Upon addition of a super-sensitizer (SS), which is different cyanine dye co-adsorbed on the surface of silver bromide grains, the fluorescence decay becomes several times faster. The results are analyzed in the framework of the “hole-trapping” supersensitization model. The effective hole trapping (electron transfer from SS to the excited J-aggregate) rate constant is found to be independent of the grain size and equal to ∼0.60 ps -1 . By fluorescence anisotropy measurement, the ultrafast energy transfer of the J-aggregates on the octahedral crystal (200 nm) is studied. The anisotropy decays are biphasic with two time constants of ∼100 fs and a few picoseconds, the short time constant being assigned to energy transfer between epitaxially arranged J-aggregates on the {111} surface.

CooA, which is a transcriptional regulator heme protein allosterically triggered by CO, is studied by femtosecond visible-pump mid-IR-probe spectroscopy. Transient bleaching upon excitation of the heme in the Soret band is detected at approximately 1979 cm(-1), which is the absorption region of the CO bound to the heme. The bleach signal shows a nonexponential decay with time constants of 56 and 290 ps, caused by the rebinding of the CO to the heme. About 98% of dissociated CO recombines geminately. The geminate recombination rate in CooA is significantly faster than those in myoglobin and hemoglobin. The angle of the bound CO with respect to the porphyrin plane is calculated to be about 78 degrees on the basis of the anisotropy measurements. A shift of the bleached mid-IR spectrum of the bound CO is detected and has a characteristic time of 160 ps. It is suggested that the spectral shift is caused by a difference in the frequency of the bound CO in different protein conformations, particularly in an active conformation and in an intermediate one, which is on the way toward an inactive conformation. Thus, the biologically relevant conformation change in CooA was traced. Possible assignment of the observed conformation change is discussed.

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 (A(0)) and its reoxidation by phylloquinone (A(1)), respectively.