Reinvestigation on primary processes of PSII-dimer from Thermosynechococcus vulcanus by femtosecond pump-probe spectroscopy.

Cyanobacterial photosynthetic apparatus efficiently capture sunlight, and the energy is subsequently transferred to photosystem I (PSI) and II (PSII), to produce electrochemical potentials. PSII is a unique membrane protein complex that photo-catalyzes oxidation of water and majorly contains photosynthetic pigments of chlorophyll a and carotenoids. In the present study, the ultrafast energy transfer and charge separation dynamics of PSII from a thermophilic cyanobacterium Thermosynechococcus vulcanus were reinvestigated by femtosecond pump-probe spectroscopic measurements under low temperature and weak intensity excitation condition. The results imply the two possible models of the energy transfers and subsequent charge separation in PSII. One is the previously suggested "transfer-to-trapped limit" model. Another model suggests that the energy transfers from core CP43 and CP47 antennas to the primary electron donor ChlD1 with time-constants of 0.71 ps and 3.28 ps at 140 K (0.17 and 1.33 ps at 296 K), respectively and that the pheophytin anion (PheoD1-) is generated with the time-constant of 43.0 ps at 140 K (14.8 ps at 296 K) upon excitation into the Qy band of chlorophyll a at 670 nm. The secondary electron transfer to quinone QA: PheoD1-QA → PheoD1QA- is observed with the time-constant of 650 ps only at 296 K. On the other hand, an inefficient ß-carotene → chlorophyll a energy transfer (33%) occurred after excitation to the S2 state of ß-carotene at 500 nm. Instead, the carotenoid triplet state appeared in an ultrafast timescale after excitation at 500 nm.

Distribution of XTdrd6/Xtr protein during oogenesis and early development in Xenopus laevis: Zygotic translation begins only in germ cells that have entered the genital ridge.

We previously identified Xenopus tudor domain containing 6/Xenopus tudor repeat (Xtdrd6/Xtr), which was exclusively expressed in the germ cells of adult Xenopus laevis. Western blot analysis showed that the XTdrd6/Xtr protein was translated in St. I/II oocytes and persisted as a maternal factor until the tailbud stage. XTdrd6/Xtr has been reported to be essential for the translation of maternal mRNA involved in oocyte meiosis. In the present study, we examined the distribution of the XTdrd6/Xtr protein during oogenesis and early development, to predict the time point of its action during development. First, we showed that XTdrd6/Xtr is localized to germinal granules in the germplasm by electron microscopy. XTdrd6/Xtr was found to be localized to the origin of the germplasm, the mitochondrial cloud of St. I oocytes, during oogenesis. Notably, XTdrd6/Xtr was also found to be localized around the nuclear membrane of St. I oocytes. This suggests that XTdrd6/Xtr may immediately interact with some mRNAs that emerge from the nucleus and translocate to the mitochondrial cloud. XTdrd6/Xtr was also detected in primordial germ cells and germ cells throughout development. Using transgenic Xenopus expressing XTdrd6/Xtr with a C-terminal FLAG tag produced by homology-directed repair, we found that the zygotic translation of the XTdrd6/Xtr protein began at St. 47/48. As germ cells are surrounded by gonadal somatic cells and are considered to enter a new differentiation stage at this phase, the newly synthesized XTdrd6/Xtr protein may regulate the translation of mRNAs involved in the new steps of germ cell differentiation.

Excited-State Dynamics of All the Mono-cis and the Major Di-cis Isomers of ß-Apo-8'-Carotenal as Revealed by Femtosecond Time-Resolved Transient Absorption Spectroscopy.

Cis isomers of carotenoids play important roles in light harvesting and photoprotection in photosynthetic bacteria, such as the reaction center in purple bacteria and the photosynthetic apparatus in cyanobacteria. Carotenoids containing carbonyl groups are involved in efficient energy transfer to chlorophyll in light-harvesting complexes, and their intramolecular charge-transfer (ICT) excited states are known to be important for this process. Previous studies, using ultrafast laser spectroscopy, have focused on the central-cis isomer of carbonyl-containing carotenoids, revealing that the ICT excited state is stabilized in polar environments. However, the relationship between the cis isomer structure and the ICT excited state has remained unresolved. In this study, we performed steady-state absorption and femtosecond time-resolved absorption spectroscopy on nine geometric isomers (7-cis, 9-cis, 13-cis, 15-cis, 13'-cis, 9,13'-cis, 9,13-cis, 13,13'-cis, and all-trans) of ß-apo-8'-carotenal, whose structures are well-defined, and discovered correlations between the decay rate constant of the S1 excited state and the S0-S1 energy gap, as well as between the position of the cis-bend and the degree of stabilization of the ICT excited state. Our results demonstrate that the ICT excited state is stabilized in polar environments in cis isomers of carbonyl-containing carotenoids and suggest that the position of the cis-bend plays an important role in the stabilization of the excited state.

Light-Stimulated Luminescence Control of Lead Halide-Based Perovskite Nanocrystals Coupled with Photochromic Molecules via Electron and Energy Transfer.

Photoswitchable nanomaterials are key materials in the development of advanced imaging techniques, such as super-resolution fluorescence microscopy. The combination of perovskite CsPbBr3 nanocrystals (NCs) with bright photoluminescence (PL) emission and diarylethenes (DAEs) with structural changes in response to ultraviolet (UV) and visible light is a promising candidate system. Herein, CsPbBr3 NCs are coupled with photochromic DAE molecules to control the PL emission from the NCs by light stimulation. The PL emission is successfully switched ON and OFF by alternating UV and visible light irradiation. Time-resolved PL emission studies suggest that Förster resonance energy transfer from CsPbBr3 NCs to the closed-ring form of DAE occurs after UV irradiation, and the PL emission is quenched. Upon visible-light irradiation, DAE is converted to the open-ring isomer, and the PL emission is restored. Femtosecond pump-probe spectroscopy reveals that light stimulation induces not only energy transfer but also photoinduced electron transfer in the NC-DAE pair on the picosecond timescale to form DAE radicals. Thus, it is suggested that the holes residing in the NCs react with the NCs, degrading the PL emission. Stable PL switching is realized using 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a hole scavenger to avoid the reaction between the holes and NCs.

A Ferroelectric Metallomesogen Exhibiting Field-Induced Slow Magnetic Relaxation.

Magnetoelectric (ME) materials exhibiting coupled electric and magnetic properties are of significant interest because of their potential use in memory storage devices, new sensors, or low-consumption devices. Herein, we report a new category of ME material that shows liquid crystal (LC), ferroelectric (FE), and field-induced single molecule magnet (SMM) behaviors. Co(II) complex incorporating alkyl chains of type [Co(3C16 -bzimpy)2 ](BF4 )2 (1; 3C16 -bzimpy=2,2'-(4-hexadecyloxy-2,6-diyl)bis(1-hexadecyl-1H-benzo[d]imidazole)) displayed a chiral smectic C mesophase in the temperature range 321 K-458 K, in which distinct FE behavior was observed, with a remnant polarization (88.3 nC cm-2 ). Complex 1 also exhibited field-induced slow magnetic relaxation behavior that reflects the large magnetic anisotropy of the Co(II) center. Furthermore, the dielectric property of 1 was able to be tuned by an external magnetic field occurring from both spin-lattice coupling and molecular orientational variation. Clearly, this multifunctional compound, combining LC, FE, and SMM properties, represents an entry to the development of a range of next-generation ME materials.

Ultrafast energy transfer dynamics of phycobilisome from Thermosynechococcus vulcanus, as revealed by ps fluorescence and fs pump-probe spectroscopies.

Cyanobacterial photosynthetic systems efficiently capture sunlight using the pigment-protein megacomplexes, phycobilisome (PBS). The energy is subsequently transferred to photosystem I (PSI) and II (PSII), to produce electrochemical potentials. In the present study, we performed picosecond (ps) time-resolved fluorescence and femtosecond (fs) pump-probe spectroscopies on the intact PBS from a thermophilic cyanobacterium, Thermosynechococcus vulcanus, to reveal excitation energy transfer dynamics in PBS. The photophysical properties of the intact PBS were well characterized by spectroscopic measurements covering wide temporal range from femtoseconds to nanoseconds. The ps fluorescence measurements excited at 570 nm, corresponding to the higher energy of the phycocyanin (PC) absorption band, demonstrated the excitation energy transfer from the PC rods to the allophycocyanin (APC) core complex as well as the energy transfer in the APC core complex. Then, the fs pump-probe measurements revealed the detailed energy transfer dynamics in the PC rods taking place in an ultrafast time scale. The results obtained in this study provide the full picture of the funnel-type excitation energy transfer with rate constants of (0.57 ps)-1 → (7.3 ps)-1 → (53 ps)-1 → (180 ps)-1 → (1800 ps)-1.

Strategies to enhance the excitation energy-transfer efficiency in a light-harvesting system using the intra-molecular charge transfer character of carotenoids.

Fucoxanthin is a carotenoid that is mainly found in light-harvesting complexes from brown algae and diatoms. Due to the presence of a carbonyl group attached to polyene chains in polar environments, excitation produces an excited intra-molecular charge transfer. This intra-molecular charge transfer state plays a key role in the highly efficient (∼95%) energy-transfer from fucoxanthin to chlorophyll a in the light-harvesting complexes from brown algae. In purple bacterial light-harvesting systems the efficiency of excitation energy-transfer from carotenoids to bacteriochlorophylls depends on the extent of conjugation of the carotenoids. In this study we were successful, for the first time, in incorporating fucoxanthin into a light-harvesting complex 1 from the purple photosynthetic bacterium, Rhodospirillum rubrum G9+ (a carotenoidless strain). Femtosecond pump-probe spectroscopy was applied to this reconstituted light-harvesting complex in order to determine the efficiency of excitation energy-transfer from fucoxanthin to bacteriochlorophyll a when they are bound to the light-harvesting 1 apo-proteins.

Excited state properties of ß-carotene analogs incorporating a lactone ring.

Carotenoids possessing a carbonyl group along their polyene backbone exhibit unique excited state properties due to the occurrence of intramolecular charge transfer (ICT) in the excited state. In fact, the ICT characteristics of naturally occurring carbonyl carotenoids play an essential role in the highly efficient energy transfer that proceeds in aquatic photosynthetic antenna systems. In the present study, we synthesized two short-chain polyene carotenoids incorporating a lactone ring, denoted as BL-7 and BL-8, having seven and eight conjugated double bonds (n = 7 and 8), respectively. The excited state properties of these compounds were directly compared to those of their non-carbonyl counterparts to clarify the role of the carbonyl group in the generation of ICT. The energies of the optically allowed S2 states for BL-7 and BL-8 were found to be more than 0.3 eV (2400 cm-1) below those of non-carbonyl short ß-carotene homologs. Ultrafast spectroscopic data demonstrated various solvent polarity-induced effects, including the appearance of stimulated emission in the near-IR region in the case of BL-7, and significant lifetime shortening of the lowest-lying singlet S1 excited states of both BL-7 and BL-8. These results suggest that these compounds exhibit ICT characteristics.

How do surrounding environments influence the electronic and vibrational properties of spheroidene?

Absorption and Raman spectra of spheroidene dissolved in various organic solvents and bound to peripheral light-harvesting LH2 complexes from photosynthetic purple bacteria Rhodobacter (Rba.) sphaeroides 2.4.1 were measured. The results showed that the peak energies of absorption and C-C and C=C stretching Raman lines are linearly proportional to the polarizability of solvents, as has already been reported. When comparing these results with those measured on LH2 complexes, it was confirmed that spheroidene is surrounded by a media with high polarizability. However, the change in the spectral width of the Raman lines, which reflect vibrational decay time, cannot be explained simply by a similar dependence of solvent polarizability. The experimental results were analyzed using a potential theoretical model. Consequently, a systematic change in the Raman line widths in the ground state can be satisfactorily explained as a function of the viscosity of the surrounding media. Even when the absorption peaks appear at the same energy, the vibrational decay time of spheroidene in the LH2 complexes is approximately 15-20 % slower than that in organic solvents.

Characterization of the intramolecular transfer state of marine carotenoid fucoxanthin by femtosecond pump-probe spectroscopy.

Fucoxanthin, containing a carbonyl group in conjugation with its polyene backbone, is a naturally occurring pigment in marine organisms and is essential to the photosynthetic light-harvesting function in brown alga and diatom. Fucoxanthin exhibits optical characteristics attributed to an intramolecular charge transfer (ICT) state that arises in polar environments due to the presence of the carbonyl group. In this study, we report the spectroscopic properties of fucoxanthin in methanol (polar and protic solvent) observed by femtosecond pump-probe measurements in the near-infrared region, where transient absorption associated with the optically allowed S2 (1(1)B u (+) ) state and stimulated emission from the strongly coupled S1/ICT state were observed following one-photon excitation to the S2 state. The results showed that the amplitude of the stimulated emission of the S1/ICT state increased with decreasing excitation energy, demonstrating that the fucoxanthin form associated with the lower energy of the steady-state absorption exhibits stronger ICT character.

Direct observation of hole shift and characterization of spin states in radical ion pairs generated from photoinduced electron transfer of (phenothiazine)(n)-anthraquinone (n = 1, 3) dyads.

Photoinduced intramolecular electron transfer of dyad PTZ3-PTZ2-PTZ1-B-AQ consisting of phenothiazine trimer (PTZ3-PTZ2-PTZ1), bicyclo[2.2.2]octane (B), and anthraquinone (AQ) was investigated. After excitation (∼20 ps) of the AQ moiety in THF, a metastable radical ion pair (RIP) PTZ3-PTZ2-PTZ1(+)-B-AQ(-) appeared at ∼620 nm. From 500 ps to 6 ns the spectrum changed to a new absorption (∼950 nm), which was assigned to the hole-shifted stable RIP state PTZ3-PTZ2(+)-PTZ1-B-AQ(-). The time constant of the hole-shift process was determined to be 6.0 ns. The hole-shifted RIP state had a lifetime (τ) of 250 ns and was characterized by spin-polarized signals as a spin-correlated radical pair (SCRP) by means of time-resolved ESR. These results were compared with those for the phenothiazine monomer analog PTZ-B-AQ, which also produced the RIP state PTZ(+)-B-AQ(-) with τ = 1.9 µs. Time-resolved ESR showed an all emission signal pattern showing the triplet mechanism of PTZ-B-(3)AQ* → (3)[PTZ(+)-B-AQ(-)]. The origin of the difference in the lifetimes between the trimer and the monomer RIP states was discussed from various points of view, including free energy difference in the RIP states, reorganization energy difference in the charge recombination process, and the spin-state difference. Of these, the spin-state difference effect provided the most reasonable explanation.

Excited-state dynamics of pentacene derivatives with stable radical substituents.

The excited-state dynamics of pentacene derivatives with stable radical substituents were evaluated in detail through transient absorption measurements. The derivatives showed ultrafast formation of triplet excited state(s) in the pentacene moiety from a photoexcited singlet state through the contributions of enhanced intersystem crossing and singlet fission. Detailed kinetic analyses for the transient absorption data were conducted to quantify the excited-state characteristics of the derivatives.

Ultrafast intramolecular relaxation dynamics of Mg- and Zn-bacteriochlorophyll a.

Ultrafast excited-state dynamics of the photosynthetic pigment (Mg-)bacteriochlorophyll a and its Zn-substituted form were investigated by steady-state absorption∕fluorescence and femtosecond pump-probe spectroscopic measurements. The obtained steady-state absorption and fluorescence spectra of bacteriochlorophyll a in solution showed that the central metal compound significantly affects the energy of the Qx state, but has almost no effect on the Qy state. Photo-induced absorption spectra were recorded upon excitation of Mg- and Zn-bacteriochlorophyll a into either their Qx or Qy state. By comparing the kinetic traces of transient absorption, ground-state beaching, and stimulated emission after excitation to the Qx or Qy state, we showed that the Qx state was substantially incorporated in the ultrafast excited-state dynamics of bacteriochlorophyll a. Based on these observations, the lifetime of the Qx state was determined to be 50 and 70 fs for Mg- and Zn-bacteriochlorophyll a, respectively, indicating that the lifetime was influenced by the central metal atom due to the change of the energy gap between the Qx and Qy states.

Ultrafast excited state dynamics of spirilloxanthin in solution and bound to core antenna complexes: identification of the S∗ and T1 states.

Ultrafast excited state dynamics of spirilloxanthin in solution and bound to the light-harvesting core antenna complexes from Rhodospirillum rubrum S1 were investigated by means of femtosecond pump-probe spectroscopic measurements. The previously proposed S∗ state of spirilloxanthin was clearly observed both in solution and bound to the light-harvesting core antenna complexes, while the lowest triplet excited state appeared only with spirilloxanthin bound to the protein complexes. Ultrafast formation of triplet spirilloxanthin bound to the protein complexes was observed upon excitation of either spirilloxanthin or bacteriochlorophyll-a. The anomalous reaction of the ultrafast triplet formation is discussed in terms of ultrafast energy transfer between spirilloxanthin and bacteriochlorophyll-a.

Core and rod structures of a thermophilic cyanobacterial light-harvesting phycobilisome.

Cyanobacteria, glaucophytes, and rhodophytes utilize giant, light-harvesting phycobilisomes (PBSs) for capturing solar energy and conveying it to photosynthetic reaction centers. PBSs are compositionally and structurally diverse, and exceedingly complex, all of which pose a challenge for a comprehensive understanding of their function. To date, three detailed architectures of PBSs by cryo-electron microscopy (cryo-EM) have been described: a hemiellipsoidal type, a block-type from rhodophytes, and a cyanobacterial hemidiscoidal-type. Here, we report cryo-EM structures of a pentacylindrical allophycocyanin core and phycocyanin-containing rod of a thermophilic cyanobacterial hemidiscoidal PBS. The structures define the spatial arrangement of protein subunits and chromophores, crucial for deciphering the energy transfer mechanism. They reveal how the pentacylindrical core is formed, identify key interactions between linker proteins and the bilin chromophores, and indicate pathways for unidirectional energy transfer.

Intramolecular charge-transfer enhances energy transfer efficiency in carotenoid-reconstituted light-harvesting 1 complex of purple photosynthetic bacteria.

In bacterial photosynthesis, the excitation energy transfer (EET) from carotenoids to bacteriochlorophyll a has a significant impact on the overall efficiency of the primary photosynthetic process. This efficiency can be enhanced when the involved carotenoid has intramolecular charge-transfer (ICT) character, as found in light-harvesting systems of marine alga and diatoms. Here, we provide insights into the significance of ICT excited states following the incorporation of a higher plant carotenoid, ß-apo-8'-carotenal, into the carotenoidless light-harvesting 1 (LH1) complex of the purple photosynthetic bacterium Rhodospirillum rubrum strain G9+. ß-apo-8'-carotenal generates the ICT excited state in the reconstituted LH1 complex, achieving an efficiency of EET of up to 79%, which exceeds that found in the wild-type LH1 complex.

Ultrafast excited state dynamics of fucoxanthin: excitation energy dependent intramolecular charge transfer dynamics.

Carotenoids containing a carbonyl group in conjugation with their polyene backbone are naturally-occurring pigments in marine organisms and are essential to the photosynthetic light-harvesting function in aquatic algae. These carotenoids exhibit spectral characteristics attributed to an intramolecular charge transfer (ICT) state that arise in polar solvents due to the presence of the carbonyl group. Here, we report the spectroscopic properties of the carbonyl carotenoid fucoxanthin in polar (methanol) and nonpolar (cyclohexane) solvents studied by steady-state absorption and femtosecond pump-probe measurements. Transient absorption associated with the optically forbidden S(1) (2(1)A) state and/or the ICT state were observed following one-photon excitation to the optically allowed S(2) (1(1)B) state in methanol. The transient absorption measurements carried out in methanol showed that the ratio of the ICT-to-S(1) state formation increased with decreasing excitation energy. We also showed that the ICT character was clearly visible in the steady-state absorption in methanol based on a Franck-Condon analysis. The results suggest that two spectroscopic forms of fucoxanthin, blue and red, exist in the polar environment.

Femtosecond transient absorption spectroscopic study of a carbonyl-containing carotenoid analogue, 2-(all-trans-retinylidene)-indan-1,3-dione.

The photophysical properties of a carbonyl-containing carotenoid analogue in an s-cis configuration, relative to the conjugated π system, 2-(all-trans-retinylidene)-indan-1,3-dione (C20Ind), were investigated by femtosecond time-resolved spectroscopy in various solvents. The lifetime of the optically forbidden S(1) state of C20Ind becomes long as solvent polarity increases. This trend is completely opposite to the situation of S(1-ICT) dynamics of carbonyl-containing carotenoids, such as peridinin and fucoxanthin. Excitation energy dependence of the transient absorption measurements shows that the transient absorption spectra in nonpolar solvents were originated from two distinct transient species, while those in polar and protic solvents are due to a single transient species. By referring to the results of MNDO-PSDCI (modified neglect of differential overlap with partial single- and double-configuration interaction) calculations, we conclude: (1) in polar and protic solvents, the S(1) state is generated following excitation up to the S(2) state; (2) in nonpolar solvents, however, both the S(1) and the (1)nπ* states are generated; and (3) C20Ind does not generate the S(1-ICT) state, despite the fact that it has two conjugated carbonyl groups.

Structural implications for a phycobilisome complex from the thermophilic cyanobacterium Thermosynechococcus vulcanus.

Phycobilisomes (PBSs) are huge, water-soluble light-harvesting complexes used by oxygenic photosynthetic organisms. The structures of some subunits of the PBSs, including allophycocyanin (APC) and phycocyanin (PC), have been solved by X-ray crystallography previously. However, there are few reports on the overall structures of PBS complexes in photosynthetic organisms. Here, we report the overall structure of the PBS complex isolated from the cyanobacterium Thermosynechococcus vulcanus, determined by negative-staining electron microscopy (EM). Intact PBS complexes were purified by trehalose density gradient centrifugation with a high-concentration phosphate buffer and then subjected to a gradient-fixation preparation using glutaraldehyde. The final map constructed by the single-particle analysis of EM images showed a hemidiscoidal structure of the PBS, consisting of APC cores and peripheral PC rods. The APC cores are composed of five cylinders: A1, A2, B, C1, and C2. Each of the cylinders is composed of three (A1 and A2), four (B), or two (C1 and C2) APC trimers. In addition, there are eight PC rods in the PBS: one bottom pair (Rb and Rb'), one top pair (Rt and Rt'), and two side pairs (Rs1/Rs1' and Rs2/Rs2'). Comparison with the overall structures of PBSs from other organisms revealed structural characteristics of T. vulcanus PBS.

Solvent vapor-induced polarity and ferroelectricity switching.

Vapor-induced crystal to crystal transformation between non-polar [Fe(sap)(acac)(sol)] (H2sap = 2-salicylideneaminophenol, acac = acethylacetate, sol = MeOH, pyridine) and polar [Fe(sap)(acac)(DMSO)] was demonstrated. It provides an example of switchable ferroelectric behaviour attributted to the structural phase transition triggered by solvent vapour.