Effects of ultraviolet-B light on photosystem II phosphoproteins in barley wild type and its chlorophyll b-less mutant chlorina f2.

The effects of ultraviolet-B light on the level and steady-state phosphorylation of photosystem II proteins have been studied in barley wild type and its chlorophyll b-less mutant chlorina f2. In the wild type, ultraviolet-B radiation is found to promote dephosphorylation of all thylakoid phosphoproteins. In addition, for reaction-centre proteins D1 and D2, dephosphorylation is paralleled by degradation. Photosystem II core proteins in the mutant are not found to be significantly phosphorylated in any experimental conditions, and loss of D1 and D2 reaction-centre proteins is slightly faster than in the wild type. These results are consistent with the possibility that phosphorylation of reaction-centre proteins affects their stability, possibly by slowing down the rate of degradation, as in the case of visible light.     

Spectroscopic investigation on the energy transfer process in photosynthetic apparatus of cyanobacteria

In this work, we employ cyanobacteria, Spirulina platensis, and separate their photosynthetic apparatus, phycobilisome (PBS), thylakoid membrane and phycobilisome-thylakoid membrane complex. The steady state absorption spectra, fluorescence spectra and corresponding deconvoluted spectra and picosecond time-resolved spectra are used to investigate the energy transfer process in phycobilisome-thylakoid membrane complex. The results on steady state spectra show chlorophylls of the photosystem II are able to transfer excitation energy to phycobilisome with Chla molecules selectively excited. The decomposition of the steady state spectra further suggest the uphill energy transfer originate from chlorophylls of photosystem II to cores of phycobilisome, while rods and cores of phycobilisome cannot receive energy from the chlorophylls of photosystem I. The time constant for the back energy transfer process is 18 ps.     

Spectroscopic evidence for triplet excitation energy transfer among carotenoids in the LH2 complex from photosynthetic bacterium Rhodopseudomonas palustris

The LH2 complex from Rhodopsudomonas (Rps.) palustris is unique in the heterogeneous carotenoid compositions. The dynamics of triplet excited state Carotenoids (3Car*) has been investigated by means of sub-microsecond time-resolved absorption spectroscopy both at physiological temperature (295 K) and at cryogenic temperature (77 K). Broad and asymmetric Tn←T-1 transient absorption was observed at room temperature following the photo-excitation of Car at 532 nm, which suggests the contribution from various carotenoid compositions having different numbers of conjugated C=C double bonds (Nc=c). The triplet absorption bands of different carotenoids, which superimposed at room temperature, could be clearly distinguished upon decreasing the temperature down to 77 K. At room temperature the shorter-wavelength side of the main Tn←T1 absorption band decayed rapidly to reach a spectral equilibration with a characteristic time constant of-1 μs, the same spectral dynamics, however, was not observed at 77 K. The     

Transfer of excitation energy in a three-dimensional-doped molecular crystal. V. Self-consistency of the temporal processes involved in energy transfer in photosynthetic units

Numerical experiments were carried out to determine the timewise self-consistency of different physical processes involved in the energy transfer in green plant photosynthetic units. A 6 × 6 × 6 array of chlorophyll-a with cubic lattice constants a = b = c = 20 Å was chosen as a model of the thylakoid disc. Another model aggregate was obtained by substituting chlorophyll-b molecules for some of the chlorophyll-a molecules. In both models, a reaction center occupied a central site in the last xy plane. Two extreme arrangements were considered for the orientation of molecules. In one, the transition moments of all molecules were directed along the y-axis. The other had chlorophyll molecules randomly oriented. The four resulting model systems were used in our investigation on exciton generation, transport, decay by fluorescence, and trapping. All excitons were assumed to be generated by a 20 ms exposure to sunlight at high altitudes. The general trends noticed from these computations are as follows: The number of excitons generated is influenced by lattice disorders. Disorders also increase the time for the establishment of an equilibrium distribution. The decay of excitons by fluorescence is always a monotonic function of time. The energy transfer is adversely affected by a lower degree of orientation in the crystal: The trapping time increases with disorder. The number of trappings decreases with the onset of fluorescence of the host molecules and the trap. From these investigations, we also made three specific observations: (1) The efficiency of exciton utilization varies from 12% for a completely random arrangement of transition dipoles to 46% for a perfectly ordered arrangement. This agrees with the experimental efficiency, about 20%. (2) The number of excitons trapped varies from one to six. This tallies with the time scale of electron transfer along the Z-scheme that requires at least two excitons trapped in about 20 ms. Thus, the photon density and the exciton transfer rate are consistent with the rates of electron transfers. (3) The trapping rate also indicates that the thylakoid disc must resemble a considerably ordered system. © 1996 John Wiley & Sons, Inc.     

Ultrafast fluorescence investigation of excitation energy transfer in different dendritic core branched structures

The mechanism of energy transport in branching structures is suggestively related to the geometry of the multichromophore architecture. In organic conjugated dendrimers, both incoherent (hopping) and coherent energy transfer processes have been observed from different dendritic architectures with different building blocks. In this communication, we report the investigation of three fundamental dendritic architectures (G0) with the same attached chromophores, but with different core atoms, C, N, and P. The synthesis of a phosphorus-containing G0 system with distyrylbenzene chromophores is provided. These three systems provide a comparison by which the relative interaction of branching chromophores can be compared on the basis of their different branching centers. Ultrafast fluorescence anisotropy measurements provide a dual measure of the geometry of the chromophores around the different central units as well as the strength of the interactions among chromophores. The nitrogen-cored system appeared to have both the strongest coupling of chromophore excitation as well as the most planar geometry of the three. Interestingly, the phosphorus system appeared to have the least planar geometry, and its interaction strength was found to be stronger than that observed for the carbon system. These results provide a comparison of the energy migration dynamics of the most common and new dendritic architectures with applications for light emission and light harvesting.     

Influence of environment induced correlated fluctuations in electronic coupling on coherent excitation energy transfer dynamics in model photosynthetic systems

Two-dimensional photon-echo experiments indicate that excitation energy transfer between chromophores near the reaction center of the photosynthetic purple bacterium Rhodobacter sphaeroides occurs coherently with decoherence times of hundreds of femtoseconds, comparable to the energy transfer time scale in these systems. The original explanation of this observation suggested that correlated fluctuations in chromophore excitation energies, driven by large scale protein motions could result in long lived coherent energy transfer dynamics. However, no significant site energy correlation has been found in recent molecular dynamics simulations of several model light harvesting systems. Instead, there is evidence of correlated fluctuations in site energy-electronic coupling and electronic coupling-electronic coupling. The roles of these different types of correlations in excitation energy transfer dynamics are not yet thoroughly understood, though the effects of site energy correlations have been well studied. In this paper, we introduce several general models that can realistically describe the effects of various types of correlated fluctuations in chromophore properties and systematically study the behavior of these models using general methods for treating dissipative quantum dynamics in complex multi-chromophore systems. The effects of correlation between site energy and inter-site electronic couplings are explored in a two state model of excitation energy transfer between the accessory bacteriochlorophyll and bacteriopheophytin in a reaction center system and we find that these types of correlated fluctuations can enhance or suppress coherence and transfer rate simultaneously. In contrast, models for correlated fluctuations in chromophore excitation energies show enhanced coherent dynamics but necessarily show decrease in excitation energy transfer rate accompanying such coherence enhancement. Finally, for a three state model of the Fenna-Matthews-Olsen light harvesting complex, we explore the influence of including correlations in inter-chromophore couplings between different chromophore dimers that share a common chromophore. We find that the relative sign of the different correlations can have profound influence on decoherence time and energy transfer rate and can provide sensitive control of relaxation in these complex quantum dynamical open systems.     

Effect of the excitation energy on the (HI)2 nonadiabatic photodissociation dynamics

The effect of the excitation energy on the nonadiabatic photodissociation dynamics of (HI)2 is explored in this work. A wave packet model is applied that simulates the photodissociation process starting from the I*-HI complex left behind after dissociation of the first HI moiety within (HI)2. The probability and product fragment state distributions of the different photodissociation pathways are analyzed in a wide range of excitation energies of the I*-HI absorption spectrum. It is found that the probability of electronically nonadiabatic transitions increases substantially (by a factor larger than two) in the range of excitation energies analyzed. This increase is due to an enhancement of the intensity of the spin-rotation coupling responsible for the nonadiabatic transitions with increasing excitation energy. A remarkably high fraction of bound, highly excited I2 photoproducts, slowly decreasing as the excitation energy increases, is also found over the range of energies studied. The I2 product state distributions show manifestations of rotational interference effects and also of rotational cooling in the case of the I2 state distributions produced upon nonadiabatic transitions. Such effects become more pronounced with increasing energy. Experimental implications of these findings are discussed.     

A comparison of the cellulolytic activities of rumen ciliate protozoa grown under different conditions

Cell-free extracts of rumen Entodiniomorphid protozoa were prepared by gentle sonication (under conditions such that the associated bacteria were not disrupted) and centrifuged (5500g, 30 min) to produce a clear, bacteria-free supernatant fluid. These preparations contained enzymes that were active against a number of cellulose preparations. They released dye from cellulose azure, produced cellobiose from or loss of turbidity of phosphoric acid-reprecipitated cellulose, produced reducing material from and loss of viscosity of carboxymethylcellulose, and produced reducing material from microcrystalline cellulose. Unfortunately with all methods the relationship between product formed and enzyme concentration was a curve with steadily decreasing slope and activities were always expressed in terms of concentration of an extract from the rumen ciliate,Epidinium ecaudatum caudatum, a series of concentrations of which were always run in parallel with other samples. Results have been calculated as specific activities relative to that of rumen-grownEpi. ec. caudatum taken as 100. Comparison has been made, where possible, between protozoa grown as a single species in the rumen of a sheep (ration 1000g hay and 100g oats/d) (referred to as R-grown), protozoa grown in culture on dried grass alone (CG-grown) and protozoa grown in culture on dried grass and ground wheat (CGW-growri) (Coleman, 1978). WithEudiplodinium maggii, the highest activity (106–135%) was obtained with CG-grown, 23–65% with R-grown, and 5–8% with CGW-grown protozoa. WithEremoplastron bovis the values were 41–49% with R-grown, 26–43% with CG-grown and 16–23% with CGW-grown protozoa. WithOstracodinium obtusum bilobum the activities were 90–210% with R-grown, 92–134% with CG-grown, and 85–110% with CGW-grown.Ophryoscolex caudatus would not grow under condition CG but had activities of 54–125% under the other two conditions.Diploplastron affine also would not grow well under condition CG, but had 20–40% activity when R-grown and 100–110% when CGW-grown. These results show that there is no consistent pattern of enzyme activities under the growth conditions used and that there may be important species differences.     

The involvement of LHCII-associated polyamines in the response of the photosynthetic apparatus to low temperature

The influence of low temperature on the structure and function of the photosynthetic apparatus was investigated in Phaseolus vulgaris L. Ten-day-old plants (grown at 26 degrees C) have been exposed to low temperature (6 degrees C) for 52 h and, then, transferred to the initial temperature (26 degrees C) for additional 30 h. Biochemical and physico-chemical measurements performed in the low temperature-treated plants showed that the response of the photosynthetic apparatus to low temperature is affected by the changes occurring in the pattern of LHCII-associated putrescine (Put) and spermine (Spm) which adjust the size of LHCII. The decrease of Put/Spm ratio, mainly due to the reduction in the quantity of LHCII-associated Put led to an increase of the LHCII, especially of the oligomeric forms. These alterations in the structure of the photosynthetic apparatus combined with the reduction in the photosynthetic electron transfer rate resulted in the inactivation of active reaction centers and the increase of dissipated energy which diminished the photosynthetic efficiency and the maximal photosynthetic rate. The transfer of plants at 26 degrees C after the low temperature treatment showed that, structurally and functionally, the photosynthetic mechanism recovered quite fast to the initial condition.     

Effects of light on the photosynthetic apparatus and a novel type of degradation of the photosystem I peripheral antenna complexes under darkness

A novel type of degradation of photosystem I peripheral antenna complexes has been observed in rice leaves under darkness in the present study. Photosynthesis, chlorophyll content, the chlorophyll a/b ratio, and relative amounts of ribulose-1,5-bisphosphate carboxylase/oxygenase decrease during dark treatment. The levels of photosystem II reaction-center complex and cytochrome f on the basis of units of chlorophyll also decline rapidly under darkness. In contrast, the levels of photosystem I reaction-center complex remain stable under darkness for six days. Low-temperature fluorescence emission spectra ascribed to photosystem I antennae clearly show a blue shift. A similar shift is also observed in the photosystem I complexes resolved with dodecyl maltoside-polyacrylamide gel electrophoresis. Moreover, polypeptide analysis of the thylakoids and photosystem I complexes isolated from the green gels shows that some polypeptides originating from photosystem I peripheral antenna complexes disappear during the dark treatment. A curve-fitting method also displays remarkable changes in the chlorophyll components between the light and dark treatments. It is likely that these results indicate the disconnection/disassembly of the photosystem I antenna as well as the photosystem II complexes induced by dark treatment. Moreover, these findings also imply the existence of different degradation mechanisms for the photosystem I and II complexes.     

Spectroscopic evidence for triplet excitation energy transfer among carotenoids in the LH2 complex from photosynthetic bacterium <Emphasis Type="Italic">Rhodopseudomonas palustris</Emphasis>

The LH2 complex from Rhodopsudomonas (Rps.) palustris is unique in the heterogeneous carotenoid compositions. The dynamics of triplet excited state Carotenoids (³Car* has been investigated by means of sub-microsecond time-resolved absorption spectroscopy both at physiological temperature (295 K) and at cryogenic temperature (77K). Broad and asymmetric T _n ←T ₁ transient absorption was observed at room temperature following the photo-excitation of Car at 532 nm, which suggests the contribution from various carotenoid compositions having different numbers of conjugated C=C double bonds (N_c=c). The triplet absorption bands of different carotenoids, which superimposed at room temperature, could be clearly distinguished upon decreasing the temperature down to 77 K. At room temperature the shorter-wavelength side of the main T_n04T₁ absorption band decayed rapidly to reach a spectral equilibration with a characteristic time constant of ∽1 μs, the same spectral dynamics, however, was not observed at 77 K. The aforementioned spectral dynamics can be explained in terms of the triplet-excitation transfer among heterogeneous carotenoid compositions. Global spectral analysis was applied to the time-resolved spectra at room temperature, which revealed two spectral components peaked at 545 and 565 nm, and assignable to the T_n04 T₁ absorption of Cars with Nc=c=11 and Nc=c=13, respectively. Surprisingly, the decay time constant of a shorter-conjugated Car, i.e. 0.72 ώs (aerobic) and 1.36 ώs (anaerobic), is smaller than that of a longer-conjugated Car, i.e. 2.12 us (aerobic) and 3.75 ώs (anaerobic), which is contradictory to the general rule of carotenoids and relative polyenes. The results are explained in terms of triplet-excitation transfer among different types of Cars. It is postulated that two Cars with different conjugation lengths coexist in an α, β-subunit in the LH2 complex.     

Angle-resolved metastable fragment yields spectra of N2 and CO in K-edge excitation energy region

Angle-resolved metastable fragments yields spectra have been measured in the N 1s ionization region of the N(2) and C 1s ionization region of CO. These spectra are compared with zero kinetic energy electron and photoelectron spectra. It has been shown that an isotropic metastable fragments yields spectra are almost identical with the ZEKE spectrum, whereas metastable fragments yields spectra with the Σ-Σ transition show similarity with photoelectron spectra. This means that these spectra clearly contain information about two shake-up mechanisms: conjugate and direct shake-up processes. All the peaks in the metastable photofragment spectra can be assigned as either satellite states or double/triple excitation states. Thus, it was shown that angle-resolved metastable photofragment spectroscopy could be used to help characterize multi-electron excitation states in general.     

Formation of HO2 radicals from the 248 nm two-photon excitation of different aromatic hydrocarbons in the presence of O2

The excitation energy dependence of HO(2) radical formation from the 248 nm irradiation of four different aromatic hydrocarbons (benzene, toluene, o-xylene, and mesitylene) in the presence of O(2) has been studied. HO(2) has been monitored at 6638.20 cm(-1) by cw-CRDS, and the formation of a short-lived, unidentified species, showing broad-band absorption around the HO(2) absorption line, has been observed. For all four hydrocarbons, the same HO(2) formation pattern has been observed: HO(2) is formed immediately on our time scale after the excitation pulse, followed by a formation of more HO(2) on a much longer time scale. Taking into account the absorption of the short-lived species, the yields of both types of HO(2) radicals are in agreement with a formation following 2-photon absorption by the aromatic hydrocarbons. The yields do not much depend on the nature of the aromatic hydrocarbon. For practical use in past and future experiments on aromatic hydrocarbons, an empirical value is given, allowing the estimation of the total concentration of HO(2) radicals formed at 40 Torr He in the presence of around [O(2)] = 1 × 10(17)cm(-3) as a function of the 248 nm excitation energy: [HO(2)]/[aromatic hydrocarbon] ≈ 2 × 10(-6) × E(2) (with E in mJ cm(-2)).     

Collisional energy transfer in the intermediate states used for optical-optical double resonance excitation of ion-pair states in I2

The optical-optical double resonance spectra of I(2) and I(2)-Xe mixtures at room temperature reported in the literature using a fixed-wavelength, broad band pump laser have now been recorded using a tuneable, narrow band source. We show that during the time of the overlapped laser pulses ( approximately 10 ns) and with 10-20 Torr of Xe there is widespread collisional energy transfer in the intermediate state and that this phenomenon offers an alternative explanation for the broad bands in the excitation spectrum, assigned to XeI(2) complexes by the authors of the earlier study (M. E. Akopyan, I. Y. Novikova, S. A. Poretsky and A. M. Pravilov, Chem. Phys., 2005, 310, 287). Dispersed emission bands, previously attributed to direct fluorescence from the ion-pair state(s) of the complexes, are re-assigned to emission from ion-pair states of the parent I(2) that are populated by collisional energy transfer out of the initially excited state.     

细菌光合反应中心突变体原初电子转移机理的理 论研究

利用量子化学DFT从头计算方法,计算经过突变的细菌光合反应中心HM202L原始电子给体和其他色素分子的电子结构,然后对其原初电子转移机理进行探讨。结果表明：1)超分子D-2A的HOMO主要是由定域在其组成单元BChl~L分子上的原子轨道组成,而它的LUMO主要是由定域在其组成单元MBPheo~M分子上的原子轨道组成。这表明它在基态的激发态时分别存在超分子内的电荷分离态[BChl~L^--MBPheo~M^+]和[BChl~L^+-MBPheo~M^-]。同时也说明了D-2A阳离子态的正电荷完全分布在组成单元细菌叶绿素分子BChl~L上,与实验事实相符。2)HM202L细菌光合反应中心原初电子转移反应存在由ABCha~L^h^*驱动的电子转移反应。     

Communication: Exciton-phonon information flow in the energy transfer process of photosynthetic complexes

Non-Markovian and nonequilibrium phonon effects are believed to be key ingredients in the energy transfer in photosynthetic complexes, especially in complexes which exhibit a regime of intermediate exciton-phonon coupling. In this work, we utilize a recently developed measure for non-Markovianity to elucidate the exciton-phonon dynamics in terms of the information flow between electronic and vibrational degrees of freedom. We study the measure in the hierarchical equation of motion approach which captures strong coupling effects and nonequilibrium molecular reorganization. We propose an additional trace distance measure for the information flow that could be extended to other master equations. We find that for a model dimer system and for the Fenna-Matthews-Olson complex the non-Markovianity is significant under physiological conditions.     

A model calculation on the transport of excitation energy in a molecular crystal

We report a model calculation of the transport of a local (site) excitation in a doped molecular crystal containing one impurity. We do not consider the impurity as a direct trap for electronic excitations (zero trap depth) but assume that exciton-phonon interaction is exclusively given by the coupling of excitons with the vibrational displacement of the impurity. The dynamical problem is solved by using a time-dependent effective potential consisting of equilibrium average exciton-phonon interaction and fluctuations around this average. Two correlation functions are computed using the slow phonon limit and assuming that the temperature of the system is 300 K. Transmission of the excitation energy over a distance of eight spacings takes place, electronically, within a few picoseconds. With the exciton-phonon interaction switched on, calculated correlation functions diminish very rapidly with increasing time, indicating that an irreversible transfer of excitonic energy to the thermal bath takes place. Thus transmission of the excitation energy over such a distance (and without a high rate of trapping) is not an efficient process.