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Using a symmetry adapted polaron transformation of the Holstein Hamiltonian, we study the interplay of electronic excitation-vibration couplings, resonance excitation transfer interactions, and temperature in the linear absorption spectra of molecular J-aggregates. Semi-analytical expressions for the spectra are derived and compared with results obtained from direct numerical diagonalization of the Hamiltonian in the two-particle basis set representation. At zero temperature, we show that our polaron transformation reproduces both the collective (exciton) and single-molecule (vibrational) optical response associated with the appropriate standard perturbation limits. Specifically, for the molecular dimer excellent agreement with the spectra from the two-particle approach for the entire range of model parameters is obtained. This is in marked contrast to commonly used polaron transformations. Upon increasing the temperature, the spectra show a transition from the collective to the individual molecular features, which results from the thermal destruction of the exciton coherence.

International audience; Cyanobacteria use chlorophyll and phycobiliproteins to harvest light. The resulting excitation energy is delivered to reaction centers (RCs), where photochemistry starts. The relative amounts of excitation energy arriving at the RCs of photosystem I (PSI) and II (PSII) depend on the spectral composition of the light. To balance the excitations in both photosystems, cyanobacteria perform state transitions to equilibrate the excitation energy. They go to state I if PSI is preferentially excited, for example after illumination with blue light (light I), and to state II after illumination with green-orange light (light II) or after dark adaptation. In this study, we performed 77-K time-resolved fluorescence spectroscopy on wild-type Synechococcus elongatus 7942 cells to measure how state transitions affect excitation energy transfer to PSI and PSII in different light conditions and to test the various models that have been proposed in literature. The time-resolved spectra show that the PSII core is quenched in state II and that this is not due to a change in excitation energy transfer from PSII to PSI (spill-over), either direct or indirect via phycobilisomes.

State transitions represent a photoacclimation process that regulates the light-driven photosynthetic reactions in response to changes in light quality/quantity. It balances the excitation between photosystem I (PSI) and II (PSII) by shuttling LHCII, the main light-harvesting complex of green algae and plants, between them. This process is particularly important in Chlamydomonas reinhardtii in which it is suggested to induce a large reorganization in the thylakoid membrane. Phosphorylation has been shown to be necessary for state transitions and the LHCII kinase has been identified. However, the consequences of state transitions on the structural organization and the functionality of the photosystems have not yet been elucidated. This situation is mainly because the purification of the supercomplexes has proved to be particularly difficult, thus preventing structural and functional studies. Here, we have purified and analysed PSI and PSII supercomplexes of C. reinhardtii in states 1 and 2, and have studied them using biochemical, spectroscopic and structural methods. It is shown that PSI in state 2 is able to bind two LHCII trimers that contain all four LHCII types, and one monomer, most likely CP29, in addition to its nine Lhcas. This structure is the largest PSI complex ever observed, having an antenna size of 340 Chls/P700. Moreover, all PSI-bound Lhcs are efficient in transferring energy to PSI. A projection map at 20 Å resolution reveals the structural organization of the complex. Surprisingly, only LHCII type I, II and IV are phosphorylated when associated with PSI, while LHCII type III and CP29 are not, but CP29 is phosphorylated when associated with PSII in state2. © 2014 The Authors The Plant Journal © 2014 John Wiley & Sons Ltd.

Near-infrared fluorescent proteins (NIR FPs) engineered from bacterial phytochromes (BphPs) are of great interest for in vivo imaging. They utilize biliverdin (BV) as a chromophore, which is a heme degradation product, and therefore they are straightforward to use in mammalian tissues. Here, we report on fluorescence properties of NIR FPs with key alterations in their BV binding sites. BphP1-FP, iRFP670 and iRFP682 have Cys residues in both PAS and GAF domains, rather than in the PAS domain alone as in wild-type BphPs. We found that NIR FP variants with Cys in the GAF or with Cys in both PAS and GAF show blue-shifted emission with long fluorescence lifetimes. In contrast, mutants with Cys in the PAS only or no Cys residues at all exhibit red-shifted emission with shorter lifetimes. Combining these results with previous biochemical and BphP1-FP structural data, we conclude that BV adducts bound to Cys in the GAF are the origin of bright blue-shifted fluorescence. We propose that the long fluorescence lifetime follows from (i) a sterically more constrained thioether linkage, leaving less mobility for ring A than in canonical BphPs, and (ii) that π-electron conjugation does not extend on ring A, making excited-state deactivation less sensitive to ring A mobility.

The PsbS protein is a critical component in the regulation of non-photochemical quenching (NPQ) in higher plant photosynthesis. Electron microscopy and image analysis of grana membrane fragments from wild type and mutant Arabidopsis plants showed that the semi-crystalline domains of photosystem II supercomplexes were identical in the presence and absence of PsbS. However, the frequency of the domains containing crystalline arrays was increased in the absence of PsbS. Conversely, there was a complete absence of such arrays in the membranes of plants containing elevated amounts of this protein. It is proposed that PsbS controls the macro-organisation of the grana membrane, providing an explanation of its role in NPQ. (C) 2009 Federation of European Biochemical Societies. Published by Elsevier B. V. All rights reserved.

A recently developed technique for dilution of the naturally high protein packing density in isolated grana membranes was applied to study the dependence of the light harvesting efficiency of photosystem (PS) II on macromolecular crowding. Slight dilution of the protein packing from 80% area fraction to the value found in intact grana thylakoids (70%) leads to an improved functionality of PSII (increased antenna size, enhanced connectivity between reaction centers). Further dilution induces a functional disconnection of light-harvesting complex (LHC) II from PSII. It is concluded that efficient light harvesting by PSII requires an optimal protein packing density in grana membranes that is close to 70%. We hypothesize that the decreased efficiency in overcrowded isolated grana thylakoids is caused by excited state quenching in LHCII, which has previously been correlated with neoxanthin distortion. Resonance Raman spectroscopy confirms this increase in neoxanthin distortion in overcrowded grana as compared with intact thylakoids. Furthermore, analysis of the changes in the antenna size in highly diluted membranes indicates a lipid-induced dissociation of up to two trimeric LHCII from PSII, leaving one trimer connected. This observation supports a hierarchy of LHCII-binding sites on PSII.

The fluorescence induction F(t) of dark-adapted chloroplasts has been studied in multi-turnover 1 s light flashes (MTFs). A theoretical expression for the initial fluorescence rise is derived from a set of rate equations that describes the sequence of transfer steps associated with the reduction of the primary quinone acceptor Q A and the release of photochemical fluorescence quenching of photosystem II (PSII). The initial F(t) rise in the hundreds of μs time range is shown to follow the theoretical function dictated by the rate constants of light excitation (k L) and release of donor side quenching (k si ). The bi-exponential function shows sigmoidicity when one of the two rate constants differs by less than one order of magnitude from the other. It is shown, in agreement with the theory, that the sigmoidicity of the fluorescence rise is variable with light intensity and mainly, if not exclusively, determined by the ratio between rate of light excitation and the rate constant of donor side quenching release.

Polylepis tarapacana forms the world's highest forest, being able to grow up to 5,200 m a.s.l. At such elevations, low temperatures, high solar radiation and water scarcity severely restrict plant survival. Our study was focused on the photosynthetic responses of Polylepis species and how they are able to cope with such a challenging environment. We performed all measurements and samplings in their natural environment. This strategy allowed us to observe unexpected patterns of daily adjustments in photosynthetic pigments, which reflect major changes in the structure and organization of the photosynthetic apparatus., Plant growth at extremely high elevations is constrained by high daily thermal amplitude, strong solar radiation and water scarcity. These conditions are particularly harsh in the tropics, where the highest elevation treelines occur. In this environment, the maintenance of a positive carbon balance involves protecting the photosynthetic apparatus and taking advantage of any climatically favourable periods. To characterize photoprotective mechanisms at such high elevations, and particularly to address the question of whether these mechanisms are the same as those previously described in woody plants along extratropical treelines, we have studied photosynthetic responses in Polylepis tarapacana Philippi in the central Andes (18°S) along an elevational gradient from 4300 to 4900 m. For comparative purposes, this gradient has been complemented with a lower elevation site (3700 m) where another Polylepis species (P. rugulosa Bitter) occurs. During the daily cycle, two periods of photosynthetic activity were observed: one during the morning when, despite low temperatures, assimilation was high; and the second starting at noon when the stomata closed because of a rise in the vapour pressure deficit and thermal dissipation is prevalent over photosynthesis. From dawn to noon there was a decrease in the content of antenna pigments (chlorophyll b and neoxanthin), together with an increase in the content of xanthophyll cycle carotenoids. These results could be caused by a reduction in the antenna size along with an increase in photoprotection. Additionally, photoprotection was enhanced by a partial overnight retention of de-epoxized xanthophylls. The unique combination of all of these mechanisms made possible the efficient use of the favourable conditions during the morning while still providing enough protection for the rest of the day. This strategy differs completely from that of extratropical mountain trees, which uncouple light-harvesting and energy-use during long periods of unfavourable, winter conditions.

In this article we report the characterization of the energy transfer process in the reconstituted isoforms of the plant light-harvesting complex II. Homotrimers of recombinant Lhcb1 and Lhcb2 and monomers of Lhcb3 were compared to native trimeric complexes. We used low-intensity femtosecond transient absorption (TA) and time-resolved fluorescence measurements at 77 K and at room temperature, respectively, to excite the complexes selectively in the chlorophyll b absorption band at 650 nm with 80 fs pulses and on the high-energy side of the chlorophyll a absorption band at 662 nm with 180 fs pulses. The subsequent kinetics was probed at 30-35 different wavelengths in the region from 635 to 700 nm. The rate constants for energy transfer were very similar, indicating that structurally the three isoforms are highly homologous and that probably none of them play a more significant role in light-harvesting and energy transfer. No signature has been found in the transient absorption measurements at 77 K for Lhcb3 which might suggest that this protein acts as a relative energy sink of the excitations in heterotrimers of Lhcb1/Lhcb2/Lhcb3. Minor differences in the amplitudes of some of the rate constants and in the absorption and fluorescence properties of some pigments were observed, which are ascribed to slight variations in the environment surrounding some of the chromophores depending on the isoform. The decay of the fluorescence was also similar for the three isoforms and multi-exponential, characterized by two major components in the ns regime and a minor one in the ps regime. In agreement with previous transient absorption measurements on native LHC II complexes, Chl b → Chl a energy transfer exhibited very fast channels but at the same time a slow component (ps). The Chls absorbing at around 660 nm exhibited both fast energy transfer which we ascribe to transfer from 'red' Chl b towards 'red' Chl a and slow transfer from 'blue' Chl a towards 'red' Chl a. The results are discussed in the context of the new available atomic models for LHC II. © Springer 2006.

Photoacclimation towards rapid changes in irradiance was studied in 2 Antarctic microalgae, the diatom Chaetoceros brevis (Bacillariophyceae) and the flagellate Pyramimonas sp. (Prasinophyceae). Both species were subjected to 3 different light regimes. Two regimes of vertical mixing (1:1 h and 3:3 h high light:low light) were superimposed on a diurnal light cycle mimicking the solar sine. For both species, maximum growth was recorded at the fastest rates of light fluctuation (1: 1 h). The light regimes had a minor effect on growth rates of the flagellate but growth in the diatom was reduced significantly at moderate rates of mixing (3:3 h). While xanthophyll cycling was active in both species, fluorescence analysis with a dual-modulated fluorometer showed that down-regulation of photosynthetic efficiency was more pronounced in Pyramimonas sp. (ca. 50%) than in C. brevis (ca. 30%). The ratio of photoprotectors to chlorophyll a was not significantly affected by the light regime, neither in the flagellate nor in the diatom. In the flagellate, state transition offered an additional means of energy dissipation during the periods of high irradiance. The diatom apparently acclimated to the average light intensity received and not to the maximum irradiance. It can be reasoned that this strategy led to photodamage during the 3:3 h cycling. It is argued that enhanced maintenance costs associated with necessary repair processes can explain the reduced growth rate that was observed. The ecological consequences for large bloom-forming diatom species are briefly discussed.

In order to maximize their use of light energy in photosynthesis, plants have molecules that act as light-harvesting antennae, which collect light quanta and deliver them to the reaction centres, where energy conversion into a chemical form takes place. The functioning of the antenna responds to the extreme changes in the intensity of sunlight encountered in nature(1-3). In shade, light is efficiently harvested in photosynthesis. However, in full sunlight, much of the energy absorbed is not needed and there are vitally important switches to specific antenna states, which safely dissipate the excess energy as heat(2,3). This is essential for plant survival(4), because it provides protection against the potential photo-damage of the photosynthetic membrane(5). But whereas the features that establish high photosynthetic efficiency have been highlighted(6), almost nothing is known about the molecular nature of the dissipative states. Recently, the atomic structure of the major plant light-harvesting antenna protein, LHCII, has been determined by X-ray crystallography(7). Here we demonstrate that this is the structure of a dissipative state of LHCII. We present a spectroscopic analysis of this crystal form, and identify the specific changes in configuration of its pigment population that give LHCII the intrinsic capability to regulate energy flow. This provides a molecular basis for understanding the control of photosynthetic light-harvesting.

The electric-field induced absorption changes (Stark effect) of reconstituted light-harvesting complex II (LHCII) in different oligomerisation states - monomers and trimers - with different xanthophyll content have been probed at 77 K. The Stark spectra of the reconstituted control samples, containing the xanthophylls lutein and neoxanthin, are very similar to previously reported spectra of native LHCII. Reconstituted LHCII, containing lutein but no neoxanthin, shows a similar electrooptical response in the Chl a region, but the Stark signal of Chl b around 650 nm amounts to at most ∼25% of that of the control samples. We conclude that neoxanthin strongly modifies the electronic states of the nearby Chl b molecules causing a large electrooptical response at 650 nm stemming from one or more Chls b in the control samples. Ambiguities about the assignment of several bands in the Soret region [Biochim. Biophys. Acta 1605 (2003) 83] are resolved and the striking difference in electric field response between the two lutein molecules is confirmed. The Stark effect in the carotenoid spectral region in both control and neoxanthin-deficient samples is almost identical, showing that the neoxanthin Stark signal is small and much less intense than the lutein Stark signal. © 2004 Elsevier B.V. All rights reserved.

We have modeled energy-transfer dynamics in the peripheral plant light-harvesting complex LHCII using both standard Redfield theory and its modification for the case of strong exciton-phonon coupling (Zhang, W. M.; Meier, T.; Chernyak, V.; Mukamel, S. J. Chem. Phys. 1998, 108, 7763). A quantitative simultaneous fit of the absorption (OD), linear dichroism (LD), steady-state fluorescence (FL) spectra at 7-293 K, and transient absorption (TA) kinetics at 77 and 293 K has been obtained using the experimental exciton-phonon spectral density to model the temperature-dependent line shape. We use configurations of the antenna (i.e., chlorophyll (Chi) a/b identities, orientations, and site energies) close to those proposed in our previous study (Novoderezhkin, V.; Salverda, J. M.; van Amerongen, H.; van Grondelle, R. J. Phys. Chem. B 2003, 107, 1893). These configurations have been further adjusted from the fit with the modified Redfield approach. The new (adjusted) models allow a better quantitative explanation of the spectral shapes. A combination of fast (femtosecond) interband energy transfer and slow (picosecond) intraband equilibration can be better reproduced as well. We paid special attention to unravel the origins of the slow components preliminarily assigned to localized states in the previous work. These "bottleneck" states have been directly visualized in this study via selective femtosecond excitation and probing at different wavelengths. In our modeling, these states are determined by two or three (depending on the model) monomeric Chls a or b shifted to the spectral region of 655-670 nm between the main absorption peaks of Chi b (650 nm) and Chl a (675 nm). In all configurations we have found these energy-shifted Chls to be bound at mixed sites (i.e., A(3), A(6), A(7), or B-3). Experiments and self-consistent modeling using the modified Redfield theory allow us to explore the participation of these states in the overall energy-transfer dynamics. This has led to a more complete and physically adequate model for the energy-transfer dynamics in LHCII.

The formation and decay of hot nuclei generated in the interaction of light projectiles (475 MeV and 2 GeV protons and 2 GeV He-3) on a series of targets (Ag-107, Au-197, Bi-209, and U-238) are studied with an apparatus combining the efficient detection of neutrons in 4 pi sr and an accurate characterization of light charged particles, intermediate-mass fragments (IMF's), and fission fragments. A two-step approach with an intranuclear cascade process for modeling the initial off-equilibrium phase of the collision followed by a classical step-by-step evaporation-including fission competition-is used to reproduce the data. It is inferred from the model, which is found to reproduce several data sensitive to heat, that nuclei with temperatures exceeding T =5 MeV are produced for a sizable part of the events, thus giving the opportunity to study the behavior of hot nuclei free from strong collective excitations which generally accompany nucleus-nucleus collisions. Most of the observed features related to particle emission or more specifically to particle evaporation are rather well accounted for by the model calculation. The evaporationlike IMF emission is generally rather weak, and does not show any rapid onset at the highest excitation energies as would have been expected in a genuine thermal multifragmentation process. Binary fission of the U-like target is shown to be a fairly probable channel at most excitation energies. Some of the characteristics of the fission channel are satisfactorily reproduced, but not all. [S0556-2813(98)02605-3].

Nuclear states in Si-28, with an initial excitation energy E*=50 MeV, were populated via the isospin T=0 reaction He-4+Mg-24 and the mixed-isospin He-3+Mg-25 reaction. In both reactions the dilepton (e(+)e(-)) and photon decay yields were measured concurrently, up to transition energies of 30 MeV. Both the dilepton and the photon yields are well described by a modified version of the statistical decay model, which includes both the photon and dilepton decays of giant resonances built on excited nuclear states. An excess of counts in the e(+)e(-) spectrum, over the converted photon yield, is observed in the energy region 17-22 MeV in the He-3 induced reaction, possibly indicating an entrance channel effect.

Background The thylakoid system in plant chloroplasts is organized into two distinct domains: grana arranged in stacks of appressed membranes and non-appressed membranes consisting of stroma thylakoids and margins of granal stacks. It is argued that the reason for the development of appressed membranes in plants is that their photosynthetic apparatus need to cope with and survive ever-changing environmental conditions. It is not known however, why different plant species have different arrangements of grana within their chloroplasts. It is important to elucidate whether a different arrangement and distribution of appressed and non-appressed thylakoids in chloroplasts are linked with different qualitative and/or quantitative organization of chlorophyll-protein (CP) complexes in the thylakoid membranes and whether this arrangement influences the photosynthetic efficiency. Results Our results from TEM and in situ CLSM strongly indicate the existence of different arrangements of pea and bean thylakoid membranes. In pea, larger appressed thylakoids are regularly arranged within chloroplasts as uniformly distributed red fluorescent bodies, while irregular appressed thylakoid membranes within bean chloroplasts correspond to smaller and less distinguished fluorescent areas in CLSM images. 3D models of pea chloroplasts show a distinct spatial separation of stacked thylakoids from stromal spaces whereas spatial division of stroma and thylakoid areas in bean chloroplasts are more complex. Structural differences influenced the PSII photochemistry, however without significant changes in photosynthetic efficiency. Qualitative and quantitative analysis of chlorophyll-protein complexes as well as spectroscopic investigations indicated a similar proportion between PSI and PSII core complexes in pea and bean thylakoids, but higher abundance of LHCII antenna in pea ones. Furthermore, distinct differences in size and arrangements of LHCII-PSII and LHCI-PSI supercomplexes between species are suggested. Conclusions Based on proteomic and spectroscopic investigations we postulate that the differences in the chloroplast structure between the analyzed species are a consequence of quantitative proportions between the individual CP complexes and its arrangement inside membranes. Such a structure of membranes induced the formation of large stacked domains in pea, or smaller heterogeneous regions in bean thylakoids. Presented 3D models of chloroplasts showed that stacked areas are noticeably irregular with variable thickness, merging with each other and not always parallel to each other.

Chlorophyll fluorescence induction of tomato leaf discs was measured at a low actinic light intensity of 10 mu mol.m(-2).s(-1) and at decreasing temperatures from 30 degrees to 0 degrees C. F-o remained constant within the temperature range assessed. In contrast, the peak of fluorescence induction, F-p, showed a strong temperature dependence. F-p increased by lowering the temperature within two temperature regions, from 30 degrees to 22 degrees C and from 14 degrees to 0 degrees C, respectively. F-p was hardly affected by temperatures between 14 degrees and 22 degrees C. Consequently, two temperature breakpoints of F-p were observed at 14 degrees and 22 degrees C.F-p reflected a maximum reduction state of the primary electron acceptor of photosystem II, Q(A). The relation between F-p and maximal Q(A) reduction was used to explain the observed temperature effects on F-p. The reduction state of Q(A) depended on both photosystem I oxidizing and photosystem II reducing activity. Consequently, breaks in the temperature response of F-p were attributed to dissimilar effects of temperature on photosystem I and photosystem II functioning.The steep increase of F-p below 14 degrees C was attributed to a low temperature induced impaired electron transport. This was sustained by the absence of a low temperature break when electron transport was inhibited by DCMU. A limitation of Q(A) oxidizing activity below 14 degrees C could be deduced from the temperature dependence of F-p of control discs.The second break in the temperature response of F-p at 22 degrees C was not caused by temperature effects on electron transport. The break of F-p at 22 degrees C was also found for F-m of electron transport inhibited leaf discs. While F-m decreased above 22 degrees C, the ratio of F-m determined at 687 and 730 nm (F(m)730/F(m)687) increased. This indicates a redistribution of excitation energy between the photosystems.Kinetic analysis of fluorescence induction traces showed a temperature effect on PSII heterogeneity. From 22 degrees to 30 degrees C, the PSII alpha fraction decreased from 70 to 50 % while the PSII beta fraction increased from 35 to 50 %. These results also suggest a temperature induced redistribution of excitation energy above 22 degrees C in favour of PSI. Implications of temperature effects on thylakoid membrane associated processes and plant functioning are discussed.

The statistical gamma-ray decay of the compound nucleus 154Dy* formed at an excitation energy of 69 MeV is studied in three angular-momentum windows [J] = 31,42 and 50h. The GDR strength function extracted from the data indicates a constant centroid energy for the resonance E(GDR) = 15.2 +/- 0.5 MeV, a small deformation of the nuclear shape for all spin regions and an increase in the resonance width with increasing angular momentum. The latter might indicate larger shape fluctuations at higher angular momenta. A systematical study of the dependence of the extracted strength function with different methods of analysis is presented, and their discrepancies discussed.

In this paper the model and simulation of primary photochemical and photo-electrochemical reactions in dark-adapted intact plant leaves is presented. A descriptive algorithm has been derived from analyses of variable chlorophyll a fluorescence and P700 oxidation kinetics upon excitation with multi-turnover pulses (MTFs) of variable intensity and duration. These analyses have led to definition and formulation of rate equations that describe the sequence of primary linear electron transfer (LET) steps in photosystem II (PSII) and of cyclic electron transport (CET) in PSI. The model considers heterogeneity in PSII reaction centers (RCs) associated with the S-states of the OEC and incorporates in a dark-adapted state the presence of a 15–35% fraction of QB-nonreducing RCs that probably is identical with the S0 fraction. The fluorescence induction algorithm (FIA) in the 10 μs–1 s excitation time range considers a photochemical O-J-D, a photo-electrochemical J-I and an I-P phase reflecting the response of the variable fluorescence to the electric trans-thylakoid potential generated by the proton pump fuelled by CET in PSI. The photochemical phase incorporates the kinetics associated with the double reduction of the acceptor pair of pheophytin (Phe) and plastoquinone QA [PheQA] in QB nonreducing RCs and the associated doubling of the variable fluorescence, in agreement with the three-state trapping model (TSTM) of PS II. The decline in fluorescence emission during the so called SMT in the 1–100 s excitation time range, known as the Kautsky curve, is shown to be associated with a substantial decrease of CET-powered proton efflux from the stroma into the chloroplast lumen through the ATPsynthase of the photosynthetic machinery.

This paper deals with kinetics and properties of variable fluorescence in leaves and thylakoids upon excitation with low intensity multi-turnover actinic light pulses corresponding with an excitation rate of about 10 Hz. These show a relatively small and amply documented rise in the sub-s time range towards the plateau level F(pl) followed by a delayed and S-shaped rise towards a steady state level F(m) which is between three and four fold the initial dark fluorescence level F(o). Properties of this retarded slow rise are i) rate of dark recovery is (1-6 s)(-1), ii) suppression by low concentration of protonophores, iii) responsiveness to complementary single turnover flash excitation with transient amplitude towards a level F(m) which is between five and six fold the initial dark fluorescence level F(o) and iv) in harmony with and quantitatively interpretable in terms of a release of photoelectrochemical quenching controlled by the trans-thylakoid proton pump powered by the light-driven Q cycle. Data show evidence for a sizeable fluorescence increase upon release of (photo) electrochemical quenching, defined as qPE. Release of qPE occurs independent of photochemical quenching defined here as qPP even under conditions at which qPP = 1. The term photochemical quenching, hitherto symbolized by qP, will require a new definition, because it incorporates in its present form a sizeable photoelectrochemical component. The same is likely to be true for definition and use of qN as an indicator of non photochemical quenching.

Time-resolved fluorescence on oligomers of the main light-harvesting complex from higher plants indicate that in vitro oligomerization leads to the formation of a weakly coupled inter-trimer chlorophyll-chlorophyll (Chl) exciton state which converts in tens of ps into a state which is spectrally broad and has a strongly far-red enhanced fluorescence spectrum. Both its lifetime and spectrum show striking similarity with a 400 ps fluorescence component appearing in intact leaves of Arabidopsis when non-photochemical quenching (NPQ) is induced. The fluorescence components with high far-red/red ratio are thus a characteristic marker for NPQ conditions in vivo. The far-red emitting state is shown to be an emissive Chl-Chl charge transfer state which plays a crucial part in the quenching. (c) 2008 Federation of European Biochemical Societies. Published by Elsevier B. V. All rights reserved.

We prepare a temperature-dependent formulation of the integrated kinetics for the overall process of photosynthesis in eukaryotic cells. To avoid complexity, the C4 plants are chosen because their rate of photosynthesis is independent of the partial pressure of O2. A systematically simplified but comprehensive scheme for both light and dark reactions is considered. The reaction rate per reaction center in the thylakoid membrane is related to the rate of exciton transfer between chlorophyll neighbors. An expression is formulated for the light reaction rate (R1'). The NADPH formation rate is related to R1' and the survival probability of the membrane. Rates of different steps in the simplified scheme can be related to each other by applying a few steady state conditions. The saturation probability of CO2 in a bundle sheath is also considered. The photochemical efficiency (phi) appears in terms of these probabilities. We find the glucose production rate as R(glucose) = (8/3) upsilon L: [corrected] R1'phi g(T)([G3P]/[P(i)]2) exp(-deltaG(E)S/RT), where g(T) is the activation quotient of the involved enzymes, G3P and P(i) represent glyceraldehyde-3-phosphate and inorganic phosphates, and deltaG(E)S is the free energy for the apparent equilibrium between G3P and glucose. This is the first time that such a comprehensive expression for R(glucose) has been derived. The probabilities are generally given by sigmoid curves. The corresponding parameters can be easily determined. The quotient g(T) incorporates a Gaussian distribution for temperature dependence and a sigmoid function describing deactivation. The theoretical plots of photochemical efficiency and glucose production rate versus temperature are in excellent agreement with the experimental ones, thereby validating the formalism.

We propose an exciton model for the peripheral plant light-harvesting complex LHCII that allows us to explain the absorption (OD) and linear dichroism (LD) spectra, the superradiance (SR), the pump-probe transient absorption (TA), the three-pulse photon echo peak shift (3PEPS), and transient grating (TG) kinetics at different excitation wavelengths. To calculate the nonlinear response we used the Liouville equation for the density matrix, expanded up to the third order with respect to the external field, with the Redfield relaxation superoperator in the exciton eigenstate basis. We found a few configurations of the antenna, with specific chlorophyll (Chl) a/b identities, orientations, and site energies, that allowed a simultaneous fit of these data while taking into account the excitonic interactions, the static disorder, and weak exciton-phonon coupling which induced dephasing and relaxation (energy transfer) between the exciton states. The spectral density of the exciton-phonon coupling adjusted from the fit allowed us to determine the time scales and pathways of energy transfer in LHCII. We find that the intraband (Chl b-->Chl b and Chl a-->Chl a) energy-transfer dynamics includes sub-picosecond (250-600 fs) exciton relaxation within dimeric or, in the Chl a band, more complicated clusters, sub-picosecond (600-800 fs) hopping between spatially separated clusters (in the a band), and "slow" (picosecond) migration between localized states. The interband (Chl b-->Chl a) transfer is characterized by the presence of very fast channels, the fastest taking only 120 fs, which connect both localized and dimeric b states with the a band. The overall relaxation/migration dynamics can be directly viewed by means of the time-dependent density matrix in the exciton and site representation. The latter representation allows us to visualize the time-dependent degree of delocalization. While the individual exciton states can be delocalized over 2-2.5 molecules in the Chl a region, thermal mixing results in a coherence size of 1.4-1.8 for the steady-state wave packet at room temperature. Altogether, we conclude that the experimentally determined dynamic and static properties of LHCII can be simulated very well on the basis of the proposed excitonic model.

Three pulse echo peak shift and transient grating (TG) measurements on the plant light-harvesting complexes LHCII and CP29 are reported. The LHCII complex is by far the most abundant light-harvesting complex in higher plants and fulfills several important physiological functions such as light-harvesting and photoprotection. Our study is focused on the light-harvesting function of LHCII and the very similar CP29 complex and reveals hitherto unresolved excitation energy transfer processes. All measurements were performed at room temperature using detergent isolated complexes from spinach leaves. Both complexes were excited in their Chl b band at 650 nm and in the blue shoulder of the Chl a band at 670 nm. Exponential,: fits to the TG and three pulse echo peak shift decay curves were used to estimate the timescales of the observed energy transfer processes. At 650 nm, the TG decay can be described with time constants of 130 fs and 2.2 ps for CP29, and 300 fs and 2.8 ps for LHCII. At 670 nm, the TG shows decay components of 230 fs and 6 ps for LHCII, and 300 fs and 5 ps for CP29. These time constants correspond to well-known energy transfer processes, from Chl b to Chl a for the 650 nm TG and from blue (670 nm) Chl a to red (680 nm) Chl a for the 670 nm TG. The peak shift decay times are entirely different. At 650 nm we find times of 150 fs and 0.5-1 ps for LHCII, and 360 fs and 3 ps for CP29, which we can associate mainly with Chl b Chl b,energy transfer. At 670 nm we find times of 140 fs and 3 ps for LHCII, and 3 ps for CP29, which we can associate with fast (only in LHCII) and slow transfer between relatively blue Chls a or Chl a states. From the occurrence of both fast Chl b Chl, bland fast Chl b --> Chl a transfer in CP29, we conclude that at least two mixed binding sites are present in this complex. A detailed comparison of our observed rates with exciton calculations on both CP29 and LHCII provides us with more insight in the location of these mixed sites. Most importantly, for CP29, we find that a Chl b pair must be present in some, but not all, complexes, on sites A(3) and B-3. For LHCII, the observed rates can best be understood if the same pair, A(3) and B-3, is involved in both fast Chl b Chl b and fast Chl a Chl a transfer. Hence, it is likely that mixed sites also occur in the native LHCII complex. Such flexibility in chlorophyll binding would agree with the general flexibility in aggregation form and xanthophylli, binding of the LHCII complex and could be of use for optimizing the role of LHCII under specific circumstances, for example under high-light conditions. Our study is the first to provide spectroscopic evidence for mixed binding sites, as well as the first to show their existence in native complexes.

The formation and decay of hot nuclei generated in the interaction of light projectiles (475 MeV and 2 GeV protons and 2 GeV He-3) on a series of targets (Ag-107, Au-197, Bi-209, and U-238) are studied with an apparatus combining the efficient detection of neutrons in 4 pi sr and an accurate characterization of light charged particles, intermediate-mass fragments (IMF's), and fission fragments. A two-step approach with an intranuclear cascade process for modeling the initial off-equilibrium phase of the collision followed by a classical step-by-step evaporation-including fission competition-is used to reproduce the data. It is inferred from the model, which is found to reproduce several data sensitive to heat, that nuclei with temperatures exceeding T =5 MeV are produced for a sizable part of the events, thus giving the opportunity to study the behavior of hot nuclei free from strong collective excitations which generally accompany nucleus-nucleus collisions. Most of the observed features related to particle emission or more specifically to particle evaporation are rather well accounted for by the model calculation. The evaporationlike IMF emission is generally rather weak, and does not show any rapid onset at the highest excitation energies as would have been expected in a genuine thermal multifragmentation process. Binary fission of the U-like target is shown to be a fairly probable channel at most excitation energies. Some of the characteristics of the fission channel are satisfactorily reproduced, but not all. [S0556-2813(98)02605-3].

The gamma-ray emission in heavy-ion fusion reactions at low bombarding energy has been studied to investigate possible entrance channel effects on the de-excitation of the compound nucleus. The analysis of the experimental data shows that the charge (N/Z ratio) and the mass asymmetries between target and projectile could influence the formation and the decay of the excited nuclear system.

Nuclear states in Si-28, with an initial excitation energy E*=50 MeV, were populated via the isospin T=0 reaction He-4+Mg-24 and the mixed-isospin He-3+Mg-25 reaction. In both reactions the dilepton (e(+)e(-)) and photon decay yields were measured concurrently, up to transition energies of 30 MeV. Both the dilepton and the photon yields are well described by a modified version of the statistical decay model, which includes both the photon and dilepton decays of giant resonances built on excited nuclear states. An excess of counts in the e(+)e(-) spectrum, over the converted photon yield, is observed in the energy region 17-22 MeV in the He-3 induced reaction, possibly indicating an entrance channel effect.

Nuclear states in ${}^{28}\mathrm{Si}$ were populated at an initial excitation energy ${E}^{*}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}50$ MeV via the isospin $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ reaction ${}^{16}\mathrm{O}{+}^{12}\mathrm{C}$. The dilepton- and photon-decay yields were measured simultaneously for the first time up to transition energies of 30 MeV. Statistical decay calculations describe well both measurements on an absolute scale. The measured absolute dilepton cross section, integrated over the transition energy region 15--30 MeV, amounts to $\ensuremath{\sigma}({e}^{+}{e}^{\ensuremath{-}})\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}84\ifmmode\pm\else\textpm\fi{}6$ nb. Implications for measuring a possible monopole strength are discussed.

Nuclear states in Si-28 were populated at an initial excitation energy E* = 50 MeV via the isospin T = 0 reaction O-16 + C-12. The dilepton- and photon-decay yields were measured simultaneously for the first time up to transition energies of 30 MeV. Statistical decay calculations describe well both measurements on an absolute scale. The measured absolute dilepton cross section, integrated over the transition energy region 15-30 MeV, amounts to sigma(e(+)e(-)) = 84 +/- 6 nb. Implications for measuring a possible monopole strength are discussed.

Evaporative [sup 1,2,3]H and [sup 4]He yields and energy spectra have been analyzed in the framework of the statistical model in ten heavy-ion reactions, spanning composite systems with 20[lt][ital Z][lt]56. Transmission coefficients derived from the ingoing-wave boundary condition (IWBCM) or optical models (OM) have been used in the calculations. The effect of using different values of deformation parameters, level-density parameter [ital a] and radii of the real part of the transmission coefficient potentials has been investigated. Except for the low-energy side of the proton spectra, the shape of energy spectra are reasonably well reproduced by both sets of transmission coefficients. While the two models are essentially equivalent for [sup 1]H and [sup 4]He cross sections, they differ significantly in the prediction of [sup 2]H and [sup 3]H cross sections. OM transmission coefficients fail in reproducing [sup 2,3]H cross sections, unless no standard parameters are used, while IWBCM transmission coefficients provide a good overall agreement. The failure of the OM is ascribed to the inclusion of nonfusion reactions in the inverse process.

Chlorophyll fluorescence induction of tomato leaf discs was measured at a low actinic light intensity of 10 mu mol.m(-2).s(-1) and at decreasing temperatures from 30 degrees to 0 degrees C. F-o remained constant within the temperature range assessed. In contrast, the peak of fluorescence induction, F-p, showed a strong temperature dependence. F-p increased by lowering the temperature within two temperature regions, from 30 degrees to 22 degrees C and from 14 degrees to 0 degrees C, respectively. F-p was hardly affected by temperatures between 14 degrees and 22 degrees C. Consequently, two temperature breakpoints of F-p were observed at 14 degrees and 22 degrees C. F-p reflected a maximum reduction state of the primary electron acceptor of photosystem II, Q(A). The relation between F-p and maximal Q(A) reduction was used to explain the observed temperature effects on F-p. The reduction state of Q(A) depended on both photosystem I oxidizing and photosystem II reducing activity. Consequently, breaks in the temperature response of F-p were attributed to dissimilar effects of temperature on photosystem I and photosystem II functioning. The steep increase of F-p below 14 degrees C was attributed to a low temperature induced impaired electron transport. This was sustained by the absence of a low temperature break when electron transport was inhibited by DCMU. A limitation of Q(A) oxidizing activity below 14 degrees C could be deduced from the temperature dependence of F-p of control discs. The second break in the temperature response of F-p at 22 degrees C was not caused by temperature effects on electron transport. The break of F-p at 22 degrees C was also found for F-m of electron transport inhibited leaf discs. While F-m decreased above 22 degrees C, the ratio of F-m determined at 687 and 730 nm (F(m)730/F(m)687) increased. This indicates a redistribution of excitation energy between the photosystems. Kinetic analysis of fluorescence induction traces showed a temperature effect on PSII heterogeneity. From 22 degrees to 30 degrees C, the PSII alpha fraction decreased from 70 to 50 % while the PSII beta fraction increased from 35 to 50 %. These results also suggest a temperature induced redistribution of excitation energy above 22 degrees C in favour of PSI. Implications of temperature effects on thylakoid membrane associated processes and plant functioning are discussed.

Photon emission in proton-induced reactions at 50 MeV with In-115 was studied. Analyses of the measured photon spectrum show that the GDR couples to the compound states as well as to pre-equilibrium states. The centroid and width of the GDR strength function were determined as E(GDR) = 15.4 +/- 0.7 MeV and GAMMA(GDR) = 6.2 +/- 0.5 MeV. From the measured gamma-ray yield above E(gamma) = 30 MeV, a value of (7.1 +/- 1.4) x 10(-5) for the hard-photon emission probability per single proton-neutron collision is inferred.

The statistical gamma-ray decay of the compound nucleus 154Dy* formed at an excitation energy of 69 MeV is studied in three angular-momentum windows [J] = 31,42 and 50h. The GDR strength function extracted from the data indicates a constant centroid energy for the resonance E(GDR) = 15.2 +/- 0.5 MeV, a small deformation of the nuclear shape for all spin regions and an increase in the resonance width with increasing angular momentum. The latter might indicate larger shape fluctuations at higher angular momenta. A systematical study of the dependence of the extracted strength function with different methods of analysis is presented, and their discrepancies discussed.

Some of the advances made in our theoretical understanding of the Z scheme of photosynthesis in green plants are reviewed here. The Z-scheme embodies a sequence of light-activated, and very fast, electron transfer processes. The molecules under consideration belong to the central diagonal arm of the Z-scheme. These are plastoquinone-histidine complexes Q(I) and Q(II), plastoquinones (PQ), cytochrome-f and Rieske protein. The electronic structures of these species have been investigated by semiempirical quantum chemical methods - CNDO/2 and INDO. In one case, the rate of electron transfer (from Q(I)(-) to Q(II), and from Q(I)(-) to Q(II)(-)) has been theoretically determined. Results of these investigations serve in two ways. On the one hand, theoretical calculations confirm the observed redox potentials, and the rate of electron transfer On the other hand, a set of fascinating information can be obtained, which clarifies several ideas about the structure, function and placement of these biological molecules in the condensed phase of chloroplast.