The room temperature emission band shape of the lowest energy chlorophyll spectral form of LHCI

Ultrafast excited state dynamics in the monomeric and trimeric photosystem I core complex of Spirulina platensis probed by two-dimensional electronic spectroscopy

Photosystem I (PSI), a naturally occurring supercomplex composed of a core part and a light-harvesting antenna, plays an essential role in the photosynthetic electron transfer chain. Evolutionary adaptation dictates a large variability in the type, number, arrangement, and absorption of the Chlorophylls (Chls) responsible for the early steps of light-harvesting and charge separation. For example, the specific location of long-wavelength Chls (referred to as red forms) in the cyanobacterial core has been intensively investigated, but the assignment of the chromophores involved is still controversial. The most red-shifted Chl a form has been observed in the trimer of the PSI core of the cyanobacterium Spirulina platensis, with an absorption centered at ∼740 nm. Here, we apply two-dimensional electronic spectroscopy to study photoexcitation dynamics in isolated trimers and monomers of the PSI core of S. platensis. By means of global analysis, we resolve and compare direct downhill and uphill excitation energy transfer (EET) processes between the bulk Chls and the red forms, observing significant differences between the monomer (lacking the most far red Chl form at 740 nm) and the trimer, with the ultrafast EET component accelerated by five times, from 500 to 100 fs, in the latter. Our findings highlight the complexity of EET dynamics occurring over a broad range of time constants and their sensitivity to energy distribution and arrangement of the cofactors involved. The comparison of monomeric and trimeric forms, differing both in the antenna dimension and in the extent of red forms, enables us to extract significant information regarding PSI functionality.

Ultrafast excited-state dynamics in land plants Photosystem I core and whole supercomplex under oxidised electron donor conditions

The kinetics of excited-state energy migration were investigated by femtosecond transient absorption in the isolated Photosystem I-Light-Harvesting Complex I (PSI-LHCI) supercomplex and in the isolated PSI core complex of spinach under conditions in which the terminal electron donor P₇₀₀ is chemically pre-oxidised. It is shown that, under these conditions, the relaxation of the excited state is characterised by lifetimes of about 0.4 ps, 4.5 ps, 15 ps, 35 ps and 65 ps in PSI-LHCI and 0.15 ps, 0.3 ps, 6 ps and 16 ps in the PSI core complex. Compartmental spectral-kinetic modelling indicates that the most likely mechanism to explain the absence of long-lived (ns) excited states is the photochemical population of a radical pair state, which cannot be further stabilised and decays non-radiatively to the ground state with time constants in the order of 6-8 ps.

Kinetics and heterogeneity of energy transfer from light harvesting complex II to photosystem I in the supercomplex isolated from Arabidopsis

State transitions are a phenomenon that maintains the excitation balance between photosystem II (PSII) and photosystem I (PSI-LHCI) by controlling their relative absorption cross-sections. Under light conditions exciting PSII preferentially, a trimeric LHCII antenna moves from PSII to PSI-LHCI to form the PSI-LHCI-LHCII supercomplex. In this work, the excited state dynamics in the PSI-LHCI and PSI-LHCI-LHCII supercomplexes isolated from Arabidopsis have been investigated by picosecond time-resolved fluorescence spectroscopy. The excited state decays were analysed using two approaches based on either (i) a sum of discrete exponentials or (ii) a continuous distribution of lifetimes. The results indicate that the energy transfer from LHCII to the bulk of the PSI antenna occurs with an average macroscopic transfer rate in the 35-65 ns^-1 interval. Yet, the most satisfactory description of the data is obtained when considering a heterogeneous population containing two PSI-LHCI-LHCII supercomplexes characterised by a transfer time of ∼15 and ∼60 ns^-1, likely due to the differences in the strength and orientation of LHCII harboured to PSI. Both these values are of the same order of magnitude of those estimated for the average energy transfer rates from the low energy spectral forms of LHCI to the bulk of the PSI antenna (15-40 ns^-1), but they are slower than the transfer from the bulk antenna of PSI to the reaction centre (>150 ns^-1), implying a relatively small kinetics bottleneck for the energy transfer from LHCII. Nevertheless, the kinetic limitation imposed by excited state diffusion has a negligible impact on the photochemical quantum efficiency of the supercomplex, which remains about 98% in the case of PSI-LHCI.

A comparison between plant photosystem I and photosystem II architecture and functioning

Oxygenic photosynthesis is indispensable both for the development and maintenance of life on earth by converting light energy into chemical energy and by producing molecular oxygen and consuming carbon dioxide. This latter process has been responsible for reducing the CO2 from its very high levels in the primitive atmosphere to the present low levels and thus reducing global temperatures to levels conducive to the development of life. Photosystem I and photosystem II are the two multi-protein complexes that contain the pigments necessary to harvest photons and use light energy to catalyse the primary photosynthetic endergonic reactions producing high energy compounds. Both photosystems are highly organised membrane supercomplexes composed of a core complex, containing the reaction centre where electron transport is initiated, and of a peripheral antenna system, which is important for light harvesting and photosynthetic activity regulation. If on the one hand both the chemical reactions catalysed by the two photosystems and their detailed structure are different, on the other hand they share many similarities. In this review we discuss and compare various aspects of the organisation, functioning and regulation of plant photosystems by comparing them for similarities and differences as obtained by structural, biochemical and spectroscopic investigations.

Wavelength dependence of the fluorescence emission under conditions of open and closed Photosystem II reaction centres in the green alga Chlorella sorokiniana

The fluorescence emission characteristics of the photosynthetic apparatus under conditions of open (F0) and closed (FM) Photosystem II reaction centres have been investigated under steady state conditions and by monitoring the decay lifetimes of the excited state, in vivo, in the green alga Chlorella sorokiniana. The results indicate a marked wavelength dependence of the ratio of the variable fluorescence, FV=FM-F0, over FM, a parameter that is often employed to estimate the maximal quantum efficiency of Photosystem II. The maximal value of the FV/FM ratio is observed between 660 and 680nm and the minimal in the 690-730nm region. It is possible to attribute the spectral variation of FV/FM principally to the contribution of Photosystem I fluorescence emission at room temperature. Moreover, the analysis of the excited state lifetime at F0 and FM indicates only a small wavelength dependence of Photosystem II trapping efficiency in vivo.

Excitation-energy transfer dynamics of higher plant photosystem I light-harvesting complexes

Photosystem I (PSI) plays a major role in the light reactions of photosynthesis. In higher plants, PSI is composed of a core complex and four outer antennas that are assembled as two dimers, Lhca1/4 and Lhca2/3. Time-resolved fluorescence measurements on the isolated dimers show very similar kinetics. The intermonomer transfer processes are resolved using target analysis. They occur at rates similar to those observed in transfer to the PSI core, suggesting competition between the two transfer pathways. It appears that each dimer is adopting various conformations that correspond to different lifetimes and emission spectra. A special feature of the Lhca complexes is the presence of an absorption band at low energy, originating from an excitonic state of a chlorophyll dimer, mixed with a charge-transfer state. These low-energy bands have high oscillator strengths and they are superradiant in both Lhca1/4 and Lhca2/3. This challenges the view that the low-energy charge-transfer state always functions as a quencher in plant Lhc's and it also challenges previous interpretations of PSI kinetics. The very similar properties of the low-energy states of both dimers indicate that the organization of the involved chlorophylls should also be similar, in disagreement with the available structural data.

Fluorescence spectral dynamics of single LHCII trimers

Single-molecule spectroscopy was employed to elucidate the fluorescence spectral heterogeneity and dynamics of individual, immobilized trimeric complexes of the main light-harvesting complex of plants in solution near room temperature. Rapid reversible spectral shifts between various emitting states, each of which was quasi-stable for seconds to tens of seconds, were observed for a fraction of the complexes. Most deviating states were characterized by the appearance of an additional, red-shifted emission band. Reversible shifts of up to 75 nm were detected. By combining modified Redfield theory with a disordered exciton model, fluorescence spectra with peaks between 670 nm and 705 nm could be explained by changes in the realization of the static disorder of the pigment-site energies. Spectral bands beyond this wavelength window suggest the presence of special protein conformations. We attribute the large red shifts to the mixing of an excitonic state with a charge-transfer state in two or more strongly coupled chlorophylls. Spectral bluing is explained by the formation of an energy trap before excitation energy equilibration is completed.

The low-energy forms of photosystem I light-harvesting complexes: spectroscopic properties and pigment-pigment interaction characteristics

In this work the spectroscopic properties of the special low-energy absorption bands of the outer antenna complexes of higher plant Photosystem I have been investigated by means of low-temperature absorption, fluorescence, and fluorescence line-narrowing experiments. It was found that the red-most absorption bands of Lhca3, Lhca4, and Lhca1-4 peak, respectively, at 704, 708, and 709 nm and are responsible for 725-, 733-, and 732-nm fluorescence emission bands. These bands are more red shifted compared to "normal" chlorophyll a (Chl a) bands present in light-harvesting complexes. The low-energy forms are characterized by a very large bandwidth (400-450 cm(-1)), which is the result of both large homogeneous and inhomogeneous broadening. The observed optical reorganization energy is untypical for Chl a and resembles more that of BChl a antenna systems. The large broadening and the changes in optical reorganization energy are explained by a mixing of an Lhca excitonic state with a charge transfer state. Such a charge transfer state can be stabilized by the polar residues around Chl 1025. It is shown that the optical reorganization energy is changing through the inhomogeneous distribution of the red-most absorption band, with the pigments contributing to the red part of the distribution showing higher values. A second red emission form in Lhca4 was detected at 705 nm and originates from a broad absorption band peaking at 690 nm. This fluorescence emission is present also in the Lhca4-N-47H mutant, which lacks the 733-nm emission band.

Photosynthesis research in Italy: a review

This historical review was compiled and edited by Giorgio Forti, whereas the other authors of the different sections are listed alphabetically after his name, below the title of the paper; they are also listed in the individual sections. This review deals with the research on photosynthesis performed in several Italian laboratories during the last 50 years; it includes research done, in collaboration, at several international laboratories, particularly USA, UK, Switzerland, Hungary, Germany, France, Finland, Denmark, and Austria. Wherever pertinent, references are provided, especially to other historical papers in Govindjee et al. [Govindjee, Beatty JT, Gest H, Allen JF (eds) (2005) Discoveries in Photosynthesis. Springer, Dordrecht]. This paper covers the physical and chemical events starting with the absorption of a quantum of light by a pigment molecule to the conversion of the radiation energy into the stable chemical forms of the reducing power and of ATP. It describes the work done on the structure, function and regulation of the photosynthetic apparatus in higher plants, unicellular algae and in photosynthetic bacteria. Phenomena such as photoinhibition and the protection from it are also included. Research in biophysics of photosynthesis in Padova (Italy) is discussed by G.M. Giacometti and G. Giacometti (2006).

Light-dependent reversal of dark-chilling induced changes in chloroplast structure and arrangement of chlorophyll-protein complexes in bean thylakoid membranes

Changes in chloroplast structure and rearrangement of chlorophyll-protein (CP) complexes were investigated in detached leaves of bean (Phaseolus vulgaris L. cv. Eureka), a chilling-sensitive plant, during 5-day dark-chilling at 1 degrees C and subsequent 3-h photoactivation under white light (200 mumol photons m(-2) s(-1)) at 22 degrees C. Although, no change in chlorophyll (Chl) content and Chl a/b ratio in all samples was observed, overall fluorescence intensity of fluorescence emission and excitation spectra of thylakoid membranes isolated from dark-chilled leaves decreased to about 50%, and remained after photoactivation at 70% of that of the control sample. Concomitantly, the ratio between fluorescence intensities of PSI and PSII (F736/F681) at 120 K increased 1.5-fold upon chilling, and was fully reversed after photoactivation. Moreover, chilling stress seems to induce a decrease of the relative contribution of LHCII fluorescence to the thylakoid emission spectra at 120 K, and an increase of that from LHCI and PSI, correlated with a decrease of stability of LHCI-PSI and LHCII trimers, shown by mild-denaturing electrophoresis. These effects were reversed to a large extent after photoactivation, with the exception of LHCII, which remained partly in the aggregated form. In view of these data, it is likely that dark-chilling stress induces partial disassembly of CP complexes, not completely restorable upon photoactivation. These data are further supported by confocal laser scanning fluorescence microscopy, which showed that regular grana arrangement observed in chloroplasts isolated from control leaves was destroyed by dark-chilling stress, and was partially reconstructed after photoactivation. In line with this, Chl a fluorescence spectra of leaf discs demonstrated that dark-chilling caused a decrease of the quantum yield PSII photochemistry (F(v)/F(m)) by almost 40% in 5 days. Complete restoration of the photochemical activity of PSII required 9 h post-chilling photoactivation, while only 3 h were needed to reconstruct thylakoid membrane organization and chloroplast structure. The latter demonstrated that the long-term dark-chilled bean leaves started to suffer from photoinhibition after transfer to moderate irradiance and temperature conditions, delaying the recovery of PSII photochemistry, independently of photo-induced reconstruction of PSII complexes.

Modelling of the electron transfer reactions in Photosystem I by electron tunnelling theory: The phylloquinones bound to the PsaA and the PsaB reaction centre subunits of PS I are almost isoenergetic to the iron–sulfur cluster FX

Photosystem I is a large macromolecular complex located in the thylakoid membranes of chloroplasts and in cyanobacteria that catalyses the light driven reduction of ferredoxin and oxidation of plastocyanin. Due to the very negative redox potential of the primary electron transfer cofactors accepting electrons, direct estimation by redox titration of the energetics of the system is hampered. However, the rates of electron transfer reactions are related to the thermodynamic properties of the system. Hence, several spectroscopic and biochemical techniques have been employed, in combination with the classical Marcus theory for electron transfer tunnelling, in order to access these parameters. Nevertheless, the values which have been presented are very variable. In particular, for the case of the tightly bound phylloquinone molecule A(1), the values of the redox potentials reported in the literature vary over a range of about 350 mV. Previous models of Photosystem I have assumed a unidirectional electron transfer model. In the present study, experimental evidence obtained by means of time resolved absorption, photovoltage, and electron paramagnetic resonance measurements are reviewed and analysed in terms of a bi-directional kinetic model for electron transfer reactions. This model takes into consideration the thermodynamic equilibrium between the iron-sulfur centre F(X) and the phylloquinone bound to either the PsaA (A(1A)) or the PsaB (A(1B)) subunit of the reaction centre and the equilibrium between the iron-sulfur centres F(A) and F(B). The experimentally determined decay lifetimes in the range of sub-picosecond to the microsecond time domains can be satisfactorily simulated, taking into consideration the edge-to-edge distances between redox cofactors and driving forces reported in the literature. The only exception to this general behaviour is the case of phylloquinone (A(1)) reoxidation. In order to describe the reported rates of the biphasic decay, of about 20 and 200 ns, associated with this electron transfer step, the redox potentials of the quinones are estimated to be almost isoenergetic with that of the iron sulfur centre F(X). A driving force in the range of 5 to 15 meV is estimated for these reactions, being slightly exergonic in the case of the A(1B) quinone and slightly endergonic, in the case of the A(1A) quinone. The simulation presented in this analysis not only describes the kinetic data obtained for the wild type samples at room temperature and is consistent with estimates of activation energy by the analysis of temperature dependence, but can also explain the effect of the mutations around the PsaB quinone binding pocket. A model of the overall energetics of the system is derived, which suggests that the only substantially irreversible electron transfer reactions are the reoxidation of A(0) on both electron transfer branches and the reduction of F(A) by F(X).

Self-assembling peptide detergents stabilize isolated photosystem I on a dry surface for an extended time

We used a class of designed peptide detergents to stabilize photosystem I (PS-I) upon extended drying under N2 on a gold-coated-Ni-NTA glass surface. PS-I is a chlorophyll-containing membrane protein complex that is the primary reducer of ferredoxin and the electron acceptor of plastocyanin. We isolated the complex from the thylakoids of spinach chloroplasts using a chemical detergent. The chlorophyll molecules associated with the PS-I complex provide an intrinsic steady-state emission spectrum between 650 and 800 nm at -196.15 degrees C that reflects the organization of the pigment-protein interactions. In the absence of detergents, a large blue shift of the fluorescence maxima from approximately 735 nm to approximately 685 nm indicates a disruption in light-harvesting subunit organization, thus revealing chlorophyll-protein interactions. The commonly used membrane protein-stabilizing detergents, N-dodecyl-beta-D-maltoside and N-octyl-beta-D-glucoside, only partially stabilized the approximately 735-nm complex with approximately 685-nm spectroscopic shift. However, prior to drying, addition of the peptide detergent acetyl-AAAAAAK at increasing concentration significantly stabilized the PS-I complex. Moreover, in the presence of acetyl-AAAAAAK, the PS-I complex is stable in a dried form at room temperature for at least 3 wk. Another peptide detergent, acetyl-VVVVVVD, also stabilized the complex but to a lesser extent. These observations suggest that the peptide detergents may effectively stabilize membrane proteins in the solid-state. These designed peptide detergents may facilitate the study of diverse types of membrane proteins.

The effect of outer antenna complexes on the photochemical trapping rate in barley thylakoid Photosystem II

We have investigated the previous suggestions in the literature that the outer antenna of Photosystem II of barley does not influence the effective photosystem primary photochemical trapping rate. It is shown by steady state fluorescence measurements at the F(0) fluorescence level of wild type and the chlorina f2 mutant, using the chlorophyll b fluorescence as a marker, that the outer antenna is thermally equilibrated with the core pigments, at room temperature, under conditions of photochemical trapping. This is in contrast with the conclusions of the earlier studies in which it was suggested that energy was transferred rapidly and irreversibly from the outer antenna to the Photosystem II core. Furthermore, the effective trapping time, determined by single photon counting, time-resolved measurements, was shown to increase from 0.17+/-0.017 ns in the chlorina Photosystem II core to a value within the range 0.42+/-0.036-0.47+/-0.044 ns for the wild-type Photosystem II with the outer antenna system. This 2.5-2.8-fold increase in the effective trapping time is, however, significantly less than that expected for a thermalized system. The data can be explained in terms of the outer antenna increasing the primary charge separation rate by about 50%.

The low energy emitting states of the Lhca4 subunit of higher plant photosystem I

The selectively red excited emission spectrum, at room temperature, of the in vitro reconstituted Lhca4, has a pronounced non-equilibrium distribution, leading to enhanced emission from the directly excited low-energy pigments. Two different emitting forms (or states), with maximal emission at 713 and 735nm (F713 and F735) and unusual spectral properties, have been identified. Both high-energy states are populated when selective excitation is into the F735 state and the fluorescence anisotropy spectrum attains the value of 0.3 in the wavelength region where both emission states are present. This indicates that the two states are on the same Lhca4 complex and have transition dipoles with similar orientation.

Carotenoid Triplet States Associated with the Long-Wavelength-Emitting Chlorophyll Forms of Photosystem I in Isolated Thylakoid Membranes

The carotenoid triplet populations associated with the long-wavelength-emitting chlorophyll forms of photosystem I (PS I)(dagger) have been investigated in isolated spinach thylakoids by means of fluorescence-detected magnetic resonance in zero field. The spectra collected in the 730-800 nm emission range can be globally fitted assuming the presence of four different carotenoid triplet states coupled to long-wavelength-emitting forms of PS I, having zero-field-splitting parameters /D/ = 0.0359 cm(-1) and /E/ = 0.00371 cm(-1), /D/ = 0.0382 cm(-1) and /E/ = 0.00388 cm(-1), /D/ = 0.0395 cm(-1) and /E/ = 0.00397 cm(-1), and /D/ = 0.0405 cm(-1) and /E/ = 0.00411 cm(-1). On the basis of the triplet-associated fluorescence emission profile, it is suggested that those triplets are associated with light-harvesting complex I, the peripheral antenna complex of PS I.

The long-wavelength chlorophyll states of plant LHCI at room temperature: a comparison with PSI-LHCI

The red antenna states of the external antenna complexes of higher plant photosystem I, known as LHCI, have been analyzed by measurement of their preequilibrium fluorescence upon direct excitation at 280 K. In addition to the previously detected F735 state, a hitherto undetected low-energy state with emission maximum around 713 nm was observed. The 280 K bandwidths (FWHM) are 55 nm for the F735 state and approximately 27 nm for the F713-nm state, much greater than for non-red-shifted antenna chlorophylls. The origin absorption band for the F735-nm state was directly detected by determination of its excitation (action) spectrum and lies at 709-710 nm. The absorption spectrum for F735, calculated using the Stepanov expression, closely overlaps the excitation spectrum, indicating that the very large Stokes shift (25 nm) is due to vibrational relaxation within the excited-state manifold and solvent effects can be excluded. Fluorescence anisotropy measurements, with direct excitation of F735, indicate that the transition dipoles of the two red states are parallel. Similar experiments performed in the long-wavelength absorbing tail of PSI-LHCI indicate the presence of emission state(s) that are red-shifted with respect to F735 of isolated LHCI. It is suggested that these are brought about by interactions between the complexes in PSI-LHCI, which occur in some yet undefined way, and which are broken upon solubilization of the component parts.