The P700-chlorophyll a-protein. Isolation and some characteristics of the complex in higher plants

Drop-casted Photosystem I/cytochrome c multilayer films for biohybrid solar energy conversion

One of the main barriers to making efficient Photosystem I-based biohybrid solar cells is the need for an electrochemical pathway to facilitate electron transfer between the P₇₀₀ reaction center of Photosystem I and an electrode. To this end, nature provides inspiration in the form of cytochrome c₆, a natural electron donor to the P₇₀₀ site. Its natural ability to access the P₇₀₀ binding pocket and reduce the reaction center can be mimicked by employing cytochrome c, which has a similar protein structure and redox chemistry while also being compatible with a variety of electrode surfaces. Previous research has incorporated cytochrome c to improve the photocurrent generation of Photosystem I using time consuming and/or specialized electrode preparation. While those methods lead to high protein areal density, in this work we use the quick and facile vacuum-assisted drop-casting technique to construct a Photosystem I/cytochrome c photoactive composite film with micron-scale thickness. We demonstrate that this simple fabrication technique can result in high cytochrome c loading and improvement in cathodic photocurrent over a drop-casted Photosystem I film without cytochrome c. In addition, we analyze the behavior of the cytochrome c/Photosystem I system at varying applied potentials to show that the improvement in performance can be attributed to enhancement of the electron transfer rate to P₇₀₀ sites and therefore the PSI turnover rate within the composite film.

Layer-by-Layer Assembly of Photosystem I and PEDOT:PSS Biohybrid Films for Photocurrent Generation

The design of electrode interfaces to achieve efficient electron transfer to biomolecules is important in many bioelectrochemical processes. Within the field of biohybrid solar energy conversion, constructing multilayered Photosystem I (PSI) protein films that maintain good electronic connection to an underlying electrode has been a major challenge. Previous shortcomings include low loadings, long deposition times, and poor connection between PSI and conducting polymers within composite films. Here, we show that PSI protein complexes can be deposited into monolayers within a 30 min timespan by leveraging the electrostatic interactions between the protein complex and the poly(3,4-ethylenedioxythiophene)-polystyrenesulfonate (PEDOT:PSS) polymer. Further, we follow a layer-by-layer (LBL) deposition procedure to produce up to 9-layer pairs of PSI and PEDOT:PSS with highly reproducible layer thicknesses as well as distinct changes in surface composition. When tested in an electrochemical cell employing ubiquinone-0 as a mediator, the photocurrent performance of the LBL films increased linearly by 83 ± 6 nA/cm² per PSI layer up to 6-layer pairs. The 6-layer pair samples yielded a photocurrent of 414 ± 13 nA/cm², after which the achieved photocurrent diminished with additional layer pairs. The turnover number (TN) of the PSI-PEDOT:PSS LBL assemblies also greatly exceeds that of drop-casted PSI multilayer films, highlighting that the rate of electron collection is improved through the systematic deposition of the protein complexes and conducting polymer. The capability to deposit high loadings of PSI between PEDOT:PSS layers, while retaining connection to the underlying electrode, shows the value in using LBL assembly to produce PSI and PEDOT:PSS bioelectrodes for photoelectrochemical energy harvesting applications.

Photocatalytic photosystem I/PEDOT composite films prepared by vapor-phase polymerization

Photosystem I (PSI) achieves photo-induced charge separation with outstanding internal quantum efficiency and has been used to improve the performance of various photoelectrochemical systems. Herein, we describe a fast and versatile technique to assemble composite films containing PSI and a chosen intrinsically conductive polymer (ICP). A mixture of PSI and a Friedel-Crafts catalyst (FeCl₃) is drop cast atop a substrate of choice. Contact with ICP monomer vapor at low temperature stimulates polymer growth throughout PSI films in minutes. We assess the effects of PSI loading on the rapid vapor-phase growth of poly(3,4-ethylenedioxythiophene) (PEDOT) within and above PSI multilayer films, and characterize the resulting film's thickness, electrochemical capacitance, and photocatalytic response. Composite films generate cathodic photocurrent when in contact with an aqueous redox electrolyte, confirming retention of the photocatalytic activity of the polymer-entrapped PSI multilayer assembly.

Enhanced photocurrent from Photosystem I upon in vitro truncation of the antennae chlorophyll

Current effects on climate change and dwindling fossil fuel reserves require new materials and methods to convert solar energy into a viable clean energy source. Recent progress in the direct conversion of light into photocurrent has been well documented using Photosystem I. In plants, PSI consists of a core complex and multiple light-harvesting complexes, denoted LHCI and LHCII. Most of the methods for isolating PSI from plants involve a selective, detergent solubilization from thylakoids followed by sucrose gradient density centrifugation. These processes isolate one variant of PSI with a specific ratio of Chl:P700. In this study, we have developed a simple and potentially scalable method for isolating multiple PSI variants using Hydroxyapatite chromatography, which has been well documented in other Photosystem I isolation protocols. By varying the wash conditions, we show that it is possible to change the Chl:P700 ratios. These different PSI complexes were cast into a PSI-Nafion-osmium polymer film that enabled their photoactivity to be measured. Photocurrent increases nearly 400% between highly washed and untreated solutions based on equal chlorophyll content. Importantly, the mild washing conditions remove peripheral Chl and some LHCI without inhibiting the photochemical activity of PSI as suggested by SDS-PAGE analysis. This result could indicate that more P700 could be loaded per surface area for biohybrid devices. Compared with other PSI isolations, this protocol also allows isolation of multiple PSI variants without loss of photochemical activity.

Electrochemical preparation of Photosystem I-polyaniline composite films for biohybrid solar energy conversion

In this work, we report for the first time the entrapment of the biomolecular supercomplex Photosystem I (PSI) within a conductive polymer network of polyaniline via electrochemical copolymerization. Composite polymer-protein films were prepared on gold electrodes through potentiostatic electropolymerization from a single aqueous solution containing both aniline and PSI. This study demonstrates the controllable integration of large membrane proteins into rapidly prepared composite films, the entrapment of such proteins was observed through photoelectrochemical analysis. PSI's unique function as a highly efficient biomolecular photodiode generated a significant enhancement in photocurrent generation for the PSI-loaded polyaniline films, compared to pristine polyaniline films, and dropcast PSI films. A comprehensive study was then performed to separately evaluate film thickness and PSI concentration in the initial polymerization solution and their effects on the net photocurrent of this novel material. The best performing composite films were prepared with 0.1 μM PSI in the polymerization solution and deposited to a film thickness of 185 nm, resulting in an average photocurrent density of 5.7 μA cm(-2) with an efficiency of 0.005%. This photocurrent output represents an enhancement greater than 2-fold over bare polyaniline films and 200-fold over a traditional PSI multilayer film of comparable thickness.

Photosystem I protein films at electrode surfaces for solar energy conversion

Over the course of a few billion years, nature has developed extraordinary nanomaterials for the efficient conversion of solar energy into chemical energy. One of these materials, photosystem I (PSI), functions as a photodiode capable of generating a charge separation with nearly perfect quantum efficiency. Because of the favorable properties and natural abundance of PSI, researchers around the world have begun to study how this protein complex can be integrated into modern solar energy conversion devices. This feature article describes some of the recent materials and methods that have led to dramatic improvements (over several orders of magnitude) in the photocurrents and photovoltages of biohybrid electrodes based on PSI, with an emphasis on the research activities in our laboratory.

Photosystem I in Langmuir-Blodgett and Langmuir-Schaefer monolayers

Photosystem I (PSI) is a membrane protein complex that generates photoinduced electrons and transfers them across the thylakoid membrane during photosynthesis. The PSI complex, separated from spinach leaves, was spread onto the air-water interface as a monolayer and transferred onto a gold electrode surface that was precoated with a self-assembled monolayer (SAM). The electrochemical properties of the transferred PSI monolayer, including cyclic voltammetry and photoinduced chronoamperometry, were measured. The results showed that PSI retained its bioactivity after the manipulation. Its capability of converting photoenergy into electrical potential was demonstrated by its reducing an electron acceptor, dichloroindophenol (DCIP), and by oxidizing an electron donor, sodium ascorbate (ASC). We have shown that the protein has two possible orientations at the water interface. The orientation distribution was determined by comparing the controlled reductive and oxidative photocurrents generated from Langmuir-Blodgett and Langmuir-Schaefer monolayers.

Photosynthetic unit size, carotenoids, and chlorophyll-protein composition of Prochloron sp., a prokaryotic green alga

Six samples of the prokaryotic, unicellular algae Prochloron sp., which occur in association with didemnid ascidians, were collected from various localities in the tropical Pacific Ocean, and their pigments and chlorophyll-protein complexes were identified and characterized. No phycobilin pigments were detected in any of the species. Chlorophylls a and b were present in ratios of a/b = 4.4-6.9. The major carotenoids were beta-carotene (70%) and zeaxanthin (20%). Minor carotenoids of one isolate were identified as echinenone, cryptoxanthin, isocryptoxanthin, mutachrome, and trihydroxy-beta-carotene; no epsilon-ring carotenoids were found in any sample. Except for the absence of glycosidic carotenoids, the overall pigment composition is typical of cyanobacteria. A chlorophyll a/b-protein complex was present in Prochloron; it was electrophoretically and spectrally indistinguishable from the light-harvesting chlorophyll a/b-protein of higher plants and green algae. It accounted for 26% (compared to approximately 50% in green plants) of the total chlorophyll; 17% was associated with a P700-chlorophyll a-protein. The photosynthetic unit size of 240 +/- 10 chlorophylls per P700 in Prochloron was about half that of eukaryotic green plants. A model is proposed for the in vivo organization of chlorophyll in Prochloron.

Photosynthetic characteristics and organization of chlorophyll in marine dinoflagellates

The photosystem I reaction center complex, the P-700-chlorophyll a-protein, has been isolated from the photosynthetic membranes of two marine dinoflagellates, Gonyaulax polyedra and Glenodinium sp., by detergent solubilization with Triton X-100. The complexes isolated from the two species were indistinguishable, exhibiting identical absorption properties (400-700 nm) at both room (300 K) and low (77 K) temperature. The room temperature, red wavelength maximum was at 675 nm. The absorption properties, kinetics of photobleaching, sodium dodecyl sulfate electrophoretic mobilities, and chlorophyll a/P-700 ratio (50 +/- 10) of the P-700-chlorophyll a-protein complexes from the two species also were essentially the same and similar to those properties characterizing P-700-chlorophyll a-protein complexes of higher plants and green algae.Photosynthetic unit sizes were determined for cells grown at 1000 muW/cm(2). Both dinoflagellates had unit sizes (total chlorophyll/P-700 ratios) of about 600, even though the distribution of chlorophyll a, chlorophyll c, and peridinin in the light-harvesting components differed in Gonyaulax and Glenodinium. The number of photosynthetic units per cell in the two species correlates directly with their photosynthetic activities. A model is presented for the distribution of chlorophyll in the photosynthetic apparatus of these dinoflagellates which accounts for the known role of the isolated pigment-protein complexes and for the known photoadaptive physiology in pigmentation and photosynthesis for these species.

Energy transfer and charge separation kinetics in photosystem I: Part 1: Picosecond transient absorption and fluorescence study of cyanobacterial photosystem I particles

The energy transfer and charge separation kinetics of a photosystem I (PS I) core particle of an antenna size of 100 chlorophyll/P700 has been studied by combined fluorescence and transient absorption kinetics with picosecond resolution. This is the first combined picosecond study of transient absorption and fluorescence carried out on a PS I particle and the results are consistent with each other. The data were analyzed by both global lifetime and global target analysis procedures. In fluorescence major lifetime components were found to be 12 and 36 ps. The shorter-lived one shows a negative amplitude at long wavelengths and is attributed to an energy transfer process between pigments in the main antenna Chl pool and a small long-wavelength Chl pool emitting around 720 nm whereas the longer-lived component is assigned to the overall charge separation lifetime. The lifetimes resolved in transient absorption are 7-8 ps, 33 ps, and [unk]1 ns. The shortest-lived one is assigned to energy transfer between the same pigment pools as observed also in fluorescence kinetics, the middle component of 33 ps to the overall charge separation, and the long-lived component to the lifetime of the oxidized primary donor P700(+). The transient absorption data indicate an even faster, but kinetically unresolved energy transfer component in the main Chl pool with a lifetime <3 ps. Several kinetic models were tested on both the fluorescence and the picosecond absorption data by global target analysis procedures. A model where the long-wave pigments are spatially and kinetically connected with the reaction center P700 is favored over a model where P700 is connected more closely with the main Chl pool. Our data show that the charge separation kinetics in these PS I particles is essentially trap limited. The relevance of our data with respect to other time-resolved studies on PS I core particles is discussed, in particular with respect to the nature and function of the long-wave pigments. From the transient absorption data we do not see any evidence for the occurrence of a reduced Chl primary electron acceptor, but we also can not exclude that possibility, provided that reoxidation of that acceptor should occur within a time <40 ps.

Excitation transport and trapping on spectrally disordered lattices

It is widely assumed that the decay of fluorescence in photosynthetic systems can be described as a sum of exponential components and that the amplitude of each component is directly related to the absorption cross-section of the antenna pigments coupled to the fluorescing species. We present exact calculations of excited state decay in two-dimensional regular lattices of different geometries containing multiple spectral forms of antenna pigments. We illustrate by these calculations that there is no simple relation between the decay amplitudes (and resulting time-resolved excitation spectra) and the steady-state absorption spectra. Only in the limit that the electronic excitations reach a rapid equilibrium among all antenna spectral forms does the excitation spectrum depend uniquely on the spectral features of the array. Using the simulations in conjunction with our recent fluorescence studies, we examine excitation transport and trapping dynamics in photosystem I and the limitations imposed by the finite time resolution in single photon counting experiments. In particular, we show that rising components, associated with excitation transfer among different spectral forms, with lifetimes <20 ps would be undetected in a typical photon counting experiment.

Functionalized nanoporous gold leaf electrode films for the immobilization of photosystem I

Plants and some types of bacteria demonstrate an elegant means to capitalize on the superabundance of solar energy that reaches our planet with their energy conversion process called photosynthesis. Seeking to harness Nature's optimization of this process, we have devised a biomimetic photonic energy conversion system that makes use of the photoactive protein complex Photosystem I, immobilized on the surface of nanoporous gold leaf (NPGL) electrodes, to drive a photoinduced electric current through an electrochemical cell. The intent of this study is to further the understanding of how the useful functionality of these naturally mass-produced, biological light-harvesting complexes can be integrated with nonbiological materials. Here, we show that the protein complexes retain their photonic energy conversion functionality after attachment to the nanoporous electrode surface and, further, that the additional PSI/electrode interfacial area provided by the NPGL allows for an increase in PSI-mediated electron transfer with respect to an analogous 2D system if the pores are sufficiently enlarged by dealloying. This increase of interfacial area is pertinent for other applications involving electron transfer between phases; thus, we also report on the widely accessible and scalable method by which the NPGL electrode films used in this study are fabricated and attached to glass and Au/Si supports and demonstrate their adaptability by modification with various self-assembled monolayers. Finally, we demonstrate that the magnitude of the PSI-catalyzed photocurrents provided by the NPGL electrode films is dependent upon the intensity of the light used to irradiate the electrodes.

Molecular Photovoltaics and the Photoactivation of Mammalian Cells

Photosynthetic reaction centers are integral plant membrane protein complexes and molecular photovoltaic structures. We report here that addition of Photosystem I (PSI)-proteoliposomes to retinoblastoma cells imparts photosensitivity to these mammalian cells, as demonstrated by light-induced movement of calcium ions. Control experiments with liposomes lacking PSI demonstrated no photosensitivity. The data demonstrate that PSI, a nanoscale molecular photovoltaic structure extracted from plants, can impart a photoresponse to mammalian cells in vitro.

Photosystem I patterning imaged by scanning electrochemical microscopy

We report the first directed adsorption of Photosystem I (PSI) on patterned surfaces containing discrete regions of methyl- and hydroxyl-terminated self-assembled monolayers (SAMs) on gold. SAM and PSI patterns are characterized by scanning electrochemical microscopy (SECM). The insulating protein complex layer blocks the electron transfer of the SECM mediator, thereby reducing the electrochemical current significantly. Uniformly and densely packed adsorbed protein layers are observed with SECM. Pattern images correlate with our previous studies where we showed that low-energy surfaces (e.g., CH3-terminated) inhibit PSI adsorption in the presence of Triton X-100, whereas high-energy surfaces (e.g., OH-terminated) enable adsorption. Therefore, a SAM pattern with alternating methyl and hydroxyl surface regions allows PSI adsorption only on the hydroxyl surface, and this is demonstrated in the resulting SECM images.

Kinetic modeling of exciton migration in photosynthetic systems. 3. Application of genetic algorithms to simulations of excitation dynamics in three-dimensional photosystem I core antenna/reaction center complexes

A procedure is described to generate and optimize the lattice models for spectrally inhomogeneous photosynthetic antenna/reaction center (RC) particles. It is based on the genetic algorithm search for the pigment spectral type distributions on the lattice by making use of steady-state and time-resolved spectroscopic input data. Upon a proper fitness definition, a family of excitation energy transfer models can be tested for their compatibility with the availability experimental data. For the case of the photosystem I core antenna (99 chlorophyll + primary electron donor pigment (P700)), three spectrally inhomogeneous three-dimensional lattice models, differing in their excitation transfer conditions, were tested. The relevant fit parameters were the pigment distribution on the lattice, the average lattice spacing of the main pool pigments, the distance of P700 and of long wavelength-absorbing (LWA) pigments to their nearest-neighbor main pool pigments, and the rate constant of charge separation from P700. For cyanobacterial PS I antenna/RC particles containing a substantial amount of LWA pigments, it is shown that the currently available experimental fluorescence data are consistent both with more migration-limited, and with more trap-limited excitation energy transfer models. A final decision between these different models requires more detailed experimental data. From all search runs about 30 different relative arrangements of P700 and LWA pigments were found. Several general features of all these different models can be noticed: 1) The reddest LWA pigment never appears next to P700. 2) The LWA pigments in most cases are spread on the surface of the lattice not far away from P700, with a pronounced tendency toward clustering of the LWA pigments. 3) The rate constant kP700 of charge separation is substantially higher than 1.2 ps-1, i.e., it exceeds the corresponding rate constant of purple bacterial RCs by at least a factor of four. 4) The excitation transfer within the main antenna pool is very rapid (less than 1 ps equilibration time), and only the equilibration with the LWA pigments is slow (about 10-12 ps). The conclusions from this extended study on three-dimensional lattices are in general agreement with the tendencies and limitations reported previously for a simpler two-dimensional array. Once more detailed experimental data are available, the procedure can be used to determine the relevant rate-limiting processes in the excitation transfer in such spectrally inhomogeneous antenna systems.

Excited state dynamics in photosystem I: effects of detergent and excitation wavelength

Femtosecond transient absorption spectroscopy has been used to investigate the energy transfer and trapping processes in both intact membranes and purified detergent-isolated particles from a photosystem II deletion mutant of the cyanobacterium Synechocystis sp. PCC 6803, which contains only the photosystem I reaction center. Processes with similar lifetimes and spectra are observed in both the membrane fragments and the detergent-isolated particles, suggesting little disruption of the core antenna resulting from the detergent treatment. For the detergent-isolated particles, three different excitation wavelengths were used to excite different distributions of pigments in the spectrally heterogeneous core antenna. Only two lifetimes of 2.7-4.3 ps and 24-28 ps, and a nondecaying component are required to describe all the data. The 24-28 ps component is associated with trapping. The trapping process gives rise to a nondecaying spectrum that is due to oxidation of the primary electron donor. The lifetimes and spectra associated with trapping and radical pair formation are independent of excitation wavelength, suggesting that trapping proceeds from an equilibrated excited state. The 2.7-4.3 ps component characterizes the evolution from the initially excited distribution of pigments to the equilibrated excited state distribution. The spectrum associated with the 2.7-4.3 ps component is therefore strongly excitation wavelength dependent. Comparison of the difference spectra associated with the spectrally equilibrated state and the radical pair state suggests that the pigments in the photosystem I core antenna display some degree of excitonic coupling.

Thirty years of fun with antenna pigment-proteins and photochemical reaction centers: A tribute to the people who have influenced my career

The author summarizes the research contributions to photosynthesis made by him, his graduate and postdoctoral students, visiting scientists and by his collaboration with other photosynthesis workers during 1964-1994. The development of isolation procedures and biochemical/biophysical characterization of antenna pigment-proteins and photochemical reaction centers are described together with the author's education and experiences as a scientific researcher. Some anecdotes hopefully add insight into what it was like to be in this area of science during the period.

Energy transfer and trapping in photosystem I reaction centers from cyanobacteria

A mutant strain of the cyanobacterium Synechocystis 6803, TolE4B, was constructed by genetic deletion of the protein that links phycobilisomes to thylakoid membranes and of the CP43 and CP47 proteins of photosystem II (PSII), leaving the photosystem I (PSI) center as the sole chromophore in the photosynthetic membranes. Both intact membrane and detergent-isolated samples of PSI were characterized by time-resolved and steady-state fluorescence methods. A decay component of approximately 25 ps dominates (99% of the amplitude) the fluorescence of the membrane sample. This result indicates that an intermediate lifetime is not associated with the intact membrane preparation and the charge separation in PSI is irreversible. The decay time of the detergent-isolated sample is similar. The 600-nm excited steady-state fluorescence spectrum displays a red fluorescence peak at approximately 703 nm at room temperature. The 450-nm excited steady-state fluorescence spectrum is dominated by a single peak around 700 nm without 680-nm "bulk" fluorescence. The experimental results were compared with several computer simulations. Assuming an antenna size of 130 chlorophyll molecules, an apparent charge separation time of approximately 1 ps is estimated. Alternatively, the kinetics could be modeled on the basis of a two-domain antenna for PSI, consistent with the available structural data, each containing approximately 65 chlorophyll a molecules. If excitation can migrate freely within each domain and communication between domains occurs only close to the reaction center, a charge separation time of 3-4 ps is obtained instead.

Kinetic modeling of exciton migration in photosynthetic systems. 2. Simulations of excitation dynamics in two-dimensional photosystem I core antenna/reaction center complexes

Kinetic modeling of the exciton migration in the cyanobacterial photosystem I core complex from Synechococcus sp. was performed by an exact solution of the Pauli master equation for exciton motion. A square two-dimensional 10 x 10 pigment lattice and a Förster dipole-dipole coupling between chromophores was assumed. We calculated decay-associated spectra and lifetimes and compared them to the corresponding experimental data from picosecond fluorescence and transient absorption obtained by global analysis. Seven spectral chlorophyll(Chl) forms, identical in shape but shifted in their absorption maximums, were used to describe the non-homogeneous broadening of the PS I-100 particle absorption spectrum. The optimized Chl lattice arrangement best reproducing the experimental decay-associated spectra as well as the steady-state fluorescence spectrum indicated the long-wavelength-absorbing Chls forming a cluster in the corner of the lattice with the reaction center (RC) placed apart at a distance of two lattice constants. The variable parameters, i.e., the charge separation rate in the RC and the lattice constant a, were found to be optimal at kRC = 2.3 ps-1 and a = 1.14 nm, respectively. The surprising conclusions of the simulations is that Chls with absorption maxima as long a 724 nm have to be taken into account to describe the time-resolved spectra of this PS I particle properly. The dependencies of the exciton decay in the model PS I particle on the excitation wavelength and on the temperature are discussed. We also show that the excited state decay of similar PS I particles that lack the long-wavelength absorbing Chls is nearly mono-exponential. Various critical factors that limit the general reliability of the conclusions of such simulations are discussed in detail.

Steady-state polarized light spectroscopy of isolated Photosystem I complexes

Monomeric and trimeric Photosystem I core complexes from the cyanobacterium Synechocystis PCC 6803 and LHC-I containing Photosystem I (PS I-200) complexes from spinach have been characterized by steady-state, polarized light spectroscopy at 77 K. The absorption spectra of the monomeric and trimeric core complexes from Synechocystis were remarkably similar, except for the amplitude of a spectral component at long wavelength, which was about twice as large in the trimeric complexes. This spectral component did not contribute significantly to the CD-spectrum. The (77 K) steady-state emission spectra showed prominent peaks at 724 nm (for the Synechocystis core complexes) and at 735 nm (for PS I-200). A comparison of the excitation spectra of the main emission band and the absorption spectra suggested that a significant part of the excitations do not pass the red pigments before being trapped by P-700. Polarized fluorescence excitation spectra of the monomeric and trimeric core complexes revealed a remarkably high anisotropy (∼0.3) above 705 nm. This suggested one or more of the following possibilities: 1) there is one red-most pigment to which all excitations are directed, 2) there are more red-most pigments but with (almost) parallel orientations, 3) there are more red-most pigments, but they are not connected by energy transfer. The high anisotropy above 705 nm of the trimeric complexes indicated that the long-wavelength pigments on different monomers are not connected by energy transfer. In contrary to the Synechocystis core complexes, the anisotropy spectrum of the LHC I containing complexes from spinach was not constant in the region of the long-wavelength pigments, and decreased significantly below 720 nm, the wavelength where the long-wavelength pigments on the core complexes start to absorb. These results suggested that in spinach the long-wavelength pigments on core and LHC-I are connected by energy transfer and have a non-parallel average Qy(0-0) transitions.

Energy transfer and trapping in the photosystem I core antenna. A temperature study

The fluorescence decay kinetics of the photosystem I-only mutant strain of Chlamydomonas reinhardtii, A4d, are used to study energy transfer and structural organization in photosystem I (PSI). Time-resolved measurements over a wide temperature range (36-295 K) have been made both on cells containing approximately 65 core chl a/P700 and an additional 60-70 chl a + b from LHC proteins and on PSI particles containing 40-50 chl a/P700. In each case, the fluorescence decay kinetics is dominated by a short component, tau 1 which is largely attributed to the lifetime of the excitations in the core complex. The results are discussed in terms of simulations of the temperature dependence of tau 1 in model systems. Spectral inhomogeneity and the temperature dependence of the spectral lineshapes are included explicitly in the simulations. Various kinds of antenna arrangements are modeled with and without the inclusion of pigments with lower absorption energies than the trap (red pigments). We conclude that funnel arrangements are not consistent with our measurements. A random model that includes one or two red pigments placed close to the trap shows temperature and wavelength dependence similar to that observed experimentally. A comparison of the temperature dependence of tau 1 for cells and PSI particles is included.

The composition and organization of photosystem I

Photosystem I, extensively studied in the past decade, was shown to be homologous in all photosynthetic organisms of the higher plants type. Its core complex was found to be highly conserved through evolution from cyanobacteria to higher plants. The genes coding for the subunits of CCI were isolated and the resulting sequences provided information about secondary structural elements. These suggested secondary structures enabled the prediction of the topology of these subunits in the photosynthetic membrane. Structural studies using both electron microscopy and X-ray crystallography, on isolated particles as well as on the complexes in the photosynthetic membrane, led to a better understanding of the overall structure of CCI. Recently two forms of three dimensional crystals of CCI were obtained. These crystals contain all the original components of CCI (both protein and pigments); these components have not been altered by crystallization. It is expected that a detailed crystallographic analysis of these crystals, together with biochemical, spectroscopical and molecular biology studies, will eventually lead to the elucidation of the high resolution structure of the photosystem I core complex and to the understanding of the exact role and mode of action of this complex in the photosynthetic membrane.

Detection of photosynthetic energy storage in a photosystem I reaction center preparation by photoacoustic spectroscopy

Thermal emission and photochemical energy storage were examined in photosystem I reaction center/core antenna complexes (about 40 Chl a/P700) using photoacoustic spectroscopy. Satisfactory signals could only be obtained from samples bound to hydroxyapatite and all samples had a low signal-to-noise ratio compared to either PS I or PS II in thylakoid membranes. The energy storage signal was saturated at low intensity (half saturation at 1.5 W m(-2)) and predicted a photochemical quantum yield of >90%. Exogenous donors and acceptors had no effect on the signal amplitudes indicating that energy storage is the result of charge separation between endogenous components. Fe(CN)6 (-3) oxidation of P700 and dithionite-induced reduction of acceptors FA-FB inhibited energy storage. These data are compatible with the hypothesis that energy storage in PS I arises from charge separation between P700 and Fe-S centers FA-FB that is stable on the time scale of the photoacoustic modulation. High intensity background light (160 W m(-2)) caused an irreversible loss of energy storage and correlated with a decrease in oxidizable P700; both are probably the result of high light-induced photoinhibition. By analogy to the low fluorescence yield of PS I, the low signal-to-noise ratio in these preparations is attributed to the short lifetime of Chl singlet excited states in PS I-40 and its indirect effect on the yield of thermal emission.

Antenna structure and excitation dynamics in photosystem I. II. Studies with mutants of Chlamydomonas reinhardtii lacking photosystem II

Using time-resolved single photon counting, fluorescence decay in photosystem I (PS I) was analyzed in mutant strains of Chlamydomonas reinhardtii that lack photosystem II. Two strains are compared: one with a wild-type PS I core antenna (120 chlorophyll a/P700) and a second showing an apparent reduction in core antenna size (60 chlorophyll a/P700). These data were calculated from the lifetimes of core antenna excited states (75 and 45 ps, respectively) and from pigment stoichiometries. Fluorescence decay in wild type PS I is composed of two components: a fast 75-ps decay that represents the photochemically limited lifetime of excited states in the core antenna, and a minor (less than 10%) 300-800 ps component that has spectral characteristics of both peripheral and core antenna pigments. Temporal and spectral properties of the fast PS I decay indicate that (a) excitations are nearly equilibrated among the range of spectral forms present in the PS I core antenna, (b) an average excitation visits a representative distribution of core antenna spectral forms on all pigment-binding subunits regardless of the origin of the excitation, (c) reduction in core antenna size does not alter the range of antenna spectral forms present, and (d) transfer from peripheral antennae to the PS I core complex is rapid (less than 5 ps).

Orientation of Photosystem-I pigments: low temperature linear dichroism spectroscopy of a highly-enriched P700 particle isolated from spinach

The linear dichroism of Photosystem I particles containing 10 chlorophylls per P700 has been investigated at 10 K. The particles were oriented by uniaxial squeezing of polyacrylamide gels. The oxidation state of P700 was altered either by incubation of the gels with redox mediators or by low temperature illumination. The QY transitions of the primary electron donor P700, of the remaining unoxidized chlorophyll in P700(+) and of a chlorophyll molecule absorbing at 686 nm, which presumably corresponds to the primary electron acceptor A0, are all preferentially oriented perpendicular to the gel squeezing direction. The QY transition of the chlorophyll forms absorbing at 670 and 675 nm appear tilted at 40 ± 5° from this orientation axis. This orientation of the various chlorophylls is compared to that previously reported for more native Photosystem I particles.

Antenna structure and excitation dynamics in photosystem I. I. Studies of detergent-isolated photosystem I preparations using time-resolved fluorescence analysis

The temporal and spectral properties of fluorescence decay in isolated photosystem I (PS I) preparations from algae and higher plants were measured using time-correlated single photon counting. Excitations in the PS I core antenna decay with lifetimes of 15-40 ps and 5-6 ns. The fast decay results from efficient photochemical quenching by P700, whereas the slow decay is attributed to core antenna complexes lacking a trap. Samples containing core and peripheral antenna complexes exhibited an additional intermediate lifetime (150-350 ps) decay. The PS I core antenna is composed of several spectral forms of chlorophyll a that are not temporally resolved in the decays. Analysis of the temporal and spectral properties of the decays provides a description of the composition, structure, and dynamics of energy transfer and trapping reactions in PS I. The core antenna size dependence of the spectral properties and the contributions of the spectral forms to the time-resolved decays show that energy is not concentrated in the longest wavelength absorbing pigments but is nearly homogenized among the spectral forms. These data suggest that the "funnel" description of antenna structure and energy transfer (Seely, G. R. 1973. J. Theor. Biol. 40:189-199) may not be applicable to the PS I core antenna.

The effect of ethylenediamine chemical modification of plastocyanin on the rate of cytochrome f oxidation and P-700+ reduction

Chemical modification of plastocyanin was carried out using ethylenediamine plus a water-soluble carbodiimide, which has the effect of replacing a negatively charged carboxylate group with a positively charged amino group at pH 6-8. The conditions were adjusted to produce a series of singly and doubly modified forms of plastocyanin. Differences in charge configuration allowed separation of these forms on a Pharmacia fast protein liquid chromatograph using a Mono Q anion exchange column. These forms were used to study the interaction of plastocyanin with its reaction partner cytochrome f. The rate of cytochrome f oxidation was progressively inhibited upon incorporation of increasing numbers of ethylenediamine moieties indicating a positively charged binding site on cytochrome f. However, differential inhibition was obtained for the various singly modified forms allowing mapping of the binding site on plastocyanin. The greatest inhibition was found for forms modified at negatively charged residues Nos. 42-45 and Nos. 59-61 which comprise a negative patch surrounding Tyr-83. In contrast, the form modified at residue No. 68, on the opposite side of the globular plastocyanin molecule, showed the least inhibition. It can be concluded that the binding site for cytochrome f is located in the vicinity of residues Nos. 42-45 and Nos. 59-61. Modification of plastocyanin at residues Nos. 42-45 showed no effect on the rate of P-700+ reduction, suggesting that these residues are not involved in the binding of Photosystem I. However, an increase in the rate of P-700+ reduction was observed for plastocyanins modified at residue No. 68 or Nos. 59-61, which is consistent with the idea that the reaction domain of Photosystem I is negatively charged and Photosystem I binds at the top of the molecule and accepts electrons via His-87 in plastocyanin. These results raise the possibility that plastocyanin can bind both cytochrome f and Photosystem I simultaneously. The effect of ethylenediamine modification on the formal potential of plastocyanin was also examined. The formal potential of control plastocyanin was found to be +372 +/- 5 mV vs. normal hydrogen electrode at pH 7. All modified forms showed a positive shift in formal potential. Singly modified forms showed increases in formal potentials between +8 and +18 mV with the largest increases being observed for plastocyanins modified at residues Nos. 42-45 or Nos. 59-61.

Antenna size dependence of fluorescence decay in the core antenna of photosystem I: estimates of charge separation and energy transfer rates

We have examined the photophysics of energy migration and trapping in photosystem I by investigating the spectral and temporal properties of the fluorescence from the core antenna chlorophylls as a function of the antenna size. Time-correlated single photon counting was used to determine the fluorescence lifetimes in the isolated P700 chlorophyll a-protein complex and in a mutant of Chlamydomonas reinhardtii that lacks the photosystem II reaction center complex. The fluorescence decay in both types of sample is dominated by a fast (15-45 psec) component that is attributed to the lifetime of excitations in the photosystem I core antenna. These excitations decay primarily by an efficient photochemical quenching on P700. The measured lifetimes show a linear relationship to the core antenna size. A linear dependence of the excitation lifetime on antenna size was predicted previously in a lattice model for excitation migration and trapping in arrays of photosynthetic pigments [Pearlstein, R.M. (1982) Photochem. Photobiol. 35, 835-844]. Based on this model, our data predict a time constant for photochemical charge separation in the photosystem I reaction center of 2.8 +/- 0.7 or 3.4 +/- 0.7 psec, assuming monomeric or dimeric P700, respectively. The predicted average single-step transfer time for excitation transfer between core antenna pigments is 0.21 +/- 0.04 psec. Under these conditions, excitation migration in photosystem I is near the diffusion limit, with each excitation making an average of 2.4 visits to the reaction center before photoconversion.

Chloramphenicol effects on chlorophyll degradation and photosystem I assembly in the chlorina CD3 wheat mutant

We previously reported that applications of chloramphenicol to the chlorina wheat mutant, CD3, decreased the leaf Chl a/b ratio and enhanced accumulations of LHC proteins and LHC complexes during greening (Duysen et al. 1985). We have now examined Chl degradation and the change in Chl a/b ratios in wheat leaves kept in the dark as a measure of LHC destruction. Chl b was stable in chloroplasts of the CD3 wheat kept in darkness up to 5 days. Chloramphenicol significantly increased Chl b accumulations and impaired Chl a degradation in both CD3 mutant and normal wheat relative to untreated plants. Our Chl data suggest that the chloramphenicol induced accumulation of the LHC complex in the mutant wheat results from enhanced processing of LHC into the membrane rather than impairment of LHC degradation. The photosystem I (PSI) fraction of the CD3 wheat mutant was examined relative to that of normal wheat after 3 days greening. PSI was deficient in 25, 26, 26.5 kD LHCI protein in the mutant but both wheats accumulated low quantities of the 27-29 kD LHCII protein as detected by Western blot analysis. Chloramphenicol enhanced accumulations of several LHCI proteins primarily near 25 kD in the mutant and the 27-29 kD LHCII protein in normal wheat. The fluorescence emission and absorbance spectra suggest that chloramphenicol enhances accumulations of dissociated LHC in the PSI preparation of normal and CD3 mutant wheat.

Sequence of two genes in pea chloroplast DNA coding for 84 and 82 kD polypeptides of the photosystem I complex

The genes encoding the two P700 chlorophyll a-apoproteins of the photosystem I complex were localized on the pea (Pisum sativum) chloroplast genome. The nucleotide sequence of the genes and the flanking regions has been determined. The genes are separated by 25 bp and are probably cotranscribed. The 5' terminal gene (psaA1) codes for a 761-residue protein (MW 84.1 kD) and the 3' terminal gene (psaA2) for a 734-residue protein (MW 82.4 kD). Both proteins are highly hydrophobic and contain eleven putative membrane-spanning domains. The homology to the corresponding polypeptides from maize are 89% and 95% for psaA1 and psaA2, respectively. A putative promoter has been identified for the psaA1 gene, and potential ribosome binding sites are present before both genes.