Long-wavelength chlorophylls in photosystem I of cyanobacteria: Origin,localization, and functions

The structural organization of photosystem I (PSI) complexes in cyanobacteria and the origin of the PSI antenna long-wavelength chlorophylls and their role in energy migration, charge separation, and dissipation of excess absorbed energy are discussed. The PSI complex in cyanobacterial membranes is organized preferentially as a trimer with the core antenna enriched with long-wavelength chlorophylls. The contents of long-wavelength chlorophylls and their spectral characteristics in PSI trimers and monomers are species-specific. Chlorophyll aggregates in PSI antenna are potential candidates for the role of the long-wavelength chlorophylls. The red-most chlorophylls in PSI trimers of the cyanobacteria Arthrospira platensis and Thermosynechococcus elongatus can be formed as a result of interaction of pigments peripherally localized on different monomeric complexes within the PSI trimers. Long-wavelength chlorophylls affect weakly energy equilibration within the heterogeneous PSI antenna, but they significantly delay energy trapping by P700. When the reaction center is open, energy absorbed by long-wavelength chlorophylls migrates to P700 at physiological temperatures, causing its oxidation. When the PSI reaction center is closed, the P700 cation radical or P700 triplet state (depending on the P700 redox state and the PSI acceptor side cofactors) efficiently quench the fluorescence of the long-wavelength chlorophylls of PSI and thus protect the complex against photodestruction.     

Dynamics of excitation in the photosystem I of cyanobacteria: transfer in the antenna, capture by the reaction site, and dissipation

The structure of a complex of photosystem I (PSI) of cyanobacteria and the mechanisms of the functioning of the antenna and PSI reaction site were described. The complex of PSI in thylakoids of cyanobacteia is organized as a trimer whose antenna is enriched in long-wave chlorophylls. The energy absorbed by these chlorophyls migrates to P700, inducing its oxidation. Long-wave chlorophyls are also involved in the dissipation of excessive energy; both the cation radical of P700 and the triplet of P700 effectively quench the fluorescence of long-wave chlorophyll of PSI. The energy exchange between the antennas of monomers in the trimer of PSI stimulates the dissipation of electron excitation energy, protecting the complex against photodestruction. The kinetics of energy migration in the antenna and charge separation in the reaction site of PSI trimers was studied using subpicosecond spectroscopy. Long-wave chlorophylls of PSI do not substantially affect the energy migration in the heterogeneous antenna of PSI but slow down the capture of energy of P700. The separation of changes in the reaction site of PSI is the most rapid among the known reaction sites.     

Stoichiometry of LHCI antenna polypeptides and characterization of gap and linker pigments in higher plants Photosystem I.

We report on the results obtained by measuring the stoichiometry of antenna polypeptides in Photosystem I (PSI) from Arabidopsis thaliana. This analysis was performed by quantification of Coomassie blue binding to individual LHCI polypeptides, fractionation by SDS/PAGE, and by the use of recombinant light harvesting complex of Photosystem I (Lhca) holoproteins as a standard reference. Our results show that a single copy of each Lhca1-4 polypeptide is present in Photosystem I. This is in agreement with the recent structural data on PSI-LHCI complex [Ben Shem, A., Frolow, F. and Nelson, N. (2003) Nature, 426, 630-635]. The discrepancy from earlier estimations based on pigment binding and yielding two copies of each LHCI polypeptide per PSI, is explained by the presence of 'gap' and 'linker' chlorophylls bound at the interface between PSI core and LHCI. We showed that these chlorophylls are lost when LHCI is detached from the PSI core moiety by detergent treatment and that gap and linker chlorophylls are both Chl a and Chl b. Carotenoid molecules are also found at this interface between LHCI and PSI core. Similar experiments, performed on PSII supercomplexes, showed that dissociation into individual pigment-proteins did not produce a significant loss of pigments, suggesting that gap and linker chlorophylls are a peculiar feature of Photosystem I.     

Are syn-ligated (bacterio)chlorophyll dimers energetic traps in light-harvesting systems?

A recent study of the stereochemical details of chlorophyll ligation in photosystem I [Balaban et al., Biochim. Biophys. Acta 1556 (2002) 197-207] has revealed that only 14 chlorophylls out of the total 96 are ligated from the same side (syn) as the 17-propionic acid residue which is esterified with phytol. The syn chlorophylls are carefully surrounding the reaction center forming the inner core antenna system and their ligands have been strongly conserved in several species during evolution. We hypothesize here that the two dimers of closely spaced syn chlorophylls which are encountered within roughly 2 nm of P700 are the ultimate energetic traps of this light-harvesting system. Structurally very similar bacteriochlorophyll a dimers are encountered within the Fenna-Matthews-Olson protein complex and within the B850 ring of the LH2 complex of purple bacteria. The non-random disposal of these dimers lends support to our hypothesis that the syn ligation coupled with a strong excitonic interaction leads to the most red-shifted pigments in light-harvesting systems. We would like to encourage both theoretical and experimental studies to either prove or disprove this intriguing structure-function conjecture in view of designing efficient artificial light-harvesting systems.     

Uphill energy transfer in photosystem I from <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis>. Time-resolved fluorescence measurements at 77 K

Energetic properties of chlorophylls in photosynthetic complexes are strongly modulated by their interaction with the protein matrix and by inter-pigment coupling. This spectral tuning is especially striking in photosystem I (PSI) complexes that contain low-energy chlorophylls emitting above 700 nm. Such low-energy chlorophylls have been observed in cyanobacterial PSI, algal and plant PSI–LHCI complexes, and individual light-harvesting complex I (LHCI) proteins. However, there has been no direct evidence of their presence in algal PSI core complexes lacking LHCI. In order to determine the lowest-energy states of chlorophylls and their dynamics in algal PSI antenna systems, we performed time-resolved fluorescence measurements at 77 K for PSI core and PSI–LHCI complexes isolated from the green alga Chlamydomonas reinhardtii. The pool of low-energy chlorophylls observed in PSI cores is generally smaller and less red-shifted than that observed in PSI–LHCI complexes. Excitation energy equilibration between bulk and low-energy chlorophylls in the PSI–LHCI complexes at 77 K leads to population of excited states that are less red-shifted (by ~?12 nm) than at room temperature. On the other hand, analysis of the detection wavelength dependence of the effective trapping time of bulk excitations in the PSI core at 77 K provided evidence for an energy threshold at ~?675 nm, above which trapping slows down. Based on these observations, we postulate that excitation energy transfer from bulk to low-energy chlorophylls and from bulk to reaction center chlorophylls are thermally activated uphill processes that likely occur via higher excitonic states of energy accepting chlorophylls.     

Spectroscopic properties of PSI-IsiA supercomplexes from the cyanobacterium Synechococcus PCC 7942

The cyanobacterium Synechococcus PCC 7942 grown under iron starvation assembles a supercomplex consisting of a trimeric Photosystem I (PSI) complex encircled by a ring of 18 CP43' or IsiA light-harvesting complexes [Nature 412 (2001) 745]. Here we present a spectroscopic characterization by temperature-dependent absorption and fluorescence spectroscopy, site-selective fluorescence spectroscopy at 5 K, and circular dichroism of isolated PSI-IsiA, PSI and IsiA complexes from this cyanobacterium grown under iron starvation. The results suggest that the IsiA ring increases the absorption cross-section of PSI by about 100%. Each IsiA subunit binds about 16-17 chlorophyll a (Chl a) molecules and serves as an efficient antenna for PSI. Each of the monomers of the trimeric PSI complex contains two red chlorophylls, which presumably give rise to one exciton-coupled dimer and at 5 K absorb and fluoresce at 703 and 713 nm, respectively. The spectral properties of these C-703 chlorophylls are not affected by the presence of the IsiA antenna ring. The spectroscopic properties of the purified IsiA complexes are similar to those of the related CP43 complex from plants, except that the characteristic narrow absorption band of CP43 at 682.5 nm is missing in IsiA.     

Characterization of a novel Photosystem I-LHCI supercomplex isolated from Chlamydomonas reinhardtii under anaerobic (State II) conditions

A novel supercomplex of Photosystem I (PSI) with light harvesting complex I (LHCI) was isolated from the green alga Chlamydomonas reinhardtii. This novel supercomplex is unique as it is the first stable supercomplex of PSI together with its external antenna. The supercomplex contains 256 chlorophylls per reaction center. The supercomplex was isolated under anaerobic conditions and may represent the State II form of the photosynthetic unit. In contrast to previously reported supercomplexes isolated in State I, which contain only 4 LHC I proteins, this supercomplex contains 10-11 LHC I proteins tightly bound to the PSI core. In contrast to plants, no LHC II is tightly bound to the PSI-LHCI supercomplex in State II. Investigation of the energy transfer from the antenna system to the reaction center core shows that the LHC supercomplexes are tightly coupled to the PSI core, not only structurally but also energetically. The excitation energy transfer kinetics are completely dominated by the fast phase, with a near-complete lack of long-lived fluorescence. This tight coupling is in contrast to all reports of energy transfer in PSI-LHCI supercomplexes (in State I), which have so far been described as weakly coupled supercomplexes with low efficiency for excitation energy transfer. These results indicate that there are large and dynamic changes of the PSI-LHCI supercomplex during the acclimation from aerobic (State I) to anaerobic (State II) conditions in Chlamydomonas.     

Optimization and evolution of light harvesting in photosynthesis: the role of antenna chlorophyll conserved between photosystem II and photosystem I

The efficiency of oxygenic photosynthesis depends on the presence of core antenna chlorophyll closely associated with the photochemical reaction centers of both photosystem II (PSII) and photosystem I (PSI). Although the number and overall arrangement of these chlorophylls in PSII and PSI differ, structural comparison reveals a cluster of 26 conserved chlorophylls in nearly identical positions and orientations. To explore the role of these conserved chlorophylls within PSII and PSI we studied the influence of their orientation on the efficiency of photochemistry in computer simulations. We found that the native orientations of the conserved chlorophylls were not optimal for light harvesting in either photosystem. However, PSII and PSI each contain two highly orientationally optimized antenna chlorophylls, located close to their respective reaction centers, in positions unique to each photosystem. In both photosystems the orientation of these optimized bridging chlorophylls had a much larger impact on photochemical efficiency than the orientation of any of the conserved chlorophylls. The differential optimization of antenna chlorophyll is discussed in the context of competing selection pressures for the evolution of light harvesting in photosynthesis.     

Remodeling of excitation energy transfer in extremophilic red algal PSI-LHCI complex during light adaptation

Photosynthetic PSI-LHCI complexes from an extremophilic red alga C. merolae grown under varying light regimes are characterized by decreasing size of LHCI antenna with increasing illumination intensity [1]. In this study we applied time-resolved fluorescence spectroscopy to characterize the kinetics of energy transfer processes in three types of PSI-LHCI supercomplexes isolated from the low (LL), medium (ML) and extreme high light (EHL) conditions. We show that the average rate of fluorescence decay is not correlated with the size of LHCI antenna and is twice faster in complexes isolated from ML-grown cells (~25–30 ps) than from both LL- and EHL-exposed cells (~50–55 ps). The difference is mainly due to a contribution of a long ~100-ps decay component detected only for the latter two PSI samples. We propose that the lack of this phase in ML complexes is caused by perfect coupling of this antenna to PSI core and lack of low-energy chlorophylls in LHCI. On the other hand, the presence of the slow, ~100-ps, fluorescence decay component in LL and EHL complexes may be due to the weak coupling between PSI core and LHCI antenna complex, and due to the presence of particularly low-energy or red chlorophylls in LHCI. Our study has revealed the remarkable functional flexibility of light harvesting strategies that have evolved in the extremophilic red algae in response to harsh or limiting light conditions involving accumulation of low energy chlorophylls that exert two distinct functions: as energy traps or as far-red absorbing light harvesting antenna, respectively.     

Functional Analyses of the Plant Photosystem I-Light-Harvesting Complex II Supercomplex Reveal That Light-Harvesting Complex II Loosely Bound to Photosystem II Is a Very Efficient Antenna for Photosystem I in State II

State transitions are an important photosynthetic short-term response that allows energy distribution balancing between photosystems I (PSI) and II (PSII). In plants when PSII is preferentially excited compared with PSI (State II), part of the major light-harvesting complex LHCII migrates to PSI to form a PSI-LHCII supercomplex. So far, little is known about this complex, mainly due to purification problems. Here, a stable PSI-LHCII supercomplex is purified from Arabidopsis thaliana and maize (Zea mays) plants. It is demonstrated that LHCIIs loosely bound to PSII in State I are the trimers mainly involved in state transitions and become strongly bound to PSI in State II. Specific Lhcb1-3 isoforms are differently represented in the mobile LHCII compared with S and M trimers. Fluorescence analyses indicate that excitation energy migration from mobile LHCII to PSI is rapid and efficient, and the quantum yield of photochemical conversion of PSI-LHCII is substantially unaffected with respect to PSI, despite a sizable increase of the antenna size. An updated PSI-LHCII structural model suggests that the low-energy chlorophylls 611 and 612 in LHCII interact with the chlorophyll 11145 at the interface of PSI. In contrast with the common opinion, we suggest that the mobile pool of LHCII may be considered an intimate part of the PSI antenna system that is displaced to PSII in State I.     

On the spectral properties and excitation dynamics of long-wavelength chlorophylls in higher-plant photosystem I

In higher-plant Photosystem I (PSI), the majority of “red” chlorophylls (absorbing at longer wavelengths than the reaction centre P₇₀₀) are located in the peripheral antenna, but contradicting reports are given about red forms in the core complex. Here we attempt to clarify the spectroscopic characteristics and quantify the red forms in the PSI core complex, which have profound implication on understanding the energy transfer and charge separation dynamics. To this end we compare the steady-state absorption and fluorescence spectra and picosecond time-resolved fluorescence kinetics of isolated PSI core complex and PSI–LHCI supercomplex from Pisum sativum recorded at 77 K. Gaussian decomposition of the absorption spectra revealed a broad band at 705 nm in the core complex with an oscillator strength of three chlorophylls. Additional absorption at 703 nm and 711 nm in PSI–LHCI indicated up to five red chlorophylls in the peripheral antenna. Analysis of fluorescence emission spectra resolved states emitting at 705, 715 and 722 nm in the core and additional states around 705–710 nm and 733 nm in PSI–LHCI. The red states compete with P₇₀₀ in trapping excitations in the bulk antenna, which occurs on a timescale of ~20 ps. The three red forms in the core have distinct decay kinetics, probably in part determined by the rate of quenching by the oxidized P₇₀₀. These results affirm that the red chlorophylls in the core complex must not be neglected when interpreting kinetic experimental results of PSI.     

Nuclear Overhauser effect studies of the conformations of MgATP bound to the active and secondary sites of muscle pyruvate kinase

MgATP binds both at the active site (site 1) and at a secondary site (site 2) on each monomer of muscle pyruvate kinase as previously found by binding studies and by X-ray analysis. Interproton distances on MgATP bound at each site have been measured by the time-dependent nuclear Overhauser effect in the absence and presence of phosphoenolpyruvate (P-enolpyruvate), which blocks ATP binding at site 1. Interproton distances at site 2 are consistent with a single conformation of bound ATP with a high antiglycosidic torsional angle (chi = 68 +/- 10 degrees) and a C3'-endo ribose pucker (delta = 90 +/- 10 degrees). Interproton distances at site 1, determined in the absence of P-enolpyruvate by assuming the averaging of distances at both sites, cannot be fit by a single adenine-ribose conformation but require the contribution of at least three low-energy structures: 62 +/- 10% low anti (chi = 30 degrees), C3'-endo; 20 +/- 8% high anti (chi = 55 degrees), O1'-endo; and 18 +/- 8% syn (chi = 217 degrees), C2'-endo. Although a different set of ATP conformations might also have fit the interproton distances, the mixture of conformations used also fits previously determined distances from Mn2+ to the protons of ATP bound at site 1 [Sloan, D. L., & Mildvan, A. S. (1976) J. Biol. Chem. 251, 2412] and is similar to the adenine-ribose portion of free Co(NH3)4ATP, which consists of 35% low anti, 51% high anti, and 14% syn [Rosevear, P. R., Bramson, H. N., O'Brian, C., Kaiser, E. T., & Mildvan, A. S. (1983) Biochemistry 22, 3439].(ABSTRACT TRUNCATED AT 250 WORDS)     

Electric field effects on red chlorophylls,beta-carotenes and P700 in cyanobacterial Photosystem I complexes

We have probed the absorption changes due to an externally applied electric field (Stark effect) of Photosystem I (PSI) core complexes from the cyanobacteria Synechocystis sp. PCC 6803, Synechococcus elongatus and Spirulina platensis. The results reveal that the so-called C719 chlorophylls in S. elongatus and S. platensis are characterized by very large polarizability differences between the ground and electronically excited states (with Tr(Deltaalpha) values up to about 1000 A(3) f(-2)) and by moderately high change in permanent dipole moments (with average Deltamu values between 2 and 3 D f(-1)). The C740 chlorophylls in S. platensis and, in particular, the C708 chlorophylls in all three species give rise to smaller Stark shifts, which are, however, still significantly larger than those found before for monomeric chlorophyll. The results confirm the hypothesis that these states originate from strongly coupled chlorophyll a molecules. The absorption and Stark spectra of the beta-carotene molecules are almost identical in all complexes and suggest similar or slightly higher values for Tr(Deltaalpha) and Deltamu than for those of beta-carotene in solution. Oxidation of P700 did not significantly change the Stark response of the carotenes and the red antenna states C719 and C740, but revealed in all PSI complexes changes around 700-705 and 690-693 nm, which we attribute to the change in permanent dipole moments of reduced P700 and the chlorophylls responsible for the strong absorption band at 690 nm with oxidized P700, respectively.     

Properties of photosystem I antenna protein complexes of the diatom Cyclotella meneghiniana

Analysis of photosystem I (PSI) complexes from Cyclotella meneghiniana cultured under different growth conditions led to the identification of three groups of antenna proteins, having molecular weights of around 19, 18, and 17 kDa. The 19-kDa proteins have earlier been demonstrated to be more peripherally bound to PSI, and their amount in the PSI complexes was significantly reduced when the iron supply in the growth medium was lowered. This polypeptide was almost missing, and thus the total amount of fucoxanthin-chlorophyll proteins (Fcps) bound to PSI was reduced as well. When treating cells with high light in addition, no further changes in antenna polypeptide composition were detected. Xanthophyll cycle pigments were found to be bound to all Fcps of PSI. However, PSI of high light cultures had a significantly higher diatoxanthin to diadinoxanthin ratio, which is assumed to protect against a surplus of excitation energy. PSI complexes from the double-stressed cultures (high light plus reduced iron supply) were slightly more sensitive against destruction by the detergent treatment. This could be seen as a higher 674-nm emission at 77 K in comparison to the PSI complexes isolated from other growth conditions. Two major emission bands of the Fcps bound to PSI at 77 K could be identified, whereby chlorophyll a fluorescing at 697 nm was more strongly coupled to the PSI core than those fluorescing at 685 nm. Thus, the build up of the PSI antenna of several Fcp components enables variable reactions to several stress factors commonly experienced by the diatoms in vivo, in particular diatoxanthin enrichment under high light and reduction of antenna size under reduced iron conditions.     

Three-dimensional reconstruction of a light-harvesting complex I-photosystem I (LHCI-PSI) supercomplex from the green alga Chlamydomonas reinhardtii. Insights into light harvesting for PSI

A supercomplex containing the photosystem I (PSI) and chlorophyll a/b light-harvesting complex I (LHCI) has been isolated using a His-tagged mutant of Chlamydomonas reinhardtii. This LHCI-PSI supercomplex contained approximately 215 chlorophyll molecules of which 175 were estimated to be chlorophyll a and 40 to be chlorophyll b, based on P700 oxidation and chlorophyll a/b ratio measurements. Its room temperature long wavelength absorption peak was at 680 nm, and it emitted chlorophyll fluorescence maximally at 715 nm (77 K). The LHCI was composed of four or more different types of Lhca polypeptides including Lhca3. No LHCII proteins or other phosphoproteins were detected in the LHCI-PSI supercomplexes suggesting that the cells from which they were isolated were in State 1. Electron microscopy of negatively stained samples followed by image analysis revealed the LHCI-PSI supercomplex to have maximal dimensions of 220 A by 180 A and to be approximately 105 A thick. An averaged top view was used to model in x-ray and electron crystallographic data for PSI and Lhca proteins respectively. We conclude that the supercomplex consists of a PSI reaction center monomer with 11 Lhca proteins arranged along the side where the PSI proteins, PsaK, PsaJ, PsaF, and PsaG are located. The estimated molecular mass for the complex is 700 kDa including the bound chlorophyll molecules. The assignment of 11 Lhca proteins is consistent with a total chlorophyll level of 215 assuming that the PSI reaction center core binds approximately 100 chlorophylls and that each Lhca subunit binds 10 chlorophylls. There was no evidence for oligomerization of Chlamydomonas PSI in contrast to the trimerization of PSI in cyanobacteria.     

Bidirectional electron transfer in photosystem I: accumulation of A0- in A-side or B-side mutants of the axial ligand to chlorophyll A0

Photosystem I contains two potential electron transfer pathways between P(700) and F(X). These branches are made up of the electron transfer chain components A, A(0), and A(1). The primary electron acceptor A(0) is a chlorophyll a monomer that could be one or both of the two chlorophyll molecules, eC-A(3)/eC-B(3), identified in the 2.5 A resolution structure. The eC-A(3)/eC-B(3) chlorophylls are both coordinated by the sulfur atom of a methionine. This coordination is highly unusual, as interactions between the acid Mg(2+) and the soft base sulfur are weak. The eC-A(3)/eC-B(3) chlorophylls also are located close to one of the connecting chlorophylls that may link the antenna and the electron transfer chain chlorophylls. Due to their location in the structure, the eC-A(3)/eC-B(3) chlorophylls may play a role in both excitation energy transfer and electron transfer. To test the role of the eC-A(3)/eC-B(3) chlorophylls in electron transfer, Met-684 of PsaA and Met-664 of PsaB have been changed to His, Ser, and Leu. Replacement of either M(A684) or M(B664) results in a significant alteration in growth phenotype. The His and Leu mutants are very light sensitive in the presence of oxygen. Growth is impaired to a greater extent in the B-side mutants. However, all of the mutants are able to grow anaerobically at comparable rates. The His and Ser mutants all accumulate PSI at a level similar to that of wild type, whereas the Leu mutants have reduced amounts of PSI. Ultrafast transient absorbance measurements show that the (A(0)(-) - A(0)) difference signal accumulates in the MH(A684) and MH(B664) mutants under neutral conditions, demonstrating that electron transfer between A(0)(-) and A(1) is blocked or significantly slowed. The results show that both the A-branch and the B-branch of the ETC are active in PSI from Chlamydomonas reinhardtii.     

Structure, function and regulation of plant photosystem I

Photosystem I (PSI) is a multisubunit protein complex located in the thylakoid membranes of green plants and algae, where it initiates one of the first steps of solar energy conversion by light-driven electron transport. In this review, we discuss recent progress on several topics related to the functioning of the PSI complex, like the protein composition of the complex in the plant Arabidopsis thaliana, the function of these subunits and the mechanism by which nuclear-encoded subunits can be inserted into or transported through the thylakoid membrane. Furthermore, the structure of the native PSI complex in several oxygenic photosynthetic organisms and the role of the chlorophylls and carotenoids in the antenna complexes in light harvesting and photoprotection are reviewed. The special role of the 'red' chlorophylls (chlorophyll molecules that absorb at longer wavelength than the primary electron donor P700) is assessed. The physiology and mechanism of the association of the major light-harvesting complex of photosystem II (LHCII) with PSI during short term adaptation to changes in light quality and quantity is discussed in functional and structural terms. The mechanism of excitation energy transfer between the chlorophylls and the mechanism of primary charge separation is outlined and discussed. Finally, a number of regulatory processes like acclimatory responses and retrograde signalling is reviewed with respect to function of the thylakoid membrane. We finish this review by shortly discussing the perspectives for future research on PSI.     

Red antenna states of Photosystem I trimers from Arthrospira platensis revealed by single-molecule spectroscopy

Single-molecule fluorescence spectroscopy at 1.4K was used to investigate the spectral properties of red (long-wavelength) chlorophylls in trimeric Photosystem I (PSI) complexes from the cyanobacterium Arthrospira platensis. Three distinct red antenna states could be identified in the fluorescence spectra of single PSI trimers from A. platensis in the presence of oxidized P700. Two of them are responsible for broad emission bands centered at 726 and 760nm. These bands are similar to those found in bulk fluorescence spectra measured at cryogenic temperatures. The broad fluorescence bands at ?726 and ?760nm belong to individual emitters that are broadened by strong electron-phonon coupling giving rise to a large Stokes-shift of about 20nm and rapid spectral diffusion. An almost perpendicular orientation of the transition dipole moments of F726 and F760 has to be assumed because direct excitation energy transfer does not occur between F726 and F760. For the first time a third red state assigned to the pool absorbing around 708nm could be detected by its zero-phonon lines. The center of the zero-phonon line distribution is found at ?714nm. The spectral properties of the three red antenna states show a high similarity to the red antenna states found in trimeric PSI of Thermosynechoccocus elongatus. Based on these findings a similar organization of the red antenna states in PSI of these two cyanobacteria is discussed.     

Chemical platinization and its effect on excitation transfer dynamics and P700 photooxidation kinetics in isolated photosystem I.

Isolated photosystem I (PSI) reaction center/core antenna complexes (PSI-40) were platinized by reduction of [PtCl6]2- at 20 degrees C and neutral pH. PSI particles were visualized directly on a gold surface by scanning tunneling microscopy (STM) before and after platinization. STM results showed that PSI particles were monomeric and roughly ellipsoidal with major and minor axes of 6 and 5 nm, respectively. Platinization deposited approximately 1000 platinum atoms on each PSI particle and made the average size significantly larger (9 x 7 nm). In addition to direct STM visualization, the presence of metallic platinum on the PSI complexes was detected by its effect of actinic shading and electrostatic shielding on P700 photooxidation and P700+ reduction. The reaction centers (P700) in both platinized and nonplatinized PSI-40 were photooxidized by light and reduced by ascorbate repeatedly, although at somewhat slower rates in platinized PSI because of the presence of platinum. The effect of platinization on excitation transfer and trapping dynamics was examined by measuring picosecond fluorescence decay kinetics in PSI-40. The fluorescence decay kinetics in both platinized and control samples can be described as a sum of three exponential components. The dominant (amplitude 0.98) and photochemically limited excitation lifetime remained the same (16 ps) before and after platinization. The excitation transfer and trapping in platinized PSI-40 was essentially as efficient as that in the control (without platinization) PSI. The platinization also did not affect the intermediate-lifetime (400-600 ps) and long-lifetime (> 2500 ps) components, which likely are related to intrinsic electron transport and to functionally uncoupled chlorophylls, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Variability of light-induced circular dichroism spectra of photosystem I complexes of cyanobacteria

Circular dichroism (CD) spectra of photosystem I (PSI) complexes of the cyanobacteria Thermosynechococcus elongatus, Arthrospira platensis and Synechocystis sp. PCC 6803 were studied. CD spectra of dark-adapted PSI trimers and monomers, measured at 77 K, show common bands at 669–670(+), 673(+), 680(−), 683–685(−), 696–697(−), 702(−) and 711(−) nm. The intensities of these bands are species specific. In addition, bands at 683–685(−) and 673(+) nm differ in intensity for trimeric and monomeric PSI complexes. CD difference spectra (P700⁺–P700) of PSI complexes at 283 K exhibit conservative bands at 701(−) and 691(+) nm due to changes in resonance interaction of chlorophylls in the reaction center upon oxidation of P700. Additional bands are observed at 671(−), 678(+), 685(−), 693(−) nm and in the region 720–725 nm those intensities correlate with intensities of analogous bands of antenna chlorophylls in dark-adapted CD spectra. It is suggested that the variability of CD difference spectra of PSI complexes is determined by changes in resonance interaction of reaction center chlorophylls with closely located antenna chlorophylls.