Photosystem I photochemistry at low temperature. Heterogeneity in pathways for electron transfer to the secondary acceptors and for recombination processes

Kinetic modeling of electron transfer reactions in photosystem I complexes of various structures with substituted quinone acceptors

The reduction kinetics of the photo-oxidized primary electron donor P₇₀₀ in photosystem I (PS I) complexes from cyanobacteria Synechocystis sp. PCC 6803 were analyzed within the kinetic model, which considers electron transfer (ET) reactions between P₇₀₀, secondary quinone acceptor A₁, iron-sulfur clusters and external electron donor and acceptors - methylviologen (MV), 2,3-dichloro-naphthoquinone (Cl₂NQ) and oxygen. PS I complexes containing various quinones in the A₁-binding site (phylloquinone PhQ, plastoquinone-9 PQ and Cl₂NQ) as well as F _X-core complexes, depleted of terminal iron-sulfur F _A/F _B clusters, were studied. The acceleration of charge recombination in F _X-core complexes by PhQ/PQ substitution indicates that backward ET from the iron-sulfur clusters involves quinone in the A₁-binding site. The kinetic parameters of ET reactions were obtained by global fitting of the P₇₀₀⁺ reduction with the kinetic model. The free energy gap ΔG ₀ between F _X and F _A/F _B clusters was estimated as -130 meV. The driving force of ET from A₁ to F _X was determined as -50 and -220 meV for PhQ in the A and B cofactor branches, respectively. For PQ in A_1A-site, this reaction was found to be endergonic (ΔG ₀ = +75 meV). The interaction of PS I with external acceptors was quantitatively described in terms of Michaelis-Menten kinetics. The second-order rate constants of ET from F _A/F _B, F _X and Cl₂NQ in the A₁-site of PS I to external acceptors were estimated. The side production of superoxide radical in the A₁-site by oxygen reduction via the Mehler reaction might comprise ≥0.3% of the total electron flow in PS I.

Long-wavelength limit of photochemical energy conversion in Photosystem I

In Photosystem I (PS I) long-wavelength chlorophylls (LWC) of the core antenna are known to extend the spectral region up to 750 nm for absorbance of light that drives photochemistry. Here we present clear evidence that even far-red light with wavelengths beyond 800 nm, clearly outside the LWC absorption bands, can still induce photochemical charge separation in PS I throughout the full temperature range from 295 to 5 K. At room temperature, the photoaccumulation of P700(+•) was followed by the absorbance increase at 826 nm. At low temperatures (T < 100 K), the formation of P700(+•)FA/B(-•) was monitored by the characteristic EPR signals of P700(+•) and FA/B(-•) and by the characteristic light-minus-dark absorbance difference spectrum in the QY region. P700 oxidation was observed upon selective excitation at 754, 785, and 808 nm, using monomeric and trimeric PS I core complexes of Thermosynechococcus elongatus and Arthrospira platensis, which differ in the amount of LWC. The results show that the LWC cannot be responsible for the long-wavelength excitation-induced charge separation at low temperatures, where thermal uphill energy transfer is frozen out. Direct energy conversion of the excitation energy from the LWC to the primary radical pair, e.g., via a superexchange mechanism, is excluded, because no dependence on the content of LWC was observed. Therefore, it is concluded that electron transfer through PS I is induced by direct excitation of a proposed charge transfer (CT) state in the reaction center. A direct signature of this CT state is seen in absorbance spectra of concentrated PS I samples, which reveal a weak and featureless absorbance band extending beyond 800 nm, in addition to the well-known bands of LWC (C708, C719 and C740) in the range between 700 and 750 nm. The present findings suggest that nature can exploit CT states for extending the long wavelength limit in PSI even beyond that of LWC. Similar mechanisms may work in other photosynthetic systems and in chemical systems capable of photoinduced electron transfer processes in general.

High Time Resolution Multifrequency EPR of Radical Pair Intermediates in Photosynthetic Reaction Centers: Structure Determination on a Nanosecond Time Scale

The primary steps of photosynthesis proceed via light-induced radical pairs as short-lived intermediates. In this paper we discuss novel coherence phenomena which can be observed for these species. It is demonstrated that the analysis of quantum beat oscillations in combination with multifrequency EPR and a magnetically oriented sample represents a powerful structural tool. We expect that this is of general interest, since the detailed structure of radical pair intermediates can be determined on a nanosecond time scale.

Conformational control of the Q(A) to Q(B) electron transfer in bacterial reaction centers: evidence for a frozen conformational landscape below -25 degrees C

The competition between the P(+)Q(A)(-) --> PQ(A) charge recombination (P, bacteriochlorophyll pair acting as primary photochemical electron donor) and the electron transfer to the secondary quinone acceptor Q(A)(-)Q(B) --> Q(A)Q(B)(-) (Q(A) and Q(B), primary and secondary electron accepting quinones) was investigated in chromatophores of Rb. capsulatus, varying the temperature down to -65 degrees C. The analysis of the flash-induced pattern for the formation of P(+)Q(A)Q(B)(-) shows that the diminished yield, when lowering the temperature, is not due to a homogeneous slowing of the rate constant k(AB) of the Q(A)(-)Q(B) --> Q(A)Q(B)(-) electron transfer but to a distribution of conformations that modulate the electron transfer rate over more than 3 orders of magnitude. This distribution appears "frozen", as no dynamic redistribution was observed over time ranges > 10 s (below -25 degrees C). The kinetic pattern was analyzed to estimate the shape of the distribution of k(AB), showing a bell-shaped band on the high rate side and a fraction of "blocked" reaction centers (RCs) with very slow k(AB). When the temperature is lowered, the high rate band moves to slower rate regions and the fraction of blocked RCs increases at the expense of the high rate band. The RCs that recombine from the P(+)Q(A)Q(B)(-) state appear temporarily converted to a state with rapid k(AB), indicating that the stabilized state described by Kleinfeld et al. (Biochemistry 1984, 23, 5780-5786) is still accessible at -60 degrees C.

Spectral and kinetic analysis of the energy coupling in the PS I-LHC I supercomplex from the green alga Chlamydomonas reinhardtii at 77 K

Energy transfer processes in the chlorophyll antenna of the PS I-LHCI supercomplexes from the green alga Chlamydomonas reinhardtii have been studied at 77 K using transient absorption spectroscopy with multicolor excitation in the 640-670 nm region. Comparison of the kinetic data obtained at low and room temperatures indicates that the slow approximately approximately 100 ps excitation equilibration phase that is characteristic of energy coupling of the LHCI peripheral antenna to the PS I core at physiological temperatures (Melkozernov AN, Kargul J, Lin S, Barber J and Blankenship RE (2004) J Phys Chem B 108: 10547-10555) is not observed in the excitation dynamics of the PS I-LHCI supercomplex at 77 K. This suggests that at low temperatures the peripheral antenna is energetically uncoupled from the PS I core antenna. Under these conditions the observed kinetic phases on the time scales from subpicoseconds to tens of picoseconds represent the superposition of the processes occurring independently in the PS I core antenna and the Chl a/b containing LHCI antenna. In the PS I-LHCI supercomplex with two uncoupled antennas the excitation is channeled to the excitation sinks formed at low temperature by clusters of red pigments. A better spectral resolution of the transient absorption spectra at 77 K results in detection of two DeltaA bands originating from the rise of photobleaching on the picosecond time scale of two clearly distinguished pools of low energy absorbing Chls in the PS I-LHCI supercomplex. The first pool of low energy pigments absorbing at 687 nm is likely to originate from the red pigments in the LHCI where the Lhca1 protein is most abundant. The second pool at 697 nm is suggested to result either from the structural interaction of the LHCI and the PS I core or from other Lhca proteins in the antenna. The kinetic data are discussed based on recent structural models of the PS I-LHCI. It is proposed that the uncoupling of pigment pools may be a control mechanism that regulates energy flow in Photosystem I.

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).

P700+- and 3P700-induced quenching of the fluorescence at 760 nm in trimeric Photosystem I complexes from the cyanobacterium Arthrospira platensis

The 5 K absorption spectrum of Photosystem I (PS I) trimers from Arthrospira platensis (old name: Spirulina platensis) exhibits long-wavelength antenna (exciton) states absorbing at 707 nm (called C707) and at 740 nm (called C740). The lowest energy state (C740) fluoresces around 760 nm (F760) at low temperature. The analysis of the spectral properties (peak position and line width) of the lowest energy transition (C740) as a function of temperature within the linear electron-phonon approximation indicates a large optical reorganization energy of approximately 110 cm(-1) and a broad inhomogeneous site distribution characterized by a line width of approximately 115 cm(-1). Linear dichroism (LD) measurements indicate that the transition dipole moment of the red-most state is virtually parallel to the membrane plane. The relative fluorescence yield at 760 nm of PS I with P700 oxidized increases only slightly when the temperature is lowered to 77 K, whereas in the presence of reduced P700 the fluorescence yield increases nearly 40-fold at 77 K as compared to that at room temperature (RT). A fluorescence induction effect could not be resolved at RT. At 77 K the fluorescence yield of PS I trimers frozen in the dark in the presence of sodium ascorbate decreases during illumination by about a factor of 5 due to the irreversible formation of (P700+)F(A/B-) in about 60% of the centers and the reversible accumulation of the longer-lived state (P700+)FX-. The quenching efficiency of different functionally relevant intermediate states of the photochemistry in PS I has been studied. The redox state of the acceptors beyond A(0) does not affect F760. Direct kinetic evidence is presented that the fluorescence at 760 nm is strongly quenched not only by P700+ but also by 3P700. Similar kinetics were observed for flash-induced absorbance changes attributed to the decay of 3P700 or P700+, respectively, and flash-induced fluorescence changes at 760 nm measured under identical conditions. A nonlinear relationship between the variable fluorescence around 760 nm and the [P700red]/[P700total] ratio was derived from titration curves of the absorbance change at 826 nm and the variable fluorescence at 760 nm as a function of the redox potential imposed on the sample solution at room temperature before freezing. The result indicates that the energy exchange between the antennae of different monomers within a PS I trimer stimulates quenching of F760 by P700+.

Species-specific differences of the spectroscopic properties of P700: analysis of the influence of non-conserved amino acid residues by site-directed mutagenesis of photosystem I from Chlamydomonas reinhardtii

We applied optical spectroscopy, magnetic resonance techniques, and redox titrations to investigate the properties of the primary electron donor P700 in photosystem I (PS I) core complexes from cyanobacteria (Thermosynechococcus elongatus, Spirulina platensis, and Synechocystis sp. PCC 6803), algae (Chlamydomonas reinhardtii CC2696), and higher plants (Spinacia oleracea). Remarkable species-specific differences of the optical properties of P700 were revealed monitoring the (3P700-P700) and (P700+.-P700) absorbance and CD difference spectra. The main bleaching band in the Qy region differs in peak position and line width for the various species. In cyanobacteria the absorbance of P700 extends more to the red compared with algae and higher plants which is favorable for energy transfer from red core antenna chlorophylls to P700 in cyanobacteria. The amino acids in the environment of P700 are highly conserved with two distinct deviations. In C. reinhardtii a Tyr is found at position PsaB659 instead of a Trp present in all other organisms, whereas in Synechocystis a Phe is found instead of a Trp at the homologous position PsaA679. We constructed several mutants in C. reinhardtii CC2696. Strikingly, no PS I could be detected in the mutant YW B659 indicating steric constraints unique to this organism. In the mutants WA A679 and YA B659 significant changes of the spectral features in the (3P700-P700), the (P700+.-P700) absorbance difference and in the (P700+.-P700) CD difference spectra are induced. The results indicate structural differences among PS I from higher plants, algae, and cyanobacteria and give further insight into specific protein-cofactor interactions contributing to the optical spectra.

Light-induced spin polarization in type I photosynthetic reaction centres

The use of light-induced spin polarization to study the structure and function of type I reaction centres is reviewed. The absorption of light by these systems generates a series of sequential radical pairs, which exhibit spin polarization as a result of the correlation of the unpaired electron spins. A description of how the polarization patterns can be used to deduce the relative orientation of the radicals is given and the most important structural results from such studies on photosystem I (PS I) are summarized. Quinone exchange experiments which demonstrate the influence of protein-cofactor interactions on the polarization patterns are discussed. The results show that there are significant differences between the binding sites of the primary quinone acceptors in PS I and purple bacterial reaction centres and suggest that pi-pi interactions probably play a more important role in PS I. Studies using spin-polarized EPR transients and spectra to investigate the electron transfer pathway and kinetics are also reviewed. The results from PS I, green-sulphur bacteria and Heliobacteria are compared and the controversy surrounding the role of a quinone in the electron transfer in the latter two systems is discussed.

Iron-sulfur clusters in type I reaction centers

Type I reaction centers (RCs) are multisubunit chlorophyll-protein complexes that function in photosynthetic organisms to convert photons to Gibbs free energy. The unique feature of Type I RCs is the presence of iron-sulfur clusters as electron transfer cofactors. Photosystem I (PS I) of oxygenic phototrophs is the best-studied Type I RC. It is comprised of an interpolypeptide [4Fe-4S] cluster, F(X), that bridges the PsaA and PsaB subunits, and two terminal [4Fe-4S] clusters, F(A) and F(B), that are bound to the PsaC subunit. In this review, we provide an update on the structure and function of the bound iron-sulfur clusters in Type I RCs. The first new development in this area is the identification of F(A) as the cluster proximal to F(X) and the resolution of the electron transfer sequence as F(X)-->F(A)-->F(B)-->soluble ferredoxin. The second new development is the determination of the three-dimensional NMR solution structure of unbound PsaC and localization of the equal- and mixed-valence pairs in F(A)(-) and F(B)(-). We provide a survey of the EPR properties and spectra of the iron-sulfur clusters in Type I RCs of cyanobacteria, green sulfur bacteria, and heliobacteria, and we summarize new information about the kinetics of back-reactions involving the iron-sulfur clusters.

Energy transfer and charge separation in photosystem I: P700 oxidation upon selective excitation of the long-wavelength antenna chlorophylls of Synechococcus elongatus

Photosystem I of the cyanobacterium Synechococcus elongatus contains two spectral pools of chlorophylls called C-708 and C-719 that absorb at longer wavelengths than the primary electron donor P700. We investigated the relative quantum yields of photochemical charge separation and fluorescence as a function of excitation wavelength and temperature in trimeric and monomeric photosystem I complexes of this cyanobacterium. The monomeric complexes are characterized by a reduced content of the C-719 spectral form. At room temperature, an analysis of the wavelength dependence of P700 oxidation indicated that all absorbed light, even of wavelengths of up to 750 nm, has the same probability of resulting in a stable P700 photooxidation. Upon cooling from 295 K to 5 K, the nonselectively excited steady-state emission increased by 11- and 16-fold in the trimeric and monomeric complexes, respectively, whereas the quantum yield of P700 oxidation decreased 2.2- and 1.7-fold. Fluorescence excitation spectra at 5 K indicate that the fluorescence quantum yield further increases upon scanning of the excitation wavelength from 690 nm to 710 nm, whereas the quantum yield of P700 oxidation decreases significantly upon excitation at wavelengths longer than 700 nm. Based on these findings, we conclude that at 5 K the excited state is not equilibrated over the antenna before charge separation occurs, and that approximately 50% of the excitations reach P700 before they become irreversibly trapped on one of the long-wavelength antenna pigments. Possible spatial organizations of the long-wavelength antenna pigments in the three-dimensional structure of photosystem I are discussed.

Strains of Synechocystis sp. PCC 6803 with altered PsaC. II. EPR and optical spectroscopic properties of FA and FB in aspartate, serine, and alanine replacements of cysteines 14 and 51

A psaC deletion mutant of the unicellular cyanobacterium Synechocystis sp. PCC 6803 was utilized to incorporate site-specific amino acid substitutions in the cysteine residues that ligate the FA and FB iron-sulfur clusters in Photosystem I (PS I). Cysteines 14 and 51 of PsaC were changed to aspartic acid (C14DPsaC, C51DPsaC, C14D/C51DPsaC), serine (C14SPsaC, C51SPsaC), and alanine (C14APsaC, C51APsaC), and the properties of FA and FB were characterized by electron paramagnetic resonance spectroscopy and time-resolved optical spectroscopy. The C14DPsaC-PS I and C14SPsaC-PS I complexes showed high levels of photoreduction of FA with g values of 2.045, 1. 944, and 1.852 after illumination at 15 K, but there was no evidence of reduced FB in the g = 2 region. The C51DPsaC-PS I and C51SPsaC-PS I complexes showed low levels of photoreduction of FB with g values of 2.067, 1.931, and 1.881 after illumination at 15 K, but there was no evidence of reduced FA in the g = 2 region. The presence of FB was inferred in C14DPsaC-PS I and C14SPsaC-PS I, and the presence of FA was inferred in C51DPsaC-PS I and C51SPsaC-PS I by magnetic interaction in the photoaccumulated spectra and by the equal spin concentration of the irreversible P700(+) cation generated by illumination at 77 K. Flash-induced optical absorbance changes at 298 K in the presence of a fast electron donor indicate that two electron acceptors function after FX in the four mutant PS I complexes at room temperature. These data suggest that a mixed-ligand [4Fe-4S] cluster is present in the mutant sites of C14X-PS I and C51X-PS I (where X = D or S), but that the proposed spin state of S = 3/2 renders the resonances undetectable in the g = 2 region. The C14APsaC-PS I, C51APsaC-PS I and C14D/C51DPsaC-PS I complexes show only the photoreduction of FX, consistent with the absence of PsaC. These results show that only those PsaC proteins that contain two [4Fe-4S] clusters are capable of assembling onto PS I cores in vivo.

Redox titration of two [4Fe-4S] clusters in the photosynthetic reaction center from the anaerobic green sulfur bacterium Chlorobium vibrioforme

Anaerobic green sulfur bacteria contain photosynthetic reaction centers analogous to photosystem I (PS I) of plants and cyanobacteria. These reaction centers, termed type I, are characterized by the presence of bound iron-sulfur clusters as the terminal electron acceptors. In this work, the iron-sulfur clusters in Chlorobium vibrioforme were studied using selective light-induced reduction protocols, spin quantifications, and chemical redox titrations coupled with EPR detection. Illumination of a dark-frozen sample at 12 K results in the appearance of a spectrum termed signal I. Chemical reduction in darkness at solution potentials between -414 mV and -492 mV results in the appearance of a different spectrum termed signal II. Illumination of these chemically poised samples at 12 K results in the appearance of signal I such that the sum of the intensity of signal I + signal II is nearly constant for every ratio of signal I/signal II. As the solution potential is lowered to -545 mV, the spectrum shifts to yet a third set of resonances, termed signal III. Concomitant with this shift is a loss of low temperature light-induced reduction of signal I. Photoaccumulation of a sample poised at a solution potential of -50 mV results also in the appearance of signal III at nearly the same spin concentration as the chemically reduced sample. Spin quantifications imply that signals I and II are both derived from the reduction of one iron-sulfur cluster, termed center I; signal III is derived from simultaneous reduction of two iron-sulfur clusters, centers I and II. By measuring the EPR signal intensities over a range of solution potentials, centers I and II were shown to have Em (pH 10.0) values of -446 mV and -501 mV, respectively. The observations are consistent with a structural and functional analogy of centers I and II with F(A) and F(B) of PS I.

Path of electron transfer in photosystem 1: direct evidence of forward electron transfer from A1 to Fe-Sx

Pulsed EPR spectroscopy and selective removal of the iron-sulfur centers in photosystem 1 have been used to study forward electron transfer from the secondary electron acceptor A1. At cryogenic temperatures where forward electron transfer is inhibited, we have observed a g = 2.003 electron spin-echo signal presenting a characteristic phase shift. This out-of-phase signal is attributed to the electron spin-polarized pair P700+/A1-, it decays with t1/e = 23 microseconds, reflecting the recombination reaction. At room temperature the out-of-phase signal is also observed, but it decays with t1/c = 200 ns in untreated photosystem 1, due to forward electron transfer from A1- to one of the iron-sulfur centers. This rate is unchanged in Fe-SA/B-depleted PS1 but is lost when the iron-sulfur center Fe-Sx is removed. In the preparations depleted of all iron-sulfur centers the out-of-phase signal decays with t1/c = 1.3 microseconds, reflecting either the back reaction or the decay of polarization. These results demonstrate that the electron transfer pathway in photosystem 1 is P700-->A1-->Fe-SX-->Fe-SA/B.

Effect of vitamin E on acetylcholine-induced current in molluscan neurons: role of cytoplasmic free calcium and arachidonic acid

Role of cytoplasmic concentration of free Ca2+ ([Ca]in) and arachidonic acid in potentiating the effect of vitamin E (DL-alpha-tocopherol) on acetylcholine receptor activity in Helix pomatia neurons was studied using a two-microelectrode intracellular recording, voltage clamp and fluorescent calcium probe fura-2 technique. Elevation of [Ca]in by intracellular injection from a microelectrode or by depolarizing pulses and application of 0.1 microM-0.1 mM vitamin E enhanced the acetylcholine-induced chloride current both in LP11 and RBc4 neurons. Application of 10 microM arachidonic acid to the same neurons decreased this current. The use of fluorescent probe showed that vitamin E did not essentially change [Ca]in, but an increase of [Ca]in intensified both the enhancing effect of vitamin E and the depressing effect of arachidonic acid. The enhancing effect of calcium influx was considerably decreased after vitamin E application. The antioxidant action of vitamin E was probably not involved in the mechanism of its enhancing effect on acetylcholine-induced current, since synthetic antioxidant, ionol, depressed acetylcholine responses. A spectrum analysis has shown the interaction between vitamin E and arachidonic acid in solution. This interaction may be considered as the molecular mechanism responsible for the prevention by vitamin E of steady arachidonic acid production from membrane phospholipids and its down-regulatory effect on acetylcholine receptor activity. Our results support this suggestion, since an inhibitor of phospholipase A2, 4-bromophenacyl bromide, mimicked the enhancing effect of vitamin E.

Spectral hole burning of the primary electron donor state of Photosystem I

Persistent photochemical hole burned profiles are reported for the primary electron donor state P700 of the reaction center of PS I. The hole profiles at 1.6 K for a wide range of burn wavelengths (λB) are broad (FWHM∼310 cm(-1)) and for the 45:1 enriched particles studied exhibit no sharp zero-phonon hole feature coincident with λB. The λB hole profiles are analyzed using the theory of Hayes et al. [J Phys Chem 1986, 90: 4928] for hole burning in the presence of arbitrarily strong linear electron-phonon coupling. A Huang-Rhys factor S in the range 4-6 and a corresponding mean phonon frequency in the range 35-50 cm(-1) together with an inhomogeneous line broadening of∼100 cm(-1) are found to provide good agreement with experiment. The zero-point level of P700(*) is predicted to lie at∼710 nm at 1.6K with an absorption maximum at∼702 nm. The hole spectra are discussed in the context of the hole spectra for the primary electron donor states of PS II and purple bacteria.

Purification and properties of the intact P-700 and Fx-containing Photosystem I core protein

The intact Photosystem I core protein, containing the psaA and psaB polypeptides, and electron transfer components P-700 through FX, was isolated from cyanobacterial and higher plant Photosystem I complexes with chaotropic agents followed by sucrose density ultracentrifugation. The concentrations of NaClO4, NaSCN, NaI, NaBr or urea required for the functional removal of the 8.9 kDa, FA/FB polypeptide was shown to be inversely related to the strength of the chaotrope. The Photosystem I core protein, which was purified to homogeniety, contains 4 mol of acid-labile sulfide and has the following properties: (i) the FX-containing core consists of the 82 and 83 kDa reaction center polypeptides but is totally devoid of the low-molecular-mass polypeptides; (ii) methyl viologen and other bipyridilium dyes have the ability to accept electrons directly from FX; (iii) the difference spectrum of FX from 400 to 900 nm is characteristic of an iron-sulfur cluster; (iv) the midpoint potential of FX, determined optically at room temperature, is 60 mV more positive than in the control; (v) there is indication by ESR spectroscopy of low-temperature heterogeneity within FX; and (vi) the heterogeneity is seen by optical spectroscopy as inefficiency in low-temperature electron flow to FX. The constraints imposed by the amount of non-heme iron and labile sulfide in the Photosystem I core protein, the cysteine content of the psaA and psaB polypeptides, and the stoichiometry of high-molecular-mass polypeptides, cause us to re-examine the possibility that FX is a [4Fe-4S] rather than a [2Fe-2S] cluster ligated by homologous cysteine residues on the psaA and psaB heterodimer.

Light-induced charge separation in Photosystem I at low temperature is not influenced by vitamin K-1

The photoreduction of iron-sulfur centers was studied at low temperature in Photosystem I particles from spinach and the cyanobacterium Synechocystis 6803, which contain various amounts of vitamin K-1 (recently tentatively identified as the acceptor A1). The irreversible charge separation that was progressively induced at low temperature between P-700 and FA (or FB) by successive laser flashes was studied at 15 K. Its maximum amount after a large number of flashes was shown to be fairly independent of the number (0, 1 or 2) of vitamins K-1 per reaction center. Moreover, the first flash yield of this charge separation was diminished by only about 50% when vitamin K-1 was completely absent from the particles by comparison with particles containing one or two vitamin K-1 per reaction center. When FA and FB were prereduced, the iron-sulfur center FX was also reversibly photoreduced at 9 K in the absence of vitamin K-1. The implications of these results for the electron pathways of Photosystem I are discussed and it is proposed that a direct electron transfer from A0- to the iron-sulfur centers is highly efficient at low temperature.