High-resolution absorbance-difference spectra of the triplet state of the primary donor P-700 in Photosystem I subchloroplast particles measured with absorbance-detected magnetic resonance at 1.2 K. Evidence that P-700 is a dimeric chlorophyll complex

Electronic Structure of the Primary Electron Donor P700+• in Photosystem I Studied by Multifrequency HYSCORE Spectroscopy at X- and Q-Band

The primary electron donor P₇₀₀ of the photosystem I (PSI) is a heterodimer consisting of two chlorophyll molecules. A series of electron-transfer events immediately following the initial light excitation leads to a stabilization of the positive charge by its cation radical form, P₇₀₀^+•. The electronic structure of P₇₀₀^+• and, in particular, its asymmetry with respect to the two chlorophyll monomers is of fundamental interest and is not fully understood up to this date. Here, we apply multifrequency X- (9 GHz) and Q-band (35 GHz) hyperfine sublevel correlation (HYSCORE) spectroscopy to investigate the electron spin density distribution in the cation radical P₇₀₀^+• of PSI from a thermophilic cyanobacterium Thermosynechococcus elongatus. Six ¹⁴N and two ¹H distinct nuclei have been resolved in the HYSCORE spectra and parameters of the corresponding nuclear hyperfine and quadrupolar hyperfine interactions were obtained by combining the analysis of HYSCORE spectral features with direct numerical simulations. Based on a close similarity of the nuclear quadrupole tensor parameters, all of the resolved ¹⁴N nuclei were assigned to six out of total eight available pyrrole ring nitrogen atoms (i.e., four in each of the chlorophylls), providing direct evidence of spin density delocalization over the both monomers in the heterodimer. Using the obtained experimental values of the ¹⁴N electron-nuclear hyperfine interaction parameters, the upper limit of the electron spin density asymmetry parameter is estimated as R_A/B^upper = 7.7 ± 0.5, while a tentative assignment of ¹⁴N observed in the HYSCORE spectra yields R_B/A = 3.1 ± 0.5.

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: the primary electron donor of photosystem I

The primary electron donor of photosystem I, P700, is a chlorophyll species that in its excited state has a potential of approximately -1.2 V. The precise chemical composition and electronic structure of P700 is still unknown. Recent evidence indicates that P700 is a dimer of one chlorophyll (Chl) a and one Chl a'. The Chl a' and Chl a are axially coordinated by His residues provided by protein subunits PsaA and PsaB, respectively. The Chl a', but not the Chl a, is also H-bonded to the protein. The H-bonding is likely responsible for selective insertion of Chl a' into the reaction center. EPR studies of P700(+*) in frozen solution and single crystals indicate a large asymmetry in the electron spin and charge distribution towards one Chl of the dimer. Molecular orbital calculations indicate that H-bonding will specifically stabilize the Chl a'-side of the dimer, suggesting that the unpaired electron would predominantly reside on the Chl a. This is supported by results of specific mutagenesis of the PsaA and PsaB axial His residues, which show that only mutations of the PsaB subunit significantly alter the hyperfine coupling constants associated with a single Chl molecule. The PsaB mutants also alter the microwave induced triplet-minus-singlet spectrum indicating that the triplet state is localized on the same Chl. Excitonic coupling between the two Chl a of P700 is weak due to the distance and overlap of the porphyrin planes. Evidence of excitonic coupling is found in PsaB mutants which show a new bleaching band at 665 nm that likely represents an increased intensity of the upper exciton band of P700. Additional properties of P700 that may give rise to its unusually low potential are discussed.

Fourier transform infrared spectroscopy of primary electron donors in type I photosynthetic reaction centers

The vibrational properties of the primary electron donors (P) of type I photosynthetic reaction centers, as investigated by Fourier transform infrared (FTIR) difference spectroscopy in the last 15 years, are briefly reviewed. The results obtained on the microenvironment of the chlorophyll molecules in P700 of photosystem I and of the bacteriochlorophyll molecules in P840 of the green bacteria (Chlorobium) and in P798 of heliobacteria are presented and discussed with special attention to the bonding interactions with the protein of the carbonyl groups and of the central Mg atom of the pigments. The observation of broad electronic transitions in the mid-IR for the cationic state of all the primary donors investigated provides evidence for charge repartition over two (B)Chl molecules. In the green sulfur bacteria and the heliobacteria, the assignments proposed for the carbonyl groups of P and P(+) are still very tentative. In contrast, the axial ligands of P700 in photosystem I have been identified and the vibrational properties of the chlorophyll (Chl) molecules involved in P700, P700(+), and (3)P700 are well described in terms of two molecules, denoted P(1) and P(2), with very different hydrogen bonding patterns. While P(1) has hydrogen bonds to both the 9-keto and the 10a-ester C=O groups and bears all the triplet character in (3)P700, the carbonyl groups of P(2) are free from hydrogen bonding. The positive charge in P700(+) is shared between the two Chl molecules with a ratio ranging from 1:1 to 2:1, in favor of P(2), depending on the temperature and the species. The localization of the triplet in (3)P700 and of the unpaired electron in P700(+) deduced from FTIR spectroscopy is in sharp contrast with that resulting from the analysis of the magnetic resonance experiments. However, the FTIR results are in excellent agreement with the most recent structural model derived from X-ray crystallography of photosystem I at 2.5 A resolution that reveals the hydrogen bonds to the carbonyl groups of the Chl in P700 as well as the histidine ligands of the central Mg atoms predicted from the FTIR data.

Secondary pair charge recombination in photosystem I under strongly reducing conditions: temperature dependence and suggested mechanism

Photoinduced electron transfer in photosystem I (PS I) proceeds from the excited primary electron donor P700 (a chlorophyll a dimer) via the primary acceptor A0 (chlorophyll a) and the secondary acceptor A1 (phylloquinone) to three [4Fe-4S] clusters, Fx, FA, and FB. Prereduction of the iron-sulfur clusters blocks electron transfer beyond A1. It has been shown previously that, under such conditions, the secondary pair P700+A1- decays by charge recombination with t1/2 approximately 250 ns at room temperature, forming the P700 triplet state (3P700) with a yield exceeding 85%. This reaction is unusual, as the secondary pair in other photosynthetic reaction centers recombines much slower and forms directly the singlet ground state rather than the triplet state of the primary donor. Here we studied the temperature dependence of secondary pair recombination in PS I from the cyanobacterium Synechococcus sp. PCC6803, which had been illuminated in the presence of dithionite at pH 10 to reduce all three iron-sulfur clusters. The reaction P700+A1- --> 3P700 was monitored by flash absorption spectroscopy. With decreasing temperature, the recombination slowed down and the yield of 3P700 decreased. In the range between 303 K and 240 K, the recombination rates could be described by the Arrhenius law with an activation energy of approximately 170 meV. Below 240 K, the temperature dependence became much weaker, and recombination to the singlet ground state became the dominating process. To explain the fast activated recombination to the P700 triplet state, we suggest a mechanism involving efficient singlet to triplet spin evolution in the secondary pair, thermally activated repopulation of the more closely spaced primary pair P700+A0- in a triplet spin configuration, and subsequent fast recombination (intrinsic rate on the order of 10(9) s(-1)) forming 3P700.

Spectral and kinetic characterization of electron acceptor A1 in a Photosystem I core devoid of iron-sulfur centers F X, F B and F A

The kinetic and spectroscopic properties of the secondary electron acceptor A1 were determined by flash absorption spectroscopy at room and cryogenic temperatures in a Photosystem I (PS I) core devoid of the iron-sulfur clusters FX, FB and FA. It was shown earlier (Warren, P.V., Golbeck, J.H. and Warden, J.T. (1993) Biochemistry 32: 849-857) that the majority of the flash-induced absorbance increase at 820 nm, reflecting formation of P700(+), decays with a t1/2 of 10 μs due to charge recombination between P700(+) and A1 (-). Following A1 (-) directly around 380 nm, where absorbance changes due to the formation of P700(+) are negligible, two major decay components were resolved in this study with t1/2 of ≈ 10 μs and 110 μs at an amplitude ratio of ≈ 2.5:1. The difference spectra between 340 and 490 nm of the two kinetic phases are highly similar, showing absorbance increases from 340 to 400 nm characteristic of the one-electron reduction of the phylloquinone A1. When measured at 10 K, the flash-induced absorbance changes around 380 nm can be fitted with two decay phases of t1/2 ≈ 15 μs and 150 μs at an amplitude ratio ≈ 1:1. The difference spectra of both kinetic phases from 340 to 400 nm are similar to those determined at 298 K and are therefore attributed to charge recombination in the pair P700(+)A1 (-). These results indicate that the backreaction between P700(+) and A1 (-) is multiphasic when FX, FB and FA are removed, and only slightly temperature dependent in the range of 298 K to 10 K.

A well resolved ODMR triplet minus singlet spectrum of P680 from PSII particles

An ADMR T-S spectrum of the primary donor (P680) of photosystem II (PSII) was obtained from anaerobically photoreduced particles. The spectrum is the best resolved obtained so far having a main bleaching band at 684 nm with a linewidth of only 100 cm-1. The view that this spectrum is produced by native homogeneous P680 unlike those obtained before is defended. A small bleaching observed at 678 nm is discussed in terms of the reaction center structure. One possible interpretation of the observations is that P680 is a very loose dimer with an exciton splitting of only 144 cm-1 corresponding to a dimer center-to-center distance of roughly 11.5 A.

Transient electron paramagnetic resonance of the triplet state of P700 in photosystem I: evidence for triplet delocalization at room temperature

Spin-polarized EPR spectra of the triplet state of P700, the primary electron donor in photosystem I (PS I), have been measured for the first time at room temperature. The measurements were performed on intact PS I from Synechococcus sp. after prereduction of all iron-sulfur centers and on vitamin K1 depleted PS I from Synechocystis 6803. The two preparations give similar spectra with a polarization pattern which indicates that the triplet state is formed via recombination of a radical pair. The axial and nonaxial zero-field splitting (zfs) parameters are found to be magnitude of D = (284 +/- 15) x 10(-4) cm-1 and magnitude of E = (22 +/- 3) x 10(-4) cm-1, respectively. The E-value is 42% smaller than in monomeric chlorophyll a, while the D-value is nearly the same. Measurements of the Synechocystis 6803 sample at 4.5 K yielded zfs parameters which are identical with those of the chlorophyll monomer, in agreement with previous results. In order to explain this behavior, it is proposed that the triplet excitation is delocalized over the two halves of a chlorophyll dimer at room temperature but appears localized on one half at low temperature. The observed zfs parameters are obtained if (1) the magnetic z-axes of the two chlorophylls are collinear, (2) the magnetic y-axes (and x-axes) of the two chlorophylls make an angle of approximately 55 degrees with each other, and (3) the admixture of charge-transfer states to 3P700 is negligible.(ABSTRACT TRUNCATED AT 250 WORDS)

Charge recombination between P700+ and A1- occurs directly to the ground state of P700 in a photosystem I core devoid of FX, FB, and FA

The charge recombination between P700+ and electron acceptor A1- was studied by flash kinetic spectroscopy in a photosystem I core devoid of iron-sulfur centers FX, FB, and FA. We showed previously that the majority of the flash-induced absorption change at 820 nm decayed with a 10-microseconds half-time, which we assigned to the disappearance of the P700 triplet formed from the backreaction of P700+ with A1- [Warren, P.V., Parrett, K.G., Warden, J.T., & Golbeck, J.H. (1990) Biochemistry 29, 6545-6550]. We have reinvestigated this assignment in the near-UV, blue, and near-IR wavelength regions. The difference spectrum from 380 to 480 nm and from 720 to 910 nm shows that the P700+ A1- charge recombination is dominated by the P700 cation rather than the P700 triplet. Accordingly, the 10-microseconds kinetic transient represents the direct backreaction of P700+ with A1-, which repopulates the ground state of P700. This is unlike a P700-FA/FB complex where, in the presence of reduced FX-, FB-, and FA-, the P700+ A1- charge recombination populates the P700 triplet state [Sétif, P., & Bottin, H. (1989) Biochemistry 28, 2689-2697]. The A1 acceptor is highly susceptible to disruption by detergents in the absence of iron-sulfur center FX. The addition of 0.1% Triton X-100 to the P700-A1 core leads to a approximately 2.5-fold increase in the magnitude of the flash-induced absorption change at 780 nm; thereafter, 85% of the absorption change decays with a 25-ns half-time and 15% decays with a 3-microseconds half-time.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluorescence detected magnetic resonance of the primary donor and inner core antenna chlorophyll in Photosystem I reaction centre protein: Sign inversion and energy transfer

The Photosystem I reaction centre protein CP1, isolated from barley using polyacrylamide gel electrophoresis showed an EPR (Electron Paramgnetic Resonance) spectrum with the polarisation pattern AEEAAE, typical of the primary donor triplet state (3)P700, created via radical pair formation and recombination. (3)P700 could also be detected by Fluorescence Detected Magnetic Resonance (FDMR) at λf > 700 nm even in the presence of a large number of chlorophyll antennae. Its zero field splitting parameters, D=282.5×10(-4) cm(-1) and E=38.5×10(-4) cm(-1), were independent of the detection wavelength, and agreed with ADMR (Absorption Detected Magnetic Resonance) and EPR values. The signs of the (3)P700 D+E and D-E transitions were positive (increase in fluorescence intensity on applying a resonance microwave field). In contrast, in the emission band 685 < λf < 700 nm FDMR spectra with negative D+E and D-E transitions were detected, and the D value was wavelength-dependent. These FDMR results support an excitation energy transfer model for CP1, derived from time-resolved fluorescence studies, in which two chlorophyll antenna forms are distinguished, with fluorescence at 685 < λf < 700 nm (inner core antennae, F690), and λf > 700 nm (low energy antenna sites, F720), in addition to the P700. The FDMR spectrum in F690 emission can be interpreted as that of (3)P700, observed via reverse singlet excitation energy transfer and added to the FDMR spectrum of the antenna triplet states generated via intramolecular intersystem crossing. This would indicate that reversible energy transfer between F690 and P700 occurs even at 4.2 K.

Primary photochemistry in photosystem-I

In this review, the main research developments that have led to the current simplified picture of photosystem I are presented. This is followed by a discussion of some conflicting reports and unresolved questions in the literature. The following points are made: (1) the evidence is contradictory on whether P700, the primary donor, is a monomer or dimer of chlorophyll although at this time the balacnce of the evidence points towards a monomeric structure for P700 when in the triplet state; (2) there is little evidence that the iron sulfur centers FA and FB act in series as tertiary acceptors and it is as likely that they act in parallel under physiological conditions; (3) a role for FX, probably another iron sulfur centrer, as an obligatory electron carrier in forward electron transfer has not been proven. Some evidence indicates that its reduction could represent a pathway different to that involving FA and FB; (4) the decay of the acceptor 'A2 (-)' as defined by optical spectroscopy corresponds with 700(+) % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOramaaBa% aaleaadaqdaaqaaiaadIfaaaaabeaaaaa!37D1!\[F_{\overline X } \] recombination under some circumstances but under other conditions it probably corresponds with P700(+) A1 (-) recombination; (5) P700(+) A1 (-) recombination as originally observed by optical spectroscopy is probably due to the decay of the P700 triplet state; (6) the acceptor A1 (-) as defined by EPR may be a special semiquinone molecule; (7) A0 is probably a chlorophyll a molecule which acts as the primary acceptor. Recombination of P700(+) A0 (-) gives rise to the P700 triplet state.A working model for electron transfer in photosystem I is presented, its general features are discussed and comparisons with other photosystems are made.

Electron spin polarization of photosynthetic reactants

Photosynthesis is the conversion of the quantum energy of light into the chemical energy of complex organic molecules and organized cellular structures in plants and in some bacteria. The processes of photosynthesis span the time domain of subpicoseconds to the millennia of slow-growing trees, its study brings together such diverse disciplines as photophysics, biochemistry, botany and ecology. In the last few decades tremendous progress has been made in understanding the multivarious chemical reactions that ultimately lead to the fixation of carbon dioxide into organic substance, yet the basic mechanism underlying the conversion of photon energy into chemical energy still remains very much an enigma. These so-called primary reactions which transduce the excitation energy of excited chlorophyll pigments into the potential energy of stabilized, separated charges on electron donor and electron acceptor molecules have been studied with a variety of physical techniques, among which fast optical spectroscopy and electron paramagnetic resonance (EPR) are prominent. This review will highlight one intriguing aspect of EPR, namely that of electron spin polarization (ESP).† It will be shown that ESP of photosynthetic primary reactants offers a unique tool to gain insight in the electrostatic and magnetic interactions that make photosynthesis work. Moreover, it will become apparent that ESP in photosynthesis has several unique traits not (yet) found in ESP of photochemical reactionsin vitro. As such, it may serve as a paradigma of ESP phenomena and will present an absorbing spectacle also for EPR spectroscopists outside photosynthesis.