Femtosecond optical studies of the primary charge separation reactions in far-red photosystem II from Synechococcus sp. PCC 7335

Primary processes of light energy conversion by Photosystem II (PSII) were studied using femtosecond broadband pump-probe absorption difference spectroscopy. Transient absorption changes of core complexes isolated from the cyanobacterium Synechococcus sp. PCC 7335 grown under far-red light (FRL-PSII) were compared with the canonical Chl a containing spinach PSII core complexes upon excitation into the red edge of the Qy band. Absorption changes of FRL-PSII were monitored at 278 K in the 400-800 nm spectral range on a timescale of 0.1-500 ps upon selective excitation at 740 nm of four chlorophyll (Chl) f molecules in the light harvesting antenna, or of one Chl d molecule at the ChlD1 position in the reaction center (RC) upon pumping at 710 nm. Numerical analysis of absorption changes and assessment of the energy levels of the presumed ion-radical states made it possible to identify PD1+ChlD1- as the predominant primary charge-separated radical pair, the formation of which upon selective excitation of Chl d has an apparent time of ∼1.6 ps. Electron transfer to the secondary acceptor pheophytin PheoD1 has an apparent time of ∼7 ps with a variety of excitation wavelengths. The energy redistribution between Chl a and Chl f in the antenna occurs within 1 ps, whereas the energy migration from Chl f to the RC occurs mostly with lifetimes of 60 and 400 ps. Potentiometric analysis suggests that in canonical PSII, PD1+ChlD1- can be partially formed from the excited (PD1ChlD1)* state.

Electrometric and Electron Paramagnetic Resonance Measurements of a Difference in the Transmembrane Electrochemical Potential: Photosynthetic Subcellular Structures and Isolated Pigment-Protein Complexes

A transmembrane difference in the electrochemical potentials of protons (ΔμH+) serves as a free energy intermediate in energy-transducing organelles of the living cell. The contributions of two components of the ΔμH+ (electrical, Δψ, and concentrational, ΔpH) to the overall ΔμH+ value depend on the nature and lipid composition of the energy-coupling membrane. In this review, we briefly consider several of the most common instrumental (electrometric and EPR) methods for numerical estimations of Δψ and ΔpH. In particular, the kinetics of the flash-induced electrometrical measurements of Δψ in bacterial chromatophores, isolated bacterial reaction centers, and Photosystems I and II of the oxygenic photosynthesis, as well as the use of pH-sensitive molecular indicators and kinetic data regarding pH-dependent electron transport in chloroplasts, have been reviewed. Further perspectives on the application of these methods to solve some fundamental and practical problems of membrane bioenergetics are discussed.

Role of pheophytin a in the primary charge separation of photosystem I from Acaryochloris marina: Femtosecond optical studies of excitation energy and electron transfer reactions

Photosystem I (PSI) of the cyanobacterium Acaryochloris marina is capable of performing an efficient photoelectrochemical conversion of far-red light due to its unique suite of cofactors. Chlorophyll d (Chl-d) has been long known as the major antenna pigment in the PSI from A. marina, while the exact cofactor composition of the reaction centre (RC) was established only recently by cryo-electron microscopy. The RC consists of four Chl-d molecules, and, surprisingly, two molecules of pheophytin a (Pheo-a), which provide a unique opportunity to resolve, spectrally and kinetically, the primary electron transfer reactions. Femtosecond transient absorption spectroscopy was here employed to observe absorption changes in the 400-860 nm spectral window occurring in the 0.1-500 ps timescale upon unselective antenna excitation and selective excitation of the Chl-d special pair P₇₄₀ in the RC. A numerical decomposition of the absorption changes, including principal component analysis, allowed the identification of P₇₄₀⁽⁺⁾Chl_d2^(-) as the primary charge separated state and P₇₄₀⁽⁺⁾Pheo_a3^(-) as the successive, secondary, radical pair. A remarkable feature of the electron transfer reaction between Chl_d2 and Pheo_a3 is the fast, kinetically unresolved, equilibrium with an estimated ratio of 1:3. The energy level of the stabilised ion-radical state P₇₄₀⁽⁺⁾Pheo_a3^(-) was determined to be ~60 meV below that of the RC excited state. In this regard, the energetics and the structural implications of the presence of Pheo-a in the electron transfer chain of PSI from A. marina are discussed, also in comparison with those of the most diffused Chl-a binding RC.

Conversion of light into electricity in a semi-synthetic system based on photosynthetic bacterial chromatophores

Chromatophores (Chr) from photosynthetic nonsulfur purple bacterium Rhodobacter sphaeroides immobilized onto a Millipore membrane filter (MF) and sandwiched between two semiconductor indium tin oxide (ITO) electrodes (termed ITO|Chr - MF|ITO) have been used to measure voltage (ΔV) induced by continuous illumination. The maximum ΔV was detected in the presence of ascorbate / N,N,N'N'-tetramethyl-p-phenylenediamine couple, coenzyme UQ₀, disaccaride trehalose and antimycin A, an inhibitor of cytochrome bc₁ complex. In doing so, the light-induced electron transfer in the reaction centers was the major source of photovoltages. The stability of the voltage signal upon prolonged irradiation (>1 h) may be due to the maintenance of a conformation that is optimal for the functioning of integral protein complexes and stabilization of lipid bilayer membranes in the presence of trehalose. Retaining ∼70 % of the original photovoltage performance on the 30th day of storage at 23 °C in the dark under air was achieved after re-injection of fresh buffer (∼40 μL) containing redox mediators into the ITO|Chr - MF|ITO system. The approach we use is easy and can be extended to other biological intact systems (cells, thylakoid membranes) capable of converting energy of light.

In memory of Vladimir Anatolievich Shuvalov (1943-2022): an outstanding biophysicist

We present here a tribute to one of the foremost biophysicists of our time, Vladimir Anatolievich Shuvalov, who made important contributions in bioenergetics, especially on the primary steps of conversion of light energy into charge-separated states in both anoxygenic and oxygenic photosynthesis. For this, he and his research team exploited pico- and femtosecond transient absorption spectroscopy, photodichroism & circular dichroism spectroscopy, light-induced FTIR (Fourier-transform infrared) spectroscopy, and hole-burning spectroscopy. We remember him for his outstanding leadership and for being a wonderful mentor to many scientists in this area. Reminiscences by many [Suleyman Allakhverdiev (Russia); Robert Blankenship (USA); Richard Cogdell (UK); Arvi Freiberg (Estonia); Govindjee Govindjee (USA); Alexander Krasnovsky, jr, (Russia); William Parson (USA); Andrei Razjivin (Russia); Jian- Ren Shen (Japan); Sergei Shuvalov (Russia); Lyudmilla Vasilieva (Russia); and Andrei Yakovlev (Russia)] have included not only his wonderful personal character, but his outstanding scientific research.

Changes in the Electron Transfer Symmetry in the Photosystem I Reaction Centers upon Removal of Iron-Sulfur Clusters

In photosynthetic reaction centers of intact photosystem I (PSI) complexes from cyanobacteria, electron transfer at room temperature occurs along two symmetrical branches of redox cofactors A and B at a ratio of ~3 : 1 in favor of branch A. Previously, this has been indirectly demonstrated using pulsed absorption spectroscopy and more directly by measuring the decay modulation frequencies of electron spin echo signals (electron spin echo envelope modulation, ESEEM), which allows to determine the distance between the separated charges of the primary electron donor P₇₀₀⁺ and phylloquinone acceptors A_1A^- and A_1B^- in the symmetric redox cofactors branches A and B. In the present work, these distances were determined using ESEEM in PSI complexes lacking three 4Fe-4S clusters, F_X, F_A, and F_B, and the PsaC protein subunit (the so-called P₇₀₀-A₁ core), in which phylloquinone molecules A_1A and A_1B serve as the terminal electron acceptors. It was shown that in the P₇₀₀-A₁ core preparations, the average distance between the centers of the P₇₀₀⁺A₁^- ion-radical pair at a temperature of 150 K in an aqueous glycerol solution and in a dried trehalose matrix, as well as in a trehalose matrix at 280 K, is ~25.5 Å, which corresponds to the symmetrical electron transfer along the A and B branches of redox cofactors at a ratio of 1 : 1. Possible reasons for the change in the electron transfer symmetry in PSI upon removal of the PsaC subunit and 4Fe-4S clusters F_X, F_A, and F_B are discussed.

Current state of the primary charge separation mechanism in photosystem I of cyanobacteria

This review analyzes new data on the mechanism of ultrafast reactions of primary charge separation in photosystem I (PS I) of cyanobacteria obtained in the last decade by methods of femtosecond absorption spectroscopy. Cyanobacterial PS I from many species harbours 96 chlorophyll a (Chl a) molecules, including six specialized Chls denoted Chl1A/Chl1B (dimer P700, or PAPB), Chl2A/Chl2B, and Chl3A/Chl3B arranged in two branches, which participate in electron transfer reactions. The current data indicate that the primary charge separation occurs in a symmetric exciplex, where the special pair P700 is electronically coupled to the symmetrically located monomers Chl2A and Chl2B, which can be considered together as a symmetric exciplex Chl2APAPBChl2B with the mixed excited (Chl2APAPBChl2B)* and two charge-transfer states P700 +Chl2A - and P700 +Chl2B -. The redistribution of electrons between the branches in favor of the A-branch occurs after reduction of the Chl2A and Chl2B monomers. The formation of charge-transfer states and the symmetry breaking mechanisms were clarified by measuring the electrochromic Stark shift of β-carotene and the absorption dynamics of PS I complexes with the genetically altered Chl 2B or Chl 2A monomers. The review gives a brief description of the main methods for analyzing data obtained using femtosecond absorption spectroscopy. The energy levels of excited and charge-transfer intermediates arising in the cyanobacterial PS I are critically analyzed.

Generation of Photoelectric Responses by Photosystem II Core Complexes in the Presence of Externally Added Cytochrome c

The effect of exogenous cytochrome c (cyt c) on kinetics of photoelectric responses (Δψ) of two types of photosystem II (PSII) core complexes (intact - PSII with active water-oxidizing complex and Mn-depleted complex) reconstituted into liposomes has been investigated by direct electrometric technique. PSII complexes were localized in the proteoliposome membranes with their donor side outward. An additional electrogenic phase was observed in the kinetics of Δψ generation in response to a laser flash besides the main fast (<0.3 µs) electrogenic component due to electron transfer from the redox-active tyrosine Y_Z to the primary quinone acceptor Q_A in the presence of oxidized cyt c (cyt c³⁺) entrapped in the internal space of proteoliposomes with intact PSII complexes. This component with characteristic time τ ≈ 40 µs and relative amplitude of ~10% of the total Δψ was attributed to the vectorial electron transfer from Q_A^- to cyt c³⁺ serving as an external acceptor. An additional electrogenic component with τ ~ 70 µs and a relative amplitude of ~20% of the total Δψ also appeared in the kinetics of Δψ formation, when cyt c²⁺ was added to the suspension of proteoliposomes containing Mn-depleted PSII core complexes. This component was attributed to the electrogenic transfer of an electron from cyt c²⁺ to photooxidized tyrosine Y_Z. These data imply that cyt c³⁺ serves as a very effective exogenous electron acceptor for Q_A^- in the case of intact PSII core complexes, and cyt c²⁺ is an extremely efficient artificial electron donor for Y_Z in the Mn-depleted PSII. The obtained data on the roles of cyt c²⁺ and cyt c³⁺ as an electron donor and acceptor for PSII, respectively, can be used to develop hybrid photoelectrochemical solar energy-converting systems based on photosynthetic pigment-protein complexes.

Symmetry breaking in photosystem I: ultrafast optical studies of variants near the accessory chlorophylls in the A- and B-branches of electron transfer cofactors

Femtosecond absorption spectroscopy of Photosystem I (PS I) complexes from the cyanobacterium Synechocystis sp. PCC 6803 was carried out on three pairs of complementary amino acid substitutions located near the second pair of chlorophyll molecules Chl_2A and Chl_2B (also termed A_-1A and A_-1B). The absorption dynamics at delays of 0.1-500 ps were analyzed by decomposition into discrete decay-associated spectra and continuously distributed exponential components. The multi-exponential deconvolution of the absorption changes revealed that the electron transfer reactions in the PsaA-N600M, PsaA-N600H, and PsaA-N600L variants near the B-branch of cofactors are similar to those of the wild type, while the PsaB-N582M, PsaB-N582H, and PsaB-N582L variants near the A-branch of cofactors cause significant alterations of the photochemical processes, making them heterogeneous and poorly described by a discrete exponential kinetic model. A redistribution of the unpaired electron between the second and the third monomers Chl_2A/Chl_2B and Chl_3A/Chl_3B was identified in the time range of 9-20 ps, and the subsequent reduction of A₁ was identified in the time range of 24-70 ps. In the PsaA-N600L and PsaB-N582H/L variants, the reduction of A₁ occurred with a decreased quantum yield of charge separation. The decreased quantum yield correlates with a slowing of the phylloquinone A₀ → A₁ reduction, but not with the initial transient spectra measured at the shortest time delay. The results support a branch competition model, where the electron is sheared between Chl_2A-Chl_3A and Chl_2B-Chl_3B cofactors before its transfer to phylloquinone in either A_1A or A_1B sites.

Conserved residue PsaB-Trp673 is essential for high-efficiency electron transfer between the phylloquinones and the iron-sulfur clusters in Photosystem I

Despite the high level of symmetry between the PsaA and PsaB polypeptides in Photosystem I, some amino acids pairs are strikingly different, such as PsaA-Gly693 and PsaB-Trp673, which are located near a cluster of 11 water molecules between the A_1A and A_1B quinones and the F_X iron-sulfur cluster. In this work, we changed PsaB-Trp673 to PsaB-Phe673 in Synechocystis sp. PCC 6803. The variant contains ~ 85% of wild-type (WT) levels of Photosystem I but is unable to grow photoautotrophically. Both time-resolved and steady-state optical measurements show that in the PsaB-W673F variant less than 50% of the electrons reach the terminal iron-sulfur clusters F_A and F_B; the majority of the electrons recombine from A_1A^- and A_1B^-. However, in those reaction centers which pass electrons forward the transfer is heterogeneous: a minor population shows electron transfer rates from A_1A^- and A_1B^- to F_X slightly slower than that of the WT, whereas a major population shows forward electron transfer rates to F_X slowed to the ~ 10 µs time range. Competition between relatively similar forward and backward rates of electron transfer from the quinones to the F_X cluster account for the relatively low yield of long-lived charge separation in the PsaB-W673F variant. A higher water content and its increased mobility observed in MD simulations in the interquinone cavity of the PsaB-W673F variant shifts the pK of PsaB-Asp575 and allows its deprotonation in situ. The heterogeneity found may be rooted in protonation state of PsaB-Asp575, which controls whether electron transfer can proceed beyond the phylloquinone cofactors.

The Mechanisms of Electrogenic Reactions in Bacterial Photosynthetic Reaction Centers: Studies in Collaboration with Alexander Konstantinov

In this review, we discuss our studies conducted in 1985-1988 in collaboration with A. A. Konstantinov, one of the top scientists in the field of membrane bioenergetics. Studying fast kinetics of membrane potential generation in photosynthetic reaction centers (RCs) of purple bacteria in response to a laser flash has made it possible to examine in detail the mechanisms of electrogenic reactions at the donor and acceptor sides of RCs. Electrogenesis associated with the intraprotein electron transfer from the exogenous secondary donors, redox dyes, and soluble cytochrome (cyt) c to the photooxidized dimer of bacteriochlorophyll P₈₇₀ was studied using proteoliposomes containing RCs from the non-sulfur purple bacterium Rhodospirillum rubrum. It was found that reduction of the secondary quinone electron acceptor Q_B accompanied by its protonation in the chromatophores from R. rubrum in response to every second light flash was electrogenic. Spectral characteristics and redox potentials of the four hemes in the tightly bound cyt c in the RC of Blastochloris viridis and electrogenic reactions associated with the electron transfer within the RC complex were identified. For the first time, relative amplitudes of the membrane potential generated in the course of individual electrogenic reactions were compared with the distances between the redox cofactors determined based on the three-dimensional structure of the Bl. viridis RC.

Trehalose matrix effects on electron transfer in Mn-depleted protein-pigment complexes of Photosystem II

The kinetics of flash-induced re-reduction of the Photosystem II (PS II) primary electron donor P₆₈₀ was studied in solution and in trehalose glassy matrices at different relative humidity. In solution, and in the re-dissolved glass, kinetics were dominated by two fast components with lifetimes in the range of 2-7 μs, which accounted for >85% of the decay. These components were ascribed to the direct electron transfer from the redox-active tyrosine Y_Z to P₆₈₀⁺. The minor slower components were due to charge recombination between the primary plastoquinone acceptor Q_A^- and P₆₈₀⁺. Incorporation of the PS II complex into the trehalose glassy matrix and its successive dehydration caused a progressive increase in the lifetime of all kinetic phases, accompanied by an increase of the amplitudes of the slower phases at the expense of the faster phases. At 63% relative humidity the fast components contribution dropped to ~50%. A further dehydration of the trehalose glass did not change the lifetimes and contribution of the kinetic components. This effect was ascribed to the decrease of conformational mobility of the protein domain between Y_Z and P₆₈₀, which resulted in the inhibition of Y_Z → P₆₈₀⁺ electron transfer in about half of the PS II population, wherein the recombination between Q_A^- and P₆₈₀⁺ occurred. The data indicate that PS II binds a larger number of water molecules as compared to PS I complexes. We conclude that our data disprove the "water replacement" hypothesis of trehalose matrix biopreservation.

Primary charge separation within the structurally symmetric tetrameric Chl2APAPBChl2B chlorophyll exciplex in photosystem I

In Photosystem I (PS I), the role of the accessory chlorophyll (Chl) molecules, Chl_2A and Chl_2B (also termed A_-1A and A_-1B), which are directly adjacent to the special pair P₇₀₀ and fork into the A- and B-branches of electron carriers, is incompletely understood. In this work, the Chl_2A and Chl_2B transient absorption ΔA₀(λ) at a time delay of 100 fs was identified by ultrafast pump-probe spectroscopy in three pairs of PS I complexes from Synechocystis sp. PCC 6803 with residues PsaA-N600 or PsaB-N582 (which ligate Chl_2B or Chl_2A through a H₂O molecule) substituted by Met, His, and Leu. The ΔA₀(λ) spectra were quantified using principal component analysis, the main component of which was interpreted as a mutation-induced shift of the equilibrium between the excited state of primary donor P₇₀₀^⁎ and the primary charge-separated state P₇₀₀⁺Chl₂^-. This equilibrium is shifted to the charge-separated state in wild-type PS I and to the excited P₇₀₀ in the PS I complexes with the substituted ligands to the Chl_2A and Chl_2B monomers. The results can be rationalized within the framework of an adiabatic model in which the P₇₀₀ is electronically coupled with the symmetrically arranged monomers Chl_2A and Chl_2B; such a structure can be considered a symmetric tetrameric exciplex Chl_2AP_AP_BChl_2B, in which the excited state (Chl_2AP_AP_BChl_2B)* is mixed with two charge-transfer states P₇₀₀⁺Chl_2A^- and P₇₀₀⁺Chl_2B^-. The electron redistribution between the two branches in favor of the A-branch apparently takes place in the picosecond time scale after reduction of the Chl_2A and Chl_2B monomers.

Generation of ion-radical chlorophyll states in the light-harvesting antenna and the reaction center of cyanobacterial photosystem I

The energy and charge-transfer processes in photosystem I (PS I) complexes isolated from cyanobacteria Thermosynechococcus elongatus and Synechocystis sp. PCC 6803 were investigated by pump-to-probe femtosecond spectroscopy. The formation of charge-transfer (CT) states in excitonically coupled chlorophyll a complexes (exciplexes) was monitored by measuring the electrochromic shift of β-carotene in the spectral range 500-510 nm. The excitation of high-energy chlorophyll in light-harvesting antenna of both species was not accompanied by immediate appearance of an electrochromic shift. In PS I from T. elongatus, the excitation of long-wavelength chlorophyll (LWC) caused a pronounced electrochromic effect at 502 nm assigned to the appearance of CT states of chlorophyll exciplexes. The formation of ion-radical pair P₇₀₀⁺A₁^- at 40 ps was limited by energy transfer from LWC to the primary donor P₇₀₀ and accompanied by carotenoid bleach at 498 nm. In PS I from Synechocystis 6803, the excitation at 720 nm produced an immediate bidentate bleach at 690/704 nm and synchronous carotenoid response at 508 nm. The bidentate bleach was assigned to the formation of primary ion-radical state P_B⁺Chl_2B^-, where negative charge is localized predominantly at the accessory chlorophyll molecule in the branch B, Chl_2B. The following decrease of carotenoid signal at ~ 5 ps was ascribed to electron transfer to the more distant molecule Chl_3B. The reduction of phylloquinone in the sites A_1A and A_1B was accompanied by a synchronous blue-shift of the carotenoid response to 498 nm, pointing to fast redistribution of unpaired electron between two branches in favor of the state P_B⁺A_1A^-.

Photovoltage generation by photosystem II core complexes immobilized onto a Millipore filter on an indium tin oxide electrode

The light-induced functioning of photosynthetic pigment-protein complex of photosystem II (PSII) is linked to the vectorial translocation of charges across the membrane, which results in the formation of voltage. Direct measurement of the light-induced voltage (∆V) generated by spinach oxygen-evolving PSII core complexes adsorbed onto a Millipore membrane filter (MF) on an indium tin oxide (ITO) electrode under continuous illumination has been performed. PSII was shown to participate in electron transfer from water to the ITO electrode, resulting in ∆V generation. No photovoltage was detected in PSII deprived of the water-oxidizing complex. The maximal and stable photoelectric signal was observed in the presence of disaccharide trehalose and 2,6-dichloro-1,4-benzoquinone, acting as a redox mediator between the primary quinone acceptor Q_A of PSII and electrode surface. Long time preservation of the steady-state photoactivity at room temperature in a simple in design ITO|PSII-MF|ITO system may be related to the retention of water molecules attached to the PSII surface in the presence of trehalose.

Control of electron transfer by protein dynamics in photosynthetic reaction centers

Trehalose and glycerol are low molecular mass sugars/polyols that have found widespread use in the protection of native protein states, in both short- and long-term storage of biological materials, and as a means of understanding protein dynamics. These myriad uses are often attributed to their ability to form an amorphous glassy matrix. In glycerol, the glass is formed only at cryogenic temperatures, while in trehalose, the glass is formed at room temperature, but only upon dehydration of the sample. While much work has been carried out to elucidate a mechanistic view of how each of these matrices interact with proteins to provide stability, rarely have the effects of these two independent systems been directly compared to each other. This review aims to compile decades of research on how different glassy matrices affect two types of photosynthetic proteins: (i) the Type II bacterial reaction center from Rhodobacter sphaeroides and (ii) the Type I Photosystem I reaction center from cyanobacteria. By comparing aggregate data on electron transfer, protein structure, and protein dynamics, it appears that the effects of these two distinct matrices are remarkably similar. Both seem to cause a "tightening" of the solvation shell when in a glassy state, resulting in severely restricted conformational mobility of the protein and associated water molecules. Thus, trehalose appears to be able to mimic, at room temperature, nearly all of the effects on protein dynamics observed in low temperature glycerol glasses.

Electrogenic reactions in Mn-depleted photosystem II core particles in the presence of synthetic binuclear Mn complexes

An electrometrical technique was used to investigate electron transfer between synthetic binuclear manganese (Mn) complexes, designated M - 2 and M - 3, and the redox-active neutral tyrosine radical (Y_Z^•) in proteoliposomes containing Mn-depleted photosystem II (PS II) core particles in response to single laser flashes. In the absence of Mn-containing compounds, the observed flash-induced membrane potential (ΔΨ) decay was mainly due to charge recombination between the reduced primary quinone acceptor Q_A^- and the oxidized Y_Z^•. More significant slowing down of the ΔΨ decay in the presence of lower concentrations of M - 2 and M - 3 associated with electron donation from Mn in the Mn-binding site to Y_Z^• indicates that these synthetic compounds are more effective electron donors than MnCl₂. The exponential fitting of the kinetics of additional electrogenic components of ΔΨ rise in the presence of Mn-containing compounds revealed the following relative amplitudes (A) and lifetimes (τ): for MnCl₂ - A∼ 3.5, τ∼150 μs, for M - 2 - A∼5%, τ∼1.4 ms, and for M - 3 - A∼5.5%, τ∼150 μs. This suggests that the efficiency of the manganese complexes in electron donation depends on the chemical nature of ligands. The experiments with EDTA-treated samples indicated that the ligands for M - 2 and M - 3 are required for their tight binding with the PS II reaction center. The obtained results demonstrate the importance of understanding the molecular mechanism(s) of flash-induced electrogenic reduction of the tyrosine radical Y_Z^• by synthetic Mn complexes capable of splitting water into oxygen and reducing equivalents.

Cyclic electron transfer around Photosystem I mediated by 2,3-dichloro-1,4-naphtoquinone and ascorbate

In this work, we investigated electron transport around the photosynthetic pigment-protein complex of Photosystem I (PS I) mediated by external high-potential electron carrier 2,3-dichloro-1,4-naphtoquinone (Cl₂ NQ) and ascorbate. It has been demonstrated that the oxidized species of Cl₂ NQ and ascorbate serve as intermediates capable of accepting electrons from the iron-sulfur cluster F_X of PS I. Reduced species of Cl₂ NQ and ascorbate are oxidized by photooxidized PS I primary donor P700+ and/or by molecular oxygen. We have found the synergistic effect of Cl₂ NQ and ascorbate on the rate of P700+ reduction. Accelerated electron flow to P700+, observed in the presence of both Cl₂ NQ and ascorbate, is explained by an increase in the reduced species of Cl₂ NQ due to electron transfer from ascorbate.

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.

Interaction of various types of photosystem I complexes with exogenous electron acceptors

Interaction of photosystem I (PS I) complexes from cyanobacteria Synechocystis sp. PCC 6803 containing various quinones in the A₁-site (phylloquinone PhQ in the wild-type strain (WT), and plastoquinone PQ or 2,3-dichloronaphthoquinone Cl ₂ NQ in the menB deletion strain) and different numbers of Fe₄S₄ clusters (intact WT and F_X-core complexes depleted of F_A/F_B centers) with external acceptors has been studied. The efficiency of interaction was estimated by measuring the light-induced absorption changes at 820 nm due to the reduction of the special pair of chlorophylls (P₇₀₀⁺) by an external acceptor(s). It was shown that externally added Cl ₂ NQ is able to effectively accept electrons from the terminal iron-sulfur clusters of PS I. Moreover, the efficiency of Cl ₂ NQ as external acceptor was higher than the efficiency of the commonly used artificial electron acceptor, methylviologen (MV) for both the intact WT PS I and for the F_X-core complexes. The comparison of the efficiency of MV interaction with different types of PS I complexes revealed gradual decrease in the following order: intact WT > menB > F_X-core. The effect of MV on the recombination kinetics in menB complexes of PS I with Cl ₂ NQ in the A₁-site differed significantly from all other PS I samples. The obtained effects are considered in terms of kinetic efficiency of electron acceptors in relation to thermodynamic and structural characteristics of PS I complexes.

Elastic Vibrations in the Photosynthetic Bacterial Reaction Center Coupled to the Primary Charge Separation: Implications from Molecular Dynamics Simulations and Stochastic Langevin Approach

Primary electron transfer reactions in the bacterial reaction center are difficult for theoretical explication: the reaction kinetics, almost unalterable over a wide range of temperature and free energy changes, revealed oscillatory features observed initially by Shuvalov and coauthors (1997, 2002). Here the reaction mechanism was studied by molecular dynamics and analyzed within a phenomenological Langevin approach. The spectral function of polarization around the bacteriochlorophyll special pair PLPM and the dielectric response upon the formation of PL(+)PM(-) dipole within the special pair were calculated. The system response was approximated by Langevin oscillators; the respective frequencies, friction, and energy coupling coefficients were determined. The protein dynamics around PL and PM were distinctly asymmetric. The polarization around PL included slow modes with the frequency 30-80 cm(-1) and the total amplitude of 130 mV. Two main low-frequency modes of protein response around PM had frequencies of 95 and 155 cm(-1) and the total amplitude of 30 mV. In addition, a slowly damping mode with the frequency of 118 cm(-1) and the damping time >1.1 ps was coupled to the formation of PL(+)PM(-) dipole. It was attributed to elastic vibrations of α-helices in the vicinity of PLPM. The proposed trapping of P excitation energy in the form of the elastic vibrations can rationalize the observed properties of the primary electron transfer reactions, namely, the unusual temperature and ΔG dependences, the oscillating phenomena in kinetics, and the asymmetry of the charge separation reactions.

Interaction of ascorbate with photosystem I

Ascorbate is one of the key participants of the antioxidant defense in plants. In this work, we have investigated the interaction of ascorbate with the chloroplast electron transport chain and isolated photosystem I (PSI), using the EPR method for monitoring the oxidized centers [Formula: see text] and ascorbate free radicals. Inhibitor analysis of the light-induced redox transients of P700 in spinach thylakoids has demonstrated that ascorbate efficiently donates electrons to [Formula: see text] via plastocyanin. Inhibitors (DCMU and stigmatellin), which block electron transport between photosystem II and Pc, did not disturb the ascorbate capacity for electron donation to [Formula: see text]. Otherwise, inactivation of Pc with CN(-) ions inhibited electron flow from ascorbate to [Formula: see text]. This proves that the main route of electron flow from ascorbate to [Formula: see text] runs through Pc, bypassing the plastoquinone (PQ) pool and the cytochrome b 6 f complex. In contrast to Pc-mediated pathway, direct donation of electrons from ascorbate to [Formula: see text] is a rather slow process. Oxidized ascorbate species act as alternative oxidants for PSI, which intercept electrons directly from the terminal electron acceptors of PSI, thereby stimulating photooxidation of P700. We investigated the interaction of ascorbate with PSI complexes isolated from the wild type cells and the MenB deletion strain of cyanobacterium Synechocystis sp. PCC 6803. In the MenB mutant, PSI contains PQ in the quinone-binding A1-site, which can be substituted by high-potential electron carrier 2,3-dichloro-1,4-naphthoquinone (Cl2NQ). In PSI from the MenB mutant with Cl2NQ in the A1-site, the outflow of electrons from PSI is impeded due to the uphill electron transfer from A1 to the iron-sulfur cluster FX and further to the terminal clusters FA/FB, which manifests itself as a decrease in a steady-state level of [Formula: see text]. The addition of ascorbate promoted photooxidation of P700 due to stimulation of electron outflow from PSI to oxidized ascorbate species. Thus, accepting electrons from PSI and donating them to [Formula: see text], ascorbate can mediate cyclic electron transport around PSI. The physiological significance of ascorbate-mediated electron transport is discussed.

O2 reduction by photosystem I involves phylloquinone under steady-state illumination

O2 reduction was investigated in photosystem I (PSI) complexes isolated from cyanobacteria Synechocystis sp. PCC 6803 wild type (WT) and menB mutant strain, which is unable to synthesize phylloquinone and contains plastoquinone at the quinone-binding site A1. PSI complexes from WT and menB mutant exhibited different dependencies of O2 reduction on light intensity, namely, the values of O2 reduction rate in WT did not reach saturation at high intensities, in contrast to the values in menB mutant. The obtained results suggest the immediate phylloquinone involvement in the light-induced O2 reduction by PSI.

Molecular dynamics study of the primary charge separation reactions in Photosystem I: effect of the replacement of the axial ligands to the electron acceptor A₀

Molecular dynamics (MD) calculations, a semi-continuum (SC) approach, and quantum chemistry (QC) calculations were employed together to investigate the molecular mechanics of ultrafast charge separation reactions in Photosystem I (PS I) of Thermosynechococcus elongatus. A molecular model of PS I was developed with the aim to relate the atomic structure with electron transfer events in the two branches of cofactors. A structural flexibility map of PS I was constructed based on MD simulations, which demonstrated its rigid hydrophobic core and more flexible peripheral regions. The MD model permitted the study of atomic movements (dielectric polarization) in response to primary and secondary charge separations, while QC calculations were used to estimate the direct chemical effect of the A(0A)/A(0B) ligands (Met or Asn in the 688/668 position) on the redox potential of chlorophylls A(0A)/A(0B) and phylloquinones A(1A)/A(1B). A combination of MD and SC approaches was used to estimate reorganization energies λ of the primary (λ₁) and secondary (λ₂ ) charge separation reactions, which were found to be independent of the active branch of electron transfer; in PS I from the wild type, λ₁ was estimated to be 390 ± 20mV, while λ₂ was estimated to be higher at 445 ± 15mV. MD and QC approaches were used to describe the effect of substituting Met688(PsaA)/Met668(PsaB) by Asn688(PsaA)/Asn668(PsaB) on the energetics of electron transfer. Unlike Met, which has limited degrees of freedom in the site, Asn was found to switch between two relatively stable conformations depending on cofactor charge. The introduction of Asn and its conformation flexibility significantly affected the reorganization energy of charge separation and the redox potentials of chlorophylls A(0A)/A(0B) and phylloquinones A(1A)/A(1B), which may explain the experimentally observed slowdown of secondary electron transfer in the M688N(PsaA) variant. This article is part of a special issue entitled: photosynthesis research for sustainability: keys to produce clean energy.

Incorporation of a high potential quinone reveals that electron transfer in Photosystem I becomes highly asymmetric at low temperature

Photosystem I (PS I) has two nearly identical branches of electron-transfer co-factors. Based on point mutation studies, there is general agreement that both branches are active at ambient temperature but that the majority of electron-transfer events occur in the A-branch. At low temperature, reversible electron transfer between P(700) and A(1A) occurs in the A-branch. However, it has been postulated that irreversible electron transfer from P(700) through A(1B) to the terminal iron-sulfur clusters F(A) and F(B) occurs via the B-branch. Thus, to study the directionality of electron transfer at low temperature, electron transfer to the iron-sulfur clusters must be blocked. Because the geometries of the donor-acceptor radical pairs formed by electron transfer in the A- and B-branch differ, they have different spin-polarized EPR spectra and echo-modulation decay curves. Hence, time-resolved, multiple-frequency EPR spectroscopy, both in the direct-detection and pulse mode, can be used to probe the use of the two branches if electron transfer to the iron-sulfur clusters is blocked. Here, we use the PS I variant from the menB deletion mutant strain of Synechocyctis sp. PCC 6803, which is unable to synthesize phylloquinone, to incorporate 2,3-dichloro-1,4-naphthoquinone (Cl(2)NQ) into the A(1A) and A(1B) binding sites. The reduction midpoint potential of Cl(2)NQ is approximately 400 mV more positive than that of phylloquinone and is unable to transfer electrons to the iron-sulfur clusters. In contrast to previous studies, in which the iron-sulfur clusters were chemically reduced and/or point mutations were used to prevent electron transfer past the quinones, we find no evidence for radical-pair formation in the B-branch. The implications of this result for the directionality of electron transfer in PS I are discussed.

Transmembrane charge transfer in photosynthetic reaction centers: some similarities and distinctions

This mini review presents a general comparison of structural and functional peculiarities of three types of photosynthetic reaction centers (RCs)--photosystem (PS) II, RC from purple bacteria (bRC) and PS I. The nature and mechanisms of the primary electron transfer reactions, as well as specific features of the charge transfer reactions at the donor and acceptor sides of RCs are considered. Comparison of photosynthetic RCs shows general similarity between the core central parts of all three types, between the acceptor sides of bRC and PS II, and between the donor sides of bRC and PS I. In the latter case, the similarity covers thermodynamic, kinetic and dielectric properties, which determine the resemblance of mechanisms of electrogenic reduction of the photooxidized primary donors. Significant distinctions between the donor and acceptor sides of PS I and PS II are also discussed. The results recently obtained in our laboratory indicate in favor of the following sequence of the primary and secondary electron transfer reactions: in PS II (bRC): Р(680)(Р(870)) → Chl(D1)(В(А)) → Phe(bPhe) → Q(A); and in PS I: Р(700) → А(0А)/A(0B) → Q(A)/Q(B).

Photosysem II: where does the light-induced voltage come from?

Photosystem II (PS II) is a biological energy transducer. The enzyme catalyses the light-driven oxidation of water and reduction of plastoquinone. The aim of this work was to review the mechanisms of electrical events in PS II. The major contribution to the total photoelectric response is due to the charge-separation between the primary chlorophyll donor P680 and quinone acceptor QA accompanied by re-reduction of P680+ by tyrosine residue YZ. The remaining part of the membrane potential is believed to be associated mainly with electron and proton transfer events due to the S-state transitions of the oxygen-evolving complex and proton uptake associated with protonation of the doubly reduced secondary quinone acceptor QB. Under certain non-physiological conditions, some other electrogenic reactions are observed, namely: proton-coupled electron transfer between QA and non-heme Fe3+ and electron transfer from the protein-water interface to the YZ radical in the presence of artificial electron donors. These data may provide a good platform for further development of artificial photosynthetic constructs and bio-inspired catalysts.

Semi-continuum electrostatic calculations of redox potentials in photosystem I

The midpoint redox potentials (E(m)) of all cofactors in photosystem I from Synechococcus elongatus as well as of the iron-sulfur (Fe(4)S(4)) clusters in two soluble ferredoxins from Azotobacter vinelandii and Clostridium acidiurici were calculated within the framework of a semi-continuum dielectric approach. The widely used treatment of proteins as uniform media with single dielectric permittivity is oversimplified, particularly, because permanent charges are considered both as a source for intraprotein electric field and as a part of dielectric polarizability. Our approach overcomes this inconsistency by using two dielectric constants: optical epsilon(o)=2.5 for permanent charges pre-existing in crystal structure, and static epsilon(s) for newly formed charges. We also take into account a substantial dielectric heterogeneity of photosystem I revealed by photoelectric measurements and a liquid junction potential correction for E(m) values of relevant redox cofactors measured in aprotic solvents. We show that calculations based on a single permittivity have the discrepancy with experimental data larger than 0.7 V, whereas E(m) values calculated within our approach fall in the range of experimental estimates. The electrostatic analysis combined with quantum chemistry calculations shows that (i) the energy decrease upon chlorophyll dimerization is essential for the downhill mode of primary charge separation between the special pair P(700) and the primary acceptor A(0); (ii) the primary donor is apparently P(700) but not a pair of accessory chlorophylls; (iii) the electron transfer from the A branch quinone Q(A) to the iron-sulfur cluster F(X) is most probably downhill, whereas that from the B branch quinone Q(B) to F(X) is essentially downhill.

Correlation of electron transfer rate in photosynthetic reaction centers with intraprotein dielectric properties

A number of the electrogenic reactions in photosystem I, photosystem II, and bacterial reaction centers (RC) were comparatively analyzed, and the variation of the dielectric permittivity (epsilon) in the vicinity of electron carriers along the membrane normal was calculated. The value of epsilon was minimal at the core of the complexes and gradually increased towards the periphery. We found that the rate of electron transfer (ET) correlated with the value of the dielectric permittivity: the fastest primary ET reactions occur in the low-polarity core of the complexes within the picosecond time range, whereas slower secondary reactions take place at the high-polarity periphery of the complexes within micro- to millisecond time range. The observed correlation was quantitatively interpreted in the framework of the Marcus theory. We calculated the reorganization energy of ET carriers using their van der Waals volumes and experimentally determined epsilon values. The electronic coupling was calculated by the empirical Moser-Dutton rule for the distance-dependent electron tunneling rate in nonadiabatic ET reactions. We concluded that the local dielectric permittivity inferred from the electrometric measurements could be quantitatively used to estimate the rate constant of ET reactions in membrane proteins with resolved atomic structure with the accuracy of less than one order of magnitude.

Long-lived coherent oscillations of the femtosecond transients in cyanobacterial photosystem I

The pulsed excitation of electronic levels coupled to specific nuclear modes by a 26 fs laser pulse at 706 nm creates a wavepacket in the nuclear space of photopystem I (PS I) of Synechocystis sp. strain PCC 6803 both in the ground state and in the one-exciton manifold. Fourier transform of transient decay curves shows several low frequency peaks. The most prominent Power Spectral Density (PSD) peaks are at omega = 49 cm(-1) and omega = 88 cm(-1). The peculiarity of the coherent wavepacket in the PS I of S. sp. strain PCC 6803 is the unique, long-lived 49 cm(-1) and 88 cm(-1) oscillations with decay times up to 10 ps. It was suggested that such a long-lived coherence is determined by a contribution of the ground state wavepacket. The dependence of these two PSD peaks on the probe wavelength resembles the profile of the transient absorption spectra of PS I. The pump-probe signal in the Soret region reflects the dynamics of the ground state wavepacket created by pulsed excitation of the Q(y)-band. It was shown that the multimode Brownian oscillator model allows a quantitative fit of the oscillatory patterns of the pump-probe signal to be obtained.

EPR study of electron transport in the cyanobacterium Synechocystis sp. PCC 6803: Oxygen-dependent interrelations between photosynthetic and respiratory electron transport chains

In this work, we investigated electron transport processes in the cyanobacterium Synechocystis sp. PCC 6803, with a special emphasis focused on oxygen-dependent interrelations between photosynthetic and respiratory electron transport chains. Redox transients of the photosystem I primary donor P700 and oxygen exchange processes were measured by the EPR method under the same experimental conditions. To discriminate between the factors controlling electron flow through photosynthetic and respiratory electron transport chains, we compared the P700 redox transients and oxygen exchange processes in wild type cells and mutants with impaired photosystem II and terminal oxidases (CtaI, CydAB, CtaDEII). It was shown that the rates of electron flow through both photosynthetic and respiratory electron transport chains strongly depended on the transmembrane proton gradient and oxygen concentration in cell suspension. Electron transport through photosystem I was controlled by two main mechanisms: (i) oxygen-dependent acceleration of electron transfer from photosystem I to NADP(+), and (ii) slowing down of electron flow between photosystem II and photosystem I governed by the intrathylakoid pH. Inhibitor analysis of P700 redox transients led us to the conclusion that electron fluxes from dehydrogenases and from cyclic electron transport pathway comprise 20-30% of the total electron flux from the intersystem electron transport chain to P700(+).

Structure of the Intermolecular Complex between Plastocyanin and Cytochrome f from Spinach

In oxygenic photosynthesis, plastocyanin shuttles electrons between the membrane-bound complexes cytochrome b6f and photosystem I. The homologous complex between cytochrome f and plastocyanin, both from spinach, is the object of this study. The solution structure of the reduced spinach plastocyanin was determined using high field NMR spectroscopy, whereas the model structure of oxidized cytochrome f was obtained by homology modeling calculations and molecular dynamics. The model structure of the intermolecular complex was calculated using the program AUTODOCK, taking into account biological information obtained from mutagenesis experiments. The best electron transfer pathway from the heme group of cytochrome f to the copper ion of plastocyanin was calculated using the program HARLEM, obtaining a coupling decay value of 1.8 x 10(-4). Possible mechanisms of interaction and electron transfer between plastocyanin and cytochrome f were discussed considering the possible formation of a supercomplex that associates one cytochrome b6f, one photosystem I, and one plastocyanin.

Electron transfer from HiPIP to the photooxidized tetraheme cytochrome subunit of Allochromatium vinosum reaction center: new insights from site-directed mutagenesis and computational studies

The kinetics of electron transfer from reduced high-potential iron-sulfur protein (HiPIP) to the photooxidized tetraheme cytochrome c subunit (THC) bound to the photosynthetic reaction center (RC) from the purple sulfur bacterium Allochromatium vinosum were studied under controlled redox conditions by flash absorption spectroscopy. At ambient redox potential Eh = +200 mV, where only the high-potential (HP) hemes of the THC are reduced, the electron transfer from HiPIP to photooxidized HP heme(s) follows second-order kinetics with rate constant k = (4.2 +/- 0.2) 10(5) M(-1) s(-1) at low ionic strength. Upon increasing the ionic strength, k increases by a maximum factor of ca. 2 at 640 mM KCl. The role of Phe48, which lies on the external surface of HiPIP close to the [Fe4S4] cluster and presumably on the electron transfer pathway to cytochrome heme(s), was investigated by site-directed mutagenesis. Substitution of Phe48 with arginine, aspartate, and histidine completely prevents electron donation. Conversely, electron transfer is still observed upon substitution of Phe48 with tyrosine and tryptophan, although the rate is decreased by more than 1 order of magnitude. These results suggest that Phe48 is located on a key protein surface patch essential for efficient electron transfer, and that the presence of an aromatic hydrophobic residue on the putative electron-transfer pathway plays a critical role. This conclusion was supported by protein docking calculations, resulting in a structural model for the HiPIP-THC complex, which involves a docking site close to the LP heme farthest from the bacteriochlorophyll special pair.

Photoelectric studies of the transmembrane charge transfer reactions in photosystem I pigment-protein complexes

The results of studies of charge transfer in cyanobacterial photosystem I (PS I) using the photoelectric method are reviewed. The electrogenicity in the PS I complex and its interaction with natural donors (plastocyanin, cytochrome c(6)), natural acceptors (ferredoxin, flavodoxin), or artificial acceptors and donors (methyl viologen and other redox dyes) were studied. The operating dielectric constant values in the vicinity of the charge transfer carriers in situ were calculated. The profile of distribution of the dielectric constant along the PS I pigment-protein complex (from plastocyanin or cytochrome c(6) through the chlorophyll dimer P700 to the acceptor complex) was estimated, and possible mechanisms of correlation between the local dielectric constant and electron transfer rate constant were discussed.

EPR study of light-induced regulation of photosynthetic electron transport in Synechocystis sp. strain PCC 6803

The kinetics of the light-induced redox changes of the photosystem 1 (PS 1) primary donor P(700) in whole cells of the cyanobacteria Synechocystis sp. PCC 6803 were studied by the electron paramagnetic resonance method. It was shown that the linear photosynthetic electron transport in cyanobacteria was controlled by two main mechanisms: (i) oxygen-dependent acceleration of electron transfer from PS 1 to NADP(+) due to activation of the Calvin cycle reactions and (ii) retardation of electron flow between two photosystems governed by a transmembrane proton gradient. In addition to the linear photosynthetic electron transport, cyanobacteria were capable of maintaining alternative pathways involving cyclic electron transfer around PS 1 and respiratory chains.

Electrogenic reduction of the primary electron donor P700 by plastocyanin in photosystem I complexes

An electrometric technique was used to investigate electron transfer between spinach plastocyanin (Pc) and photooxidized primary electron donor P700 in photosystem I (PS I) complexes from the cyanobacterium Synechocystis sp. PCC 6803. In the presence of Pc, the fast unresolvable kinetic phase of membrane potential generation related to electron transfer between P700 and the terminal iron-sulfur acceptor F(B) was followed by additional electrogenic phases in the microsecond and millisecond time scales, which contribute approximately 20% to the overall electrogenicity. These phases are attributed to the vectorial electron transfer from Pc to the protein-embedded chlorophyll dimer P700(+) within the PsaA/PsaB heterodimer. The observed rate constant of the millisecond kinetic phase exhibited a saturation profile at increasing Pc concentration, suggesting the formation of a transient complex between Pc and PS I with the dissociation constant K(d) of about 80 microM. A small but detectable fast electrogenic phase was observed at high Pc concentration. The rate constant of this phase was independent of Pc concentration, indicating that it is related to a first-order process.

Electrogenic proton transfer in Rhodobacter sphaeroides reaction centers: effect of coenzyme Q(10) substitution by decylubiquinone in the Q(B) binding site

An electrometric technique was used to investigate the effect of coenzyme Q(10) (UQ), substitution by decylubiquinone (dQ) at the Q(B) binding site of reaction centers (UQ-RC and dQ-RC, respectively) on the electrogenic proton transfer kinetics upon Q(B) reduction in Rhodobacter sphaeroides chromatophores. Unlike dQ-RC, the kinetics of the second flash-induced proton uptake in UQ-RC clearly deviated from the mono-exponential one. The activation energy (about 30 kJ/mol) and the pH profile of the kinetics in dQ-RC were similar to those in UQ-RC, with the power law approximation used in the latter case. The interpretation of the data presumed the quinone translocation between the two binding positions within the Q(B) site. It is proposed that the native isoprenyl side chain (in contrast to decyl chain) favors the equilibrium binding of neutral quinone at the redox-active 'proximal' position, but causes a higher barrier for the hydroquinone movement from 'proximal' to 'distal' position.

Effect of D2O and cryosolvents on the redox properties of bacteriochlorophyll dimer and electron transfer processes in Rhodobacter sphaeroides reaction centers

Effects of environmental changes on the reaction pattern of excitation energy trapping and transformation into the "stable" radical pair P+Q(A)-, have been analyzed in isolated reaction centers of the anoxygenic purple bacterium Rhodobacter sphaeroides. The following results were obtained: (a) replacement of exchangeable protons by deuterons significantly retarded the electron transfer steps of primary charge separation, leading to the radical pair P+I- and of the subsequent reoxidation of I- by the quinone acceptor Q(A) but has virtually no effect on the midpoint potential of P/P+ that was found to be 430+/-20 mV; (b) addition of 70% (v/v) glycerol causes a shift of Em by about 30 mV towards higher values whereas the kinetics of the electron transfer reactions remain almost unaffected; (c) in the presence of the cryoprotectant DMSO, a combined effect arises, i.e. a retardation of the electron transfer kinetics comparable to that induced by H/D exchange and simultaneously an upshift of the Em value to 475+/-20 mV, resembling the action of glycerol. These results are discussed within the framework of effects on the midpoint potential due to the dielectric constant of the medium and changes of the charge distribution in the vicinity of the redox groups and the influence of relaxation processes on electron transfer reactions.

Recruitment of a foreign quinone into the A1 site of photosystem I. Altered kinetics of electron transfer in phylloquinone biosynthetic pathway mutants studied by time-resolved optical, EPR, and electrometric techniques

Interruption of the menA or menB gene in Synechocystis sp. PCC 6803 results in the incorporation of a foreign quinone, termed Q, into the A(1) site of photosystem I with a number of experimental indicators identifying Q as plastoquinone-9. A global multiexponential analysis of time-resolved optical spectra in the blue region shows the following three kinetic components: 1) a 3-ms lifetime in the absence of methyl viologen that represents charge recombination between P700(+) and an FeS(-) cluster; 2) a 750-microseconds lifetime that represents electron donation from an FeS(-) cluster to methyl viologen; and 3) an approximately 15-microseconds lifetime that represents an electrochromic shift of a carotenoid pigment. Room temperature direct detection transient EPR studies of forward electron transfer show a spectrum of P700(+) Q(-) during the lifetime of the spin polarization and give no evidence of a significant population of P700(+) FeS(-) for t </= 2-3 microseconds. The UV difference spectrum measured 5 microseconds after a flash shows a maximum at 315 nm, a crossover at 280 nm, and a minimum at 255 nm as well as a shoulder at 290-295 nm, all of which are characteristic of the plastoquinone-9 anion radical. Kinetic measurements that monitor Q at 315 nm show a major phase of forward electron transfer to the FeS clusters with a lifetime of approximately 15 microseconds, which matches the electrochromic shift at 485 nm of the carotenoid, as well as an minor phase with a lifetime of approximately 250 microseconds. Electrometric measurements show similar biphasic kinetics. The slower kinetic phase can be detected using time-resolved EPR spectroscopy and has a spectrum characteristic of a semiquinone anion radical. We estimate the redox potential of plastoquinone-9 in the A(1) site to be more oxidizing than phylloquinone so that electron transfer from Q(-) to F(X) is thermodynamically unfavorable in the menA and menB mutants.

Reduction and protonation of the secondary quinone acceptor of Rhodobacter sphaeroides photosynthetic reaction center: kinetic model based on a comparison of wild-type chromatophores with mutants carrying Arg-->Ile substitution at sites 207 and 217 in the L-subunit

After the light-induced charge separation in the photosynthetic reaction center (RC) of Rhodobacter sphaeroides, the electron reaches, via the tightly bound ubiquinone QA, the loosely bound ubiquinone Q(B) After two subsequent flashes of light, Q(B) is reduced to ubiquinol Q(B)H2, with a semiquinone anion Q-(B) formed as an intermediate after the first flash. We studied Q(B)H2 formation in chromatophores from Rb. sphaeroides mutants that carried Arg-->Ile substitution at sites 207 and 217 in the L-subunit. While Arg-L207 is 17 A away from Q(B), Arg-L217 is closer (9 A) and contacts the Q(B)-binding pocket. From the pH dependence of the charge recombination in the RC after the first flash, we estimated deltaG(AB), the free energy difference between the Q-(A)Q(B) and Q(A)Q-(B) states, and pK212, the apparent pK of Glu-L212, a residue that is only 4 A away from Q(B). As expected, the replacement of positively charged arginines by neutral isoleucines destabilized the Q-(B) state in the L217RI mutant to a larger extent than in the L207RI one. Also as expected, pK212 increased by approximately 0.4 pH units in the L207RI mutant. The value of pK212 in the L217RI mutant decreased by 0.3 pH units, contrary to expectations. The rate of the Q-(A)Q-(B)-->Q(A)Q(B)H2 transition upon the second flash, as monitored by electrometry via the accompanying changes in the membrane potential, was two times faster in the L207RI mutant than in the wild-type, but remained essentially unchanged in the L217RI mutant. To rationalize these findings, we developed and analyzed a kinetic model of the Q-(A)Q-(B)-->Q(A)Q(B)H2 transition. The model properly described the available experimental data and provided a set of quantitative kinetic and thermodynamic parameters of the Q(B) turnover. The non-electrostatic, 'chemical' affinity of the QB site to protons proved to be as important for the attracting protons from the bulk, as the appropriate electrostatic potential. The mutation-caused changes in the chemical proton affinity could be estimated from the difference between the experimentally established pK2J2 shifts and the expected changes in the electrostatic potential at Glu-L212, calculable from the X-ray structure of the RC. Based on functional studies, structural data and kinetic modeling, we suggest a mechanistic scheme of the QB turnover. The detachment of the formed ubiquinol from its proximal position next to Glu-L212 is considered as the rate-limiting step of the reaction cycle.