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The kinetics of the primary electron donor P700+ and the quinone acceptor A1– redox transitions were simultaneously studied for the first time in the time range of 200 μs-10 ms using high-frequency pulse Q-band EPR spectroscopy at cryogenic temperatures in various complexes of photosystem I (PSI) from the cyanobacterium Synechocystis sp. PCC 6803. In the A1-core PSI complexes that lack 4Fe4S clusters, the kinetics of the A1– and P700+ signals disappearance at 100 K were similar and had a characteristic time of τ ≈ 500 μs, caused by charge recombination in the P700+A1A– ion-radical pair in the A branch of redox cofactors. The kinetics of the backward electron transfer from A1B– to P700+ in the B branch of redox cofactors with τ < 100 μs could not be resolved due to time limitations of the method. In the native PSI complexes with a full set of redox cofactors and in the FX-core complexes, containing the 4Fe4S cluster FX, the kinetics of the A1– signal was significantly faster than that of the P700+ signal. The disappearance of the A1– signal had a characteristic time of 280-350 μs; it was suggested that, in addition to the backward electron transfer from A1A– to P700+ with τ ≈ 500 μs, its kinetics also includes the forward electron transfer from A1A– to the 4Fe4S cluster FX, which had slowed down to 150-200 μs. In the kinetics of P700+ reduction, it was possible to distinguish components caused by the backward electron transfer from A1– (τ ≈ 500 μs) and from 4Fe4S clusters (τ = 1 ms for the FX-core and τ > 5 ms for native complexes). These results are in qualitative agreement with the data on the kinetics of P700+ reduction obtained previously using pulse absorption spectrometry at cryogenic temperatures. [ABSTRACT FROM AUTHOR]

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 A0A/A0B ligands (Met or Asn in the 688/668 position) on the redox potential of chlorophylls A0A/A0B and phylloquinones A1A/A1B. 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 ± 20 mV, while λ2 was estimated to be higher at 445 ± 15 mV. MD and QC approaches were used to describe the effect of substituting Met688PsaA/Met668PsaB by Asn688PsaA/Asn668PsaB 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 A0A/A0B and phylloquinones A1A/A1B, which may explain the experimentally observed slowdown of secondary electron transfer in the M688NPsaA variant. This article is part of a Special Issue entitled: Photosynthesis research for sustainability: Keys to produce clean energy. [ABSTRACT FROM AUTHOR]

The kinetics of flash-induced re-reduction of the Photosystem II (PS II) primary electron donor P 680 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 P 680 +. The minor slower components were due to charge recombination between the primary plastoquinone acceptor Q A − and P 680 +. 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 P 680 , which resulted in the inhibition of Y Z → P 680 + electron transfer in about half of the PS II population, wherein the recombination between Q A − and P 680 + 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. [Display omitted] • Photosystem II in glassy trehalose matrices at variable humidity was studied. • Forward electron transfer and charge recombination were slowed down in trehalose. • Dehydration inhibited protein conformational mobility and Y Z → P 680 electron transfer. • The data disprove "water replacement" hypothesis of trehalose matrix biopreservation. [ABSTRACT FROM AUTHOR]

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 A1A– and A1B– 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, FX, FA, and FB, and the PsaC protein subunit (the so-called P700–A1 core), in which phylloquinone molecules A1A and A1B 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 FX, FA, and FB are discussed. [ABSTRACT FROM AUTHOR]

Abstract This work aims to fully elucidate the effects of a trehalose glassy matrix on electron transfer reactions in cyanobacterial Photosystem I (PS I). Forward and backward electron transfer rates from A 1A − and A 1B − to F X, and charge recombination rates from A 0 −, A 1B −, A 1A −, F X −, and [F A /F B ]− to P 700 + were measured in P 700 –F A /F B complexes, P 700 –F X cores, and P 700 –A 1 cores, both in liquid and in a trehalose glassy matrix at 11% humidity. By comparing CONTIN-resolved kinetic events over 6 orders of time in increasingly simplified versions of PS I at 480 nm, a wavelength that reports primarily A 1A −/A 1B − oxidation, and over 9 orders of time at 830 nm, a wavelength that reports P 700 + reduction and A 0 − oxidation, assignments could be made for nearly all of the resolved kinetic phases. Trehalose-embedded PS I samples demonstrated partially arrested forward electron transfer. The fractions of complexes in which electron transfer did not proceed beyond A 0 , A 1 and F X were 53%, 16% and 22%, respectively, with only 10% of electrons reaching the terminal F A /F B clusters. The ~10 μs and ~150 μs components in both liquid and trehalose-embedded PS I were assigned to recombination between A 1B − and P 700 + and between A 1A − and P 700 +, respectively. The kinetics and amplitudes of these resolved kinetic phases in liquid and trehalose-embedded PS I samples could be well-fitted by a kinetic model that allowed us to calculate the asymmetrical contribution of the A 1A − and A 1B − quinones to the electrochromic signal at 480 nm. Possible reasons for these effects are discussed. Highlights • Photosystem I embedded in trehalose glass demonstrates arrested electron transfer. • Forward rates from the A 1A −/A 1B − quinones to F X are slower than in liquid medium. • Recombination rates from the A 1A −/A 1B − quinones to P 700 + are similar to liquid medium. • Recombination from F X − to P 700 + is heterogeneous and splits into several components. • The kinetics and amplitudes of the reactions can be well-fitted by a kinetic model. [ABSTRACT FROM AUTHOR]

Matrix effects on long-range electron transfer were studied in cyanobacterial Photosystem I (PS I) complexes, embedded into trehalose glasses at different hydration levels. W-band EPR studies demonstrated, via nitroxide spin probes, structural homogeneity of the dry PS I-trehalose matrix and no alteration of cofactors' distance and relative orientation under temperature and matrix variation. In dry trehalose glasses at room temperature (RT), PS I was stable for months. Flash-induced charge recombination kinetics were examined by high-field time-resolved EPR and optical spectroscopies. The kinetics in hydrated PS I-trehalose glasses mostly reflected the reduction of the photooxidized primary donor P 700 •+ by the reduced terminal iron-sulfur clusters. Upon dehydration, the P 700 •+ decay accelerated and became more distributed. Continuous distributions of lifetimes τ were extracted from the kinetics by two numerical approaches: a maximum entropy method (MemExp program) and a constrained regularization method (CONTIN program). Both analyses revealed that upon dehydration the contribution of the two slowest components (lifetimes τ ~ 300 ms and ~ 60 ms), attributed to P 700 •+ [F A /F B ] − recombination, decreased in parallel with the increase of the fastest component ( τ ~ 150 μs), and of additional distributed phases with intermediate lifetimes. Dehydration at RT mimicked the effects of freezing water-glycerol PS I systems, suggesting an impairment of PS I protein dynamics in the dry trehalose glass. Similar effects were observed previously in bacterial reaction centers. The work presented for PS I provides new insights into the crucial issue of protein-matrix interactions for protein functionality as controlled by hydrogen-bond networks of the hydration shell. [ABSTRACT FROM AUTHOR]