Effect of charge distribution over a chlorophyll dimer on the redox potential of P680 in photosystem II as studied by density functional theory calculations

The effect of charge distribution over a chlorophyll dimer on the redox potential of P680 in photosystem II was studied by density functional theory calculations using the P680 coordinates in the X-ray structure. From the calculated ionization potentials of the dimer and the monomeric constituents, the decrease in the redox potential by charge delocalization over the dimer was estimated to be approximately 140 mV. Such charge delocalization was previously observed in the isolated D1-D2-Cyt b 559 complexes, whereas the charge was primarily localized on P D1 in the core complexes. The calculated potential decrease of approximately 140 mV can explain the inhibition of Y Z oxidation in the former complexes and in turn implies that the charge localization on P D1 upon formation of the core complex increases the P680 potential to the level necessary for water oxidation.     

Density functional theory calculations on the thermodynamic properties of polynitrosoprismanes

A series of polynitrosoprismanes, C(6)H(6 - n )(NO)( n ) (n?=?1-6), considered as high energy density compounds (HEDCs), have been designed computationally. We calculated the electronic structures, the heats of formation, the specific enthalpies of combustion, the bond dissociation energies, and the strain energies of the title compounds using density functional theory (DFT) with the 6-311G** basis set. It was found that the ΔE (LUMO-HOMO) values of the title compounds decrease as the number of nitroso groups increase, and the energy gaps of the prismane derivatives are much lower than that of TATB. Their high positive heats of formation indicate that polynitrosoprismanes can store a great deal of energy. Furthermore, the HOFs for the nitrosoprismane series were observed to decrease until three nitroso groups were connected to the prismane skeleton. For the polynitrosoprismanes, the trigger bond was confirmed to be the C-C bond in the skeleton. According to our calculations, all nitrosoprismanes appear to have large strain energies, and these calculations can provide basic information that may prove useful for the molecular design of novel high energy density materials.     

Biosynthesis of chlorophyll P-680 of photosystem II in greening leaves of plants adapted to heat shock

In etiolated pea and maize leaves illuminated after incubation at 38 degreesC, a new dark reaction was shown manifested in the bathochromic shift of spectral bands and accompanied by esterification of the product of protochlorophyllide photochemical reduction--Chld 684/676: Chld 684/676 --> Chl 688/680. After completion of the reaction a rapid (20-30 sec) quenching of the fluorescence of the reaction product (Chl 688/680) was observed. The reaction Chld 684/676 --> Chl 688/680 is inhibited under anaerobic conditions and in the presence of cyanide; the reaction accompanied by Chl 688/680 fluorescence quenching is not observed in pea mutants with impaired function of photosystem II reaction centers. The spectral properties of the formed Chl form with the absorption maximum at 680 nm, fluorescence quenching, and simultaneous synthesis of pheophytin suggest that the reaction is connected with the chlorophyll of photosystem II reaction center--P-680.     

Determination of depth of immersion of chlorophyll P680, pheophytin, and a secondary electron donor in the subchloroplast preparations of photosystem II from pea

The immersion depths (r) of the main functional components of the reaction centres of the Photosystem 2 in the thylakoid membranes were determined by ESR at 77K. It was shown that P680(+), Pheo(-) (pheophytin), and Z(+) (secondary electron donor) the r value was 2-4, 4-7 and 14-20 A, respectively. On the basis of these and reference data a model of location of the Photosystem 2 reaction centre components in the photosynthetic membrane was suggested.     

Probing the lowest energy chlorophyll a states of Photosystem II via selective spectroscopy: new insights on P680

We present the wavelength dependence of homogeneous holewidths of persistent spectral holes burnt in O₂-evolving Photosystem II core complexes isolated from spinach, in the temperature range 2.5–8 K. The data supports the assignment that those chlorophylls which undergo persistent spectral hole-burning are specific CP43 and CP47-trap states that transfer their excitation energy to the reaction center. The lifetime-limited holewidths show that when PS II is in the S₁(Q_A⁻) (closed) state, the CP43/CP47-trap states have excited-state lifetimes in the range from 70 to 270 ps. These lifetimes correspond to excitation transfer rates to the reaction center, and are far slower than required for models in which the PS II reaction center (P680) acts as a ‘shallow-trap’ for excitations. For wavelengths at which both traps absorb, the hole shape is clearly a composite of two Lorentzians, corresponding to hole-burning in both states simultaneously. The temperature dependence of the homogeneous holewidth does not follow the usual T^1.3 dependence found in many chlorophyll–protein systems. Our data indicates T ² temperature dependence, typically found in crystalline systems where the chromophore is coupled to localized phonon modes.     

Induction kinetics of photosystem I-activated P700 oxidation in plant leaves and their dependence on pre-energization

Absorbance changes ΔA ₈₁₀ were measured in pea (Pisum sativum L., cv. Premium) leaves to track redox transients of chlorophyll P700 during and after irradiation with far red (FR) light under various preillumination conditions in the absence and presence of inhibitors and protonophorous uncoupler of photosynthetic electron transport. It was shown that cyclic electron transport (CET) in chloroplasts of pea leaves operates at its highest rate after preillumination of leaves with white light and is strongly suppressed after preillumination with FR light. The FR light-induced suppression was partly released during prolonged dark adaptation. Upon FR illumination of dark-adapted leaves, the induction of CET was observed, during which CET activity increased to the peak from the low level and then decreased gradually. The kinetics of P700 oxidation induced by FR light of various intensities in leaves preilluminated with white light were fit to empirical sigmoid curves containing two variables. In leaves treated with a protonophore FCCP, the amplitude of FR light-induced changes ΔA ₈₁₀ was strongly suppressed, indicating that the rate of CET is controlled by the pH gradient across the thylakoid membrane.     

Effect of preillumination on the P-680+ reduction kinetics in chloride-free photosystem II membranes

The re-reduction course of P-680⁺, the photooxidized PS II primary donor, was measured as a function of excitation number in Cl⁻-depleted PS II membranes. After the 1st and 2nd excitations the signal amplitude of P-680⁺ is small, indicating a submicrosecond reduction of P-680⁺ by Z, the secondary donor of PS II. After the 3rd excitation, however, a larger P-680⁺ signal with a 40–50 μs half-life is observed. The slow decay of this signal is attributed to a back-reaction with a reduced acceptor in the presence of the Z⁺S₂ state on the donor side. The state Z⁺S₂ has a lifetime longer than 300 ms and its formation was found to depend on the presence of the abnormal S₂ state created by the 1st excitation. The P-680 data and thermoluminescence measurements show that the S-state advancement beyond S₂ is blocked in the absence of Cl⁻ and that the Cl⁻-free abnormal S₂ state has a lifetime about 10-times longer than the normal S₂ state.     

Directed mutagenesis indicates that the donor to P+680 in photosystem II is tyrosine-161 of the D1 polypeptide

Photosystem II contains two redox-active tyrosines. One of these, YZ, reduces the reaction center chlorophyll, P680, and transfers the oxidizing equivalent to the oxygen-evolving complex. The second, YD, has a long-lived free radical state of unknown function. We recently established that YD is Tyr-160 of the D2 polypeptide by site-directed mutagenesis of a psbD gene in the unicellular cyanobacterium Synechocystis 6803 [Debus, R. J., Barry, B. A., Babcock, G. T., & McIntosh, L. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 427-430]. YZ is most likely the symmetry-related Tyr-161 of the D1 polypeptide. To test this hypothesis, we have changed Tyr-161 to phenylalanine by site-directed mutagenesis of a psbA gene in Synechocystis. The resulting mutant assembles PSII, as judged by its ability to produce the stable Y+D radical, but is unable to grow photosynthetically and exhibits altered fluorescence properties. The nature of the fluorescence change indicates that forward electron transfer to P+680 is disrupted in the mutant. These results provide strong support for our identification of Tyr-161 in the D1 polypeptide with YZ.     

Molecular origin of the pH dependence of tyrosine D oxidation kinetics and radical stability in photosystem II

A role for redox-active tyrosines has been demonstrated in many important biological processes, including water oxidation carried out by photosystem II (PSII) of oxygenic photosynthesis. The rates of tyrosine oxidation and reduction and the Tyr/Tyr reduction potential are undoubtedly controlled by the immediate environment of the tyrosine, with the coupling of electron and proton transfer, a critical component of the kinetic and redox behavior. It has been demonstrated by Faller et al. that the rate of oxidation of tyrosine D (Tyr(D)) at room temperature and the extent of Tyr(D) oxidation at cryogenic temperatures, following flash excitation, dramatically increase as a function of pH with a pK(a) of approximately 7.6 [Faller et al. 2001 Proc. Natl. Acad. Sci. USA 98, 14368-14373; Faller et al. 2001 Biochemistry 41, 12914-12920]. In this work, we investigated, using FTIR difference spectroscopy, the mechanistic reasons behind this large pH dependence. These studies were carried out on Mn-depleted PSII core complexes isolated from Synechocystis sp. PCC 6803, WT unlabeled and labeled with (13)C(6)-, or (13)C(1)(4)-labeled tyrosine, as well as on the D2-Gln164Glu mutant. The main conclusions of this work are that the pH-induced changes involve the reduced Tyr(D) state and not the oxidized Tyr(D)() state and that Tyr(D) does not exist in the tyrosinate form between pH 6 and 10. We can also exclude a change in the protonation state of D2-His189 as being responsible for the large pH dependence of Tyr(D) oxidation. Indeed, our data are consistent with D2-His189 being neutral both in the Tyr(D) and Tyr(D)() states in the whole pH6-10 range. We show that the interactions between reduced Tyr(D) and D2-His189 are modulated by the pH. At pH greater than 7.5, the nu(CO) mode frequency of Tyr(D) indicates that Tyr(D) is involved in a strong hydrogen bond, as a hydrogen bond donor only, in a fraction of the PSII centers. At pH below 7.5, the hydrogen-bonding interaction formed by Tyr(D) is weaker and Tyr(D) could be also involved as a hydrogen bond acceptor, according to calculations performed by Takahashi and Noguchi [J. Phys. Chem. B 2007 111, 13833-13844]. The involvement of Tyr(D) in this strong hydrogen-bonding interaction correlates with the ability to oxidize Tyr(D) at cryogenic temperatures and rapidly at room temperature. A strong hydrogen-bonding interaction is also observed at pH 6 in the D2-Gln164Glu mutant, showing that the residue at position D2-164 regulates the properties of Tyr(D.) The IR data point to the role of a protonatable group(s) (with a pK(a) of approximately 7) other than D2-His189 and Tyr(D), in modifying the characteristics of the Tyr(D) hydrogen-bonding interactions, and hence its oxidation properties. It remains to be determined whether the strong hydrogen-bonding interaction involves D2-His189 and if Tyr(D) oxidation involves the same proton transfer route at low and at high pH.     

Molecular origin of the pH dependence of tyrosine D oxidation kinetics and radical stability in photosystem II

A role for redox-active tyrosines has been demonstrated in many important biological processes, including water oxidation carried out by photosystem II (PSII) of oxygenic photosynthesis. The rates of tyrosine oxidation and reduction and the Tyr/Tyr reduction potential are undoubtedly controlled by the immediate environment of the tyrosine, with the coupling of electron and proton transfer, a critical component of the kinetic and redox behavior. It has been demonstrated by Faller et al. that the rate of oxidation of tyrosine D (Tyr_D) at room temperature and the extent of Tyr_D oxidation at cryogenic temperatures, following flash excitation, dramatically increase as a function of pH with a pK_a of ≈ 7.6 [Faller et al. 2001 Proc. Natl. Acad. Sci. USA 98, 14368-14373; Faller et al. 2001 Biochemistry 41, 12914-12920]. In this work, we investigated, using FTIR difference spectroscopy, the mechanistic reasons behind this large pH dependence. These studies were carried out on Mn-depleted PSII core complexes isolated from Synechocystis sp. PCC 6803, WT unlabeled and labeled with ¹³C₆-, or ¹³C₁(4)-labeled tyrosine, as well as on the D2-Gln164Glu mutant. The main conclusions of this work are that the pH-induced changes involve the reduced Tyr_D state and not the oxidized Tyr_D state and that Tyr_D does not exist in the tyrosinate form between pH 6 and 10. We can also exclude a change in the protonation state of D2-His189 as being responsible for the large pH dependence of Tyr_D oxidation. Indeed, our data are consistent with D2-His189 being neutral both in the Tyr_D and Tyr_D states in the whole pH6-10 range. We show that the interactions between reduced Tyr_D and D2-His189 are modulated by the pH. At pH greater than 7.5, the ν(CO) mode frequency of Tyr_D indicates that Tyr_D is involved in a strong hydrogen bond, as a hydrogen bond donor only, in a fraction of the PSII centers. At pH below 7.5, the hydrogen-bonding interaction formed by Tyr_D is weaker and Tyr_D could be also involved as a hydrogen bond acceptor, according to calculations performed by Takahashi and Noguchi [J. Phys. Chem. B 2007 111, 13833-13844]. The involvement of Tyr_D in this strong hydrogen-bonding interaction correlates with the ability to oxidize Tyr_D at cryogenic temperatures and rapidly at room temperature. A strong hydrogen-bonding interaction is also observed at pH 6 in the D2-Gln164Glu mutant, showing that the residue at position D2-164 regulates the properties of Tyr_D. The IR data point to the role of a protonatable group(s) (with a pK_a of ≈ 7) other than D2-His189 and Tyr_D, in modifying the characteristics of the Tyr_D hydrogen-bonding interactions, and hence its oxidation properties. It remains to be determined whether the strong hydrogen-bonding interaction involves D2-His189 and if Tyr_D oxidation involves the same proton transfer route at low and at high pH.     

Cationic state distribution over the chlorophyll d-containing P(D1)/P(D2) pair in photosystem II

Most of the chlorophyll (Chl) cofactors in photosystem II (PSII) from Acaryochloris marina are Chld, although a few Chla molecules are also present. To evaluate the possibility that Chla may participate in the P(D1)/P(D2) Chl pair in PSII from A. marina, the P(D1)(?+)/P(D2)(?+) charge ratio was investigated using the PSII crystal structure analyzed at 1.9-? resolution, while considering all possibilities for the Chld-containing P(D1)/P(D2) pair, i.e., Chld/Chld, Chla/Chld, and Chld/Chla pairs. Chld/Chld and Chla/Chld pairs resulted in a large P(D1)(?+) population relative to P(D2)(?+), as identified in Chla/Chla homodimer pairs in PSII from other species, e.g., Thermosynechococcus elongatus PSII. However, the Chld/Chla pair possessed a P(D1)(?+)/P(D2)(?+) ratio of approximately 50/50, which is in contrast to previous spectroscopic studies on A. marina PSII. The present results strongly exclude the possibility that the Chld/Chla pair serves as P(D1)/P(D2) in A. marina PSII. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.     

Phosphorescence analysis of the chlorophyll triplet state in preparations of photosystem II

The low-temperature (77 K) phosphorescence of chlorophyll (Chl) in the reaction centres (D1D2-cyt b559-particles) and the core complexes of photosystem II isolated from higher plants was studied. Two phosphorescence spectral bands with the emission maxima at 950 and 977 nm, excitation maxima at 666 and 675-680 nm, and the lifetimes equal to 2 and 1.5 ms, respectively, were registered. The data indicate that the phosphorescence corresponds to the triplet Chl a molecules spatially separated from carotenoids. In samples treated by potassium ferricyanide and frozen under illumination by red light, the intensities of both bands were reduced, but the decrease of the short-wavelength 950-nm band was much more pronounced. This allows an assumption that the short-wavelength phosphorescence belongs to Chl a molecules, which are more accessible for ferricyanide because they are located on the surface of the chlorophyll-protein complexes, whereas the long-wavelength phosphorescence is emitted by the Chl molecules located inside the D1D2 heterodimer and therefore, is more protected by protein macromolecules.     

Functional implications on the mechanism of the function of photosystem II including water oxidation based on the structure of photosystem II

The structure of photosystem I at 3.8 A resolution illustrated the main structural elements of the water-oxidizing photosystem II complex, including the constituents of the electron transport chain. The location of the Mn cluster within the complex has been identified for the first time to our knowledge. At this resolution, no individual atoms are visible, however, the electron density of the Mn cluster can be used to discuss both the present models of the Mn cluster as revealed from various spectroscopic methods and the implications for the mechanisms of water oxidation. Twenty-six chlorophylls from the antenna system of photosystem II have been identified. They are arranged in two layers, one close to the stromal side and one close to the lumenal side. Comparing the structure of the antenna system of photosystem II with the chlorophyll arrangement in photosystem I, which was recently determined at 2.5 A resolution shows that photosystem II lacks the central domain of the photosystem I antenna, which is discussed in respect of the repair cycle of photosystem II due to photoinhibition.     

Replacement of tyrosine D with phenylalanine affects the normal proton transfer pathways for the reduction of P680+ in oxygen-evolving photosystem II particles from Chlamydomonas

We have probed the electrostatics of P680(+) reduction in oxygenic photosynthesis using histidine-tagged and histidine-tagged Y(D)-less Photosystem II cores. We make two main observations: (i) that His-tagged Chlamydomonas cores show kinetics which are essentially identical to those of Photosystem II enriched thylakoid membranes from spinach; (ii) that the microsecond kinetics, previously shown to be proton/hydrogen transfer limited [Schilstra et al. (1998) Biochemistry 37, 3974-3981], are significantly different in Y(D)-less Chlamydomonas particles when compared with both the His-tagged Chlamydomonas particles and the spinach membranes. The oscillatory nature of the kinetics in both Chlamydomonas samples is normal, indicating that S-state cycling is unaffected by either the histidine-tagging or the replacement of tyrosine D with phenylalanine. We propose that the effects on the proton-coupled electron transfers of P680(+) reduction in the absence of Y(D) are likely to be due to pK shifts of residues in a hydrogen-bonded network of amino acids in the vicinity of Y(Z). Tyrosine D is 35 A from Y(Z) and yet has a significant influence on proton-coupled electron transfer events in the vicinity of Y(Z). This finding emphasizes the delicacy of the proton balance that Photosystem II has to achieve during the water splitting process.     

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

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

Influence of histidine-198 of the D1 subunit on the properties of the primary electron donor, P680, of photosystem II in Thermosynechococcus elongatus

The influence of the histidine axial ligand to the PD1 chlorophyll of photosystem II on the redox potential and spectroscopic properties of the primary electron donor, P680, was investigated in mutant oxygen-evolving photosystem II (PSII) complexes purified from the thermophilic cyanobacterium Thermosynechococcus elongatus. To achieve this aim, a mutagenesis system was developed in which the psbA1 and psbA2 genes encoding D1 were deleted from a His-tagged CP43 strain (to generate strain WT*) and mutations D1-H198A and D1-H198Q were introduced into the remaining psbA3 gene. The O2-evolving activity of His-tagged PSII isolated from WT* was found to be significantly higher than that measured from His-tagged PSII isolated from WT in which psbA1 is expected to be the dominantly expressed form. PSII purified from both the D1-H198A and D1-H198Q mutants exhibited oxygen-evolving activity as high as that from WT*. Surprisingly, a variety of kinetic and spectroscopic measurements revealed that the D1-H198A and D1-H198Q mutations had little effect on the redox and spectroscopic properties of P680, in contrast to the earlier results from the analysis of the equivalent mutants constructed in Synechocystis sp. PCC 6803 [B.A. Diner, E. Schlodder, P.J. Nixon, W.J. Coleman, F. Rappaport, J. Lavergne, W.F. Vermaas, D.A. Chisholm, Site-directed mutations at D1-His198 and D2-His197 of photosystem II in Synechocystis PCC 6803: sites of primary charge separation and cation and triplet stabilization, Biochemistry 40 (2001) 9265-9281]. We conclude that the nature of the axial ligand to PD1 is not an important determinant of the redox and spectroscopic properties of P680 in T. elongatus.     

Effect of the dielectric constant of the medium on the photooxidation of chlorophyll a by parabenzoquinone]

Spectrophotometric study was carried out of the effect of medium polarity on the parabenzoquinone photooxidation of chlorophyll "a" at lower temperature using as solvents toluene and CCl4 containing different quantities of polaric solvents, methanole, ethanole, amilic alkohol, acetone. It is shown that the photoreaction with the formation of primary photooxidized form of the pigment starts only if some amount of the polar component is in the medium. The data are in favour of the suggestion that chlorophyll and quinone in non-polar medium form an reaction complex which disintegrates under the given conditions at a definite value of medium polarity.