Chlorophyll Photosystem Research Articles

A Simple Expression for the Screening of Excitonic Couplings between Chlorophylls as Inferred for Photosystem I Trimers.

The Coulomb coupling between transition densities of the pigments in photosynthetic pigment-protein complexes, termed excitonic coupling, is a key factor for the description of optical spectra and energy transfer. A challenging question is the quantification of the screening of the excitonic coupling by the optical polarizability of the environment. We use the equivalence between the sophisticated quantum chemical polarizable continuum (PCM) model and the simple electrostatic Poisson-TrEsp approach to analyze the distance and orientation dependence of the dielectric screening between chlorophylls in photosystem I trimers. On the basis of these calculations we find that the vacuum couplings Vmn(0) and the couplings in the dielectric medium Vmn=fmnVmn(0) are related by the empirical screening factor fmn=0.60+39.6θ(|κmn|-1.17)exp(-0.56Rmn/Å), where κmn is the usual orientational factor of the dipole-dipole coupling between the pigments, Rmn is the center-to-center distance, and the Heaviside-function θ(|κmn|-1.17) ensures that the exponential distance dependence only contributes for in-line type dipole geometries. We are confident that the present expression can be applied also to other pigment-protein complexes with chlorophyll or related pigments of similar shape. The variance between the Poisson-TrEsp and the approximate coupling values is found to decrease by a factor of 8 and 3-4 using the present expression, instead of an exponential distance dependent or constant screening factor, respectively, assumed previously in the literature.

Dissection of respiratory and cyclic electron transport in Synechocystis sp. PCC 6803.

Cyclic electron transport (CET) is an attractive hypothesis for regulating photosynthetic electron transport and producing the additional ATP in oxygenic phototrophs. The concept of CET has been established in the last decades, and it is proposed to function in the progenitor of oxygenic photosynthesis, cyanobacteria. The in vivo activity of CET is frequently evaluated either from the redox state of the reaction center chlorophyll in photosystem (PS) I, P700, in the absence of PSII activity or by comparing PSI and PSII activities through the P700 redox state and chlorophyll fluorescence, respectively. The evaluation of CET activity, however, is complicated especially in cyanobacteria, where CET shares the intersystem chain, including plastoquinone, cytochrome b6/f complex, plastocyanin, and cytochrome c6, with photosynthetic linear electron transport (LET) and respiratory electron transport (RET). Here we sought to distinguish the in vivo electron transport rates in RET and CET in the cyanobacterium Synechocystis sp. PCC 6803. The reduction rate of oxidized P700 (P700+) decreased to less than 10% when PSII was inhibited, indicating that PSII is the dominant electron source to PSI but P700+ is also reduced by electrons derived from other sources. The oxidative pentose phosphate (OPP) pathway functions as the dominant electron source for RET, which was found to be inhibited by glycolaldehyde (GA). In the condition where the OPP pathway and respiratory terminal oxidases were inhibited by GA and KCN, the P700+ reduction rate was less than 1% of that without any inhibitors. This study indicate that the electron transport to PSI when PSII is inhibited is dominantly derived from the OPP pathway in Synechocystis sp. PCC 6803.

Direct Evidence for Excitation Energy Transfer Limitations Imposed by Low-Energy Chlorophylls in Photosystem I-Light Harvesting Complex I of Land Plants.

The overall efficiency of photosynthetic energy conversion depends both on photochemical and excitation energy transfer processes from extended light-harvesting antenna networks. Understanding the trade-offs between increase in the antenna cross section and bandwidth and photochemical conversion efficiency is of central importance both from a biological perspective and for the design of biomimetic artificial photosynthetic complexes. Here, we employ two-dimensional electronic spectroscopy to spectrally resolve the excitation energy transfer dynamics and directly correlate them with the initial site of excitation in photosystem I–light harvesting complex I (PSI-LHCI) supercomplex of land plants, which has both a large antenna dimension and a wide optical bandwidth extending to energies lower than the peak of the reaction center chlorophylls. Upon preferential excitation of the low-energy chlorophylls (red forms), the average relaxation time in the bulk supercomplex increases by a factor of 2–3 with respect to unselective excitation at higher photon energies. This slowdown is interpreted in terms of an excitation energy transfer limitation from low-energy chlorophyll forms in the PSI-LHCI. These results aid in defining the optimum balance between the extension of the antenna bandwidth to the near-infrared region, which increases light-harvesting capacity, and high photoconversion quantum efficiency.

Characterization of the low-temperature triplet state of chlorophyll in photosystem II core complexes: Application of phosphorescence measurements and Fourier transform infrared spectroscopy

Phosphorescence measurements at 77 K and light-induced FTIR difference spectroscopy at 95 K were applied to study of the triplet state of chlorophyll a (3Chl) in photosystem II (PSII) core complexes isolated from spinach. Using both methods, 3Chl was observed in the core preparations with doubly reduced primary quinone acceptor QA. The spectral parameters of Chl phosphorescence resemble those in the isolated PSII reaction centers (RCs). The main spectral maximum and the lifetime of the phosphorescence corresponded to 955±1nm and of 1.65±0.05ms respectively; in the excitation spectrum, the absorption maxima of all core complex pigments (Chl, pheophytin a (Pheo), and β-carotene) were observed. The differential signal at 1667(−)/1628(+)cm−1 reflecting a downshift of the stretching frequency of the 131-keto C=O group of Chl was found to dominate in the triplet-minus-singlet FTIR difference spectrum of core complexes. Based on FTIR results and literature data, it is proposed that 3Chl is mostly localized on the accessory chlorophyll that is in triplet equilibrium with P680. Analysis of the data suggests that the Chl triplet state responsible for the phosphorescence and the FTIR difference spectrum is mainly generated due to charge recombination in the reaction center radical pair P680+PheoD1−, and the energy and temporal parameters of this triplet state as well as the molecular environment and interactions of the triplet-bearing Chl molecule are similar in the PSII core complexes and isolated PSII RCs.

Long-wavelength chlorophylls in photosystem I of cyanobacteria: Origin, localization, and functions

The structural organization of photosystem I (PSI) complexes in cyanobacteria and the origin of the PSI antenna long-wavelength chlorophylls and their role in energy migration, charge separation, and dissipation of excess absorbed energy are discussed. The PSI complex in cyanobacterial membranes is organized preferentially as a trimer with the core antenna enriched with long-wavelength chlorophylls. The contents of long-wavelength chlorophylls and their spectral characteristics in PSI trimers and monomers are species-specific. Chlorophyll aggregates in PSI antenna are potential candidates for the role of the long-wavelength chlorophylls. The red-most chlorophylls in PSI trimers of the cyanobacteria Arthrospira platensis and Thermosynechococcus elongatus can be formed as a result of interaction of pigments peripherally localized on different monomeric complexes within the PSI trimers. Long-wavelength chlorophylls affect weakly energy equilibration within the heterogeneous PSI antenna, but they significantly delay energy trapping by P700. When the reaction center is open, energy absorbed by long-wavelength chlorophylls migrates to P700 at physiological temperatures, causing its oxidation. When the PSI reaction center is closed, the P700 cation radical or P700 triplet state (depending on the P700 redox state and the PSI acceptor side cofactors) efficiently quench the fluorescence of the long-wavelength chlorophylls of PSI and thus protect the complex against photodestruction.

On the dimerization of chlorophyll in photosystem II

In photosystem II, absorbed light energy is transferred to a reaction centre consisting of chlorophyll units. Release of an electron from the reaction centre is the starting point for the charge separation and electron transport chain in PSII. Crystal structures of the reaction centre have identified two chlorophyll monomers forming a dimer with a partial structural overlap, thus being stabilized by van der Waals interactions. However, the magnitude of this interaction is not accurately known. In this work, the structure of the chlorophyll dimer has been optimized for the first time using dispersion-corrected density functional theory (B3LYP-DCP) revealing the magnitude of dimerization to be approximately -17 kcal mol(-1). The dispersion interaction is shown to be of great significance for the chlorophyll dimer stabilization. In addition, the redox potential of the chlorophyll dimer is calculated to be 1283 mV in good agreement with recent experimental data.

Preferential pathways for light-trapping involving β-ligated chlorophylls

The magnesium atom of chlorophylls (Chls) is always five- or six-coordinated within chlorophyll–protein complexes which are the main light-harvesting systems of plants, algae and most photosynthetic bacteria. Due to the presence of stereocenters and the axial ligation of magnesium the two faces of Chls are diastereotopic. It has been previously recognized that the α-configuration having the magnesium ligand on the opposite face of the 17-propionic acid moiety is more frequently encountered and is more stable than the more seldom β-configuration that has the magnesium ligand on the same face [T.S. Balaban, P. Fromme, A.R. Holzwarth, N. Krauβ, V.I. Prokhorenko, Relevance of the diastereotopic ligation of magnesium atoms in chlorophylls in Photosystem I, Biochim. Biophys. Acta (Bioenergetics), 1556 (2002) 197–207; T. Oba, H. Tamiaki, Which side of the π-macrocycle plane of (bacterio)chlorophylls is favored for binding of the fifth ligand? Photosynth. Res. 74 (2002) 1–10]. In photosystem I only 14 Chls out of a total of 96 are in a β-configuration and these occupy preferential positions around the reaction center. We have now analyzed the α/β dichotomy in the homodimeric photosystem II based on the 2.9 Å resolution crystal structure [A. Guskov, J. Kern, A. Gabdulkhakov, M. Broser, A. Zouni, W. Saenger, Cyanobacterial photosystem II at 2.9 Å resolution: role of quinones, lipids, channels and chloride, Nature Struct. Mol. Biol. 16 (2009) 334–342] and find that out of 35 Chls in each monomer only 9 are definitively in the β-configuration, while 4 are uncertain. Ab initio calculations using the approximate coupled-cluster singles-and-doubles model CC2 [O. Christiansen, H. Koch, P. Jørgensen, The second-order approximate coupled cluster singles and doubles model CC2, Chem. Phys. Lett. 243 (1995) 409–418] now correctly predict the absorption spectra of Chls a and b and conclusively show for histidine, which is the most frequent axial ligand of magnesium in chlorophyll–protein complexes, that only slight differences (< 4 nm) are encountered between the α- and β-configurations. Significant red shifts (up to 50 nm) can, however, be encountered in excitonically coupled β–β-Chl dimers. Surprisingly, in both photosystems I and II very similar “special” β–β dimers are encountered at practically the same distances from P700 and P680, respectively. In purple bacteria LH2, the B850 ring is composed exclusively of such tightly coupled β-bacteriochlorophylls a. A statistical analysis of the close contacts with the protein matrix (< 5 Å) shows significant differences between the α- and β-configurations and the subunit providing the axial magnesium ligand. The present study allows us to conclude that the excitation energy transfer in light-harvesting systems, from a peripheral antenna towards the reaction center, may follow preferential pathways due to structural reasons involving β-ligated Chls.

Room temperature photooxidation of beta-carotene and peripheral chlorophyll in photosystem II reaction centre.

Differential kinetic absorption spectra were measured during actinic illumination of photosystem II reaction centres and core complexes in the presence of electron acceptors silicomolybdate and ferricyanide. The spectra of samples with ferricyanide differ from those with both ferricyanide and silicomolybdate. Near-infrared spectra show temporary beta-carotene and peripheral chlorophyll oxidation during room temperature actinic illumination. Peripheral chlorophyll is photooxidized even after decay of beta-carotene oxidation activity and significant reduction of beta-carotene content in both reaction centres and photosystem II core complexes. Besides, new carotenoid cation is observed after about 1 s of actinic illumination in the reaction centres when silicomolybdate is present. Similar result was observed in PSII core complexes. HPLC analyses of illuminated reaction centres reveal several novel carotenoids, whereas no new carotenoid species were observed in HPLC of illuminated core complexes. Our data support the proposal that pigments of inner antenna are a sink of cations originating in the photosystem II reaction centre.

Protective dissipation of excess absorbed energy by photosynthetic apparatus of cyanobacteria: role of antenna terminal emitters

Two mechanisms of photoprotective dissipation of the excessively absorbed energy by photosynthetic apparatus of cyanobacteria are described that divert energy from reaction centers. Energy dissipation, monitored as nonphotochemical fluorescence quenching, occurs at different steps of energy transfer within the phycobilisomes or core antenna of photosystem I. Although these mechanisms differ significantly, in both cases, energy dissipates mainly from terminal emitters: allophycocyanin B or core membrane linker protein (L(CM)) in phycobilisomes, or the longest-wavelength chlorophylls in photosystem I antenna. It is supposed that carotenoid-induced energy dissipation in phycobilisomes is triggered by light-induced transformation of the nonquenched state of antenna into quenched state due to conformation changes caused by orange carotinoid-binding protein (OCP)-phycobilisome interaction. Fluorescence of the longest-wavelength chlorophylls of photosystem I antenna is strongly quenched by P700 cation radical or by P700 triplet state, dependent on redox state of the acceptor side cofactors of photosystem I.

Di(μ-oxo) Binuclear Manganese(III,IV) Poly(bipyridyl) Complexes Bearing Four Ruthenium(II) Photoactive Units: Synthesis, Characterization, and Photoinduced Electron-Transfer Properties

In order to model the photoinduced electron-transfer reactions from the manganese cluster to the photoactive P680 chlorophylls in photosystem II, three heterohexanuclear complexes, [Mn2III,IVO2[RuII(bpy)2(Ln)]4]11+ [bpy = 2,2'-bipyridine, n = 2 (1a), 4 (1b), 6 (1c)], in which one MnIII,IV(micro-O)2 center is covalently linked to four RuII(bpy)3-like moieties by bridged bis(bipyridine) Ln ligands, have been synthesized and characterized. The electrochemical, photophysical, and photochemical properties of these complexes have been investigated in CH3CN. The cyclic voltammograms and rotating-disk electrode curves of the three complexes show the presence of two very close successive reversible oxidation processes corresponding to the Mn2III,IV/Mn2IV,IV and RuII/RuIII redox couples (estimated E1/2 approximately 0.82 and 0.90 V, respectively). The lower potential of the Mn2III,IV subunit compared to those of the RuII moieties indicates that the RuIII species can act as an efficient oxidant toward the Mn2III,IV core. The two oxidized forms of the complexes [Mn2IV,IVO2[RuII(bpy)2(Ln)]4]12+ (2a-c) and [Mn2IV,IVO2[RuIII(bpy)2(Ln)]4]16+ (3a-c) obtained in good yields (>90% for 2a-c and >85% for 3a-c) by sequential electrolyses are very stable. Photophysical studies show that the 3MLCT excited state of the Ru(bpy)3 centers is moderately quenched by the Mn2III,IV(micro-O)2 core (15-25% depending on the length of the bridging alkyl chain). Nevertheless, this energy transfer can be easily short-circuited in the presence of an external irreversible electron acceptor like the (4-bromophenyl)diazonium cation, by an electron transfer leading, in a stepwise fashion, to the stable one- and five-electron-oxidized species 2a-c and 3a-c, respectively, also in good yields, under continuous irradiation of the solutions. Electro- and photoinduced oxidation experiments have been followed by UV-visible and electron paramagnetic resonance spectroscopy.

Does Herbicide Resistance Have a Cost in Brassica rapa?

The evolution of resistance to pesticides has become a classic example of natural selection at work (Palumbi, 2001). As humans chemically modify their environment, organisms with short generation times respond. Examples include resistance of bacteria to antibiotics, of insects to insecticides, and of weeds to herbicides. Here the focus is on resistance to herbicides in weedy plants. In particular, genetic strains of rapid-cycling Brassica rapa offer opportunities to study resistance to the herbicide atrazine, a triazine herbicide. In the presence of the herbicide, individuals of the resistant strain survive whereas individuals of the susceptible strain die. This experiment has been used in a college-level biology class but may, also be applicable to an Advanced Placement Biology class in high school. In addition to the biological content, students gain experience in hypothesis formation and testing, experimental design, and statistical analysis. Herbicide resistance may come with a cost to the organism. Resistance to triazine herbicides is often dependent on a mutation that also inhibits photosynthesis (Warwick, 1991; Holt et al., 1993). For atrazine to kill a plant, it first binds to the polypeptide psbA, located in the thylacoid membrane of chloroplasts. In resistant plants of Amaranthus hybridus, the herbicide fails to bind to psbA. One mutation, in the chloroplast DNA coding for psbA, accounts for the difference between susceptible and resistant plants (Hirschberg & McIntosh, 1983). The mutation that confers resistance also inhibits electron transfer in Photosystem II and under certain environmental conditions will inhibit the rate of photosynthesis (Dekker & Sharkey, 1992). Photosystem II is part of the energy-capturing apparatus of chloroplasts. When chlorophyll in Photosystem II absorbs a photon of light, it initiates a complex cascade of electron transport that eventually captures the energy in chemical form in the chloroplast. If electron transport is inhibited, less energy, can be captured (see Campbell & Reece, 2002, for a lucid and detailed explanation). In the absence of the herbicide, resistant organisms may be disadvantaged. Reduction of photosynthesis will lead to reduced biomass accumulation and may negatively affect competitive ability and fitness (Warwick, 1991; Warwick & Black, 1994). Williams et al. (1995) worked with two strains of jimsonweed (Datura stramonium) that were either susceptible or resistant to the triazine herbicides. They found that resistant jimsonweeds, when grown alone, had lower total biomass and lower reproductive biomass than susceptible jimsonweeds. When jimsonweed was grown in competition with maize, its total biomass and reproductive biomass were still lower than without competition. Resistant jimsonweeds were more negatively affected by competition than susceptible jimsonweeds. Cost of resistance is defined as the reduction in fitness of resistant plants grown in the absence of herbicide. Brassica rapa, in the form of Wisconsin Fast Plants[TM] (Hafner, 1990), is ideal for studying the cost of herbicide resistance. An atrazine-resistant strain (Aaa, Rci, zr, atrazine resistant) can be grown alone and in combination with an atrazine-susceptible strain without the use of herbicide. Cost of resistance can be measured as the reduction in growth and/or reproduction of the resistant strain relative to the susceptible strain. Competitive ability can be measured by comparing plants of one strain grown alone with plants of the same strain grown in combination with the second strain. When two plants of the same strain are grown together they experience intrastrain competition, whereas a resistant plant and a susceptible plant growing together experience inter-strain competition. Based on the experiment described below, students should be able to answer the questions: * Does herbicide resistance have a cost in terms of growth and reproduction in rapid-cycling Brassica rapa? …

Circular dichroism of the peripheral chlorophylls in photosystem II reaction centers revealed by electrochemical oxidation

Visible absorption spectra and circular dichroism (CD) of the red absorption band of isolated photosystem II reaction centers were measured at room temperature during progressive bleaching by electrochemical oxidation, in comparison with aerobic photochemical destruction, and with anaerobic photooxidation in the presence of the artificial electron acceptor silicomolybdate. Initially, selective bleaching of peripheral chlorophylls absorbing at 672 nm was obtained by electrochemical oxidation at +0.9 V, whereas little selectivity was observed at higher potentials. Illumination in the presence of silicomolybdate did not cause a bleaching but a spectral broadening of the 672-nm band was observed, apparently in response to the oxidation of carotene. The 672-nm absorption band is shown to exhibit a positive CD, which accounts for the 674-nm shoulder in CD spectra at low temperature. The origin of this CD is discussed in view of the observation that all CD disappears with the 680-nm absorption band during aerobic photodestruction.

The orientations of core antenna chlorophylls in photosystem II are optimized to maximize the quantum yield of photosynthesis

In photosystem II (PSII) the probability that energy absorbed by core antenna chlorophyll (Chl) is transferred to the reaction center (RC) is extremely high. Although close proximity between antenna Chl ensures a high transfer efficiency, relative pigment orientation can fractionally modify it. This level of refinement has often been assumed to be superfluous as so many subsequent processes limit the overall efficiency of photosynthesis. Nevertheless, did natural selection act on the most efficient step of energy conversion in PSII by optimizing the orientation of antenna Chl? Our Monte Carlo simulations sampled the orientation space of Chls in kinetic models for excitation energy transfer based on the X-ray structures of PSII from Thermosynechococcus vulcanus and Synechocystis elongatus. Our results revealed that the orientations of key antenna Chls are optimized to maximize photosynthesis while the orientations of the two peripheral RC Chls (Chl Z) are not.

Deciphering the 820 nm signal: redox state of donor side and quantum yield of Photosystem I in leaves.

By recording leaf transmittance at 820 nm and quantifying the photon flux density of far red light (FRL) absorbed by long-wavelength chlorophylls of Photosystem I (PS I), the oxidation kinetics of electron carriers on the PS I donor side was mathematically analyzed in sunflower (Helianthus annuus L.), tobacco (Nicotiana tabacum L.) and birch (Betula pendula Roth.) leaves. PS I donor side carriers were first oxidized under FRL, electrons were then allowed to accumulate on the PS I donor side during dark intervals of increasing length. After each dark interval the electrons were removed (titrated) by FRL. The kinetics of the 820 nm signal during the oxidation of the PS I donor side was modeled assuming redox equilibrium among the PS I donor pigment (P700), plastocyanin (PC), and cytochrome f plus Rieske FeS (Cyt f + FeS) pools, considering that the 820 nm signal originates from P700(+) and PC(+). The analysis yielded the pool sizes of P700, PC and (Cyt f + FeS) and associated redox equilibrium constants. PS I density varied between 0.6 and 1.4 mumol m(-2). PS II density (measured as O(2) evolution from a saturating single-turnover flash) ranged from 0.64 to 2.14 mumol m(-2). The average electron storage capacity was 1.96 (range 1.25 to 2.4) and 1.16 (range 0.6 to 1.7) for PC and (Cyt f + FeS), respectively, per P700. The best-fit electrochemical midpoint potential differences were 80 mV for the P700/PC and 25 mV for the PC/Cyt f equilibria at 22 degrees C. An algorithm relating the measured 820 nm signal to the redox states of individual PS I donor side electron carriers in leaves is presented. Applying this algorithm to the analysis of steady-state light response curves of net CO(2) fixation rate and 820 nm signal shows that the quantum yield of PS I decreases by about half due to acceptor side reduction at limiting light intensities before the donor side becomes oxidized at saturating intensities. Footnote:

Relevance of the diastereotopic ligation of magnesium atoms of chlorophylls in Photosystem I

The central magnesium (Mg) atoms of natural occurring tetrapyrroles such as chlorophylls (Chls) and bacteriochlorophylls (BChls) are typically five-coordinated, a fact which leads to the formation of diastereoisomers if the Mg–ligand bond is stable on the time scale of the observation method. This possibility has only been briefly addressed before in a CD-study of BChl c aggregates [T.S. Balaban, A.R. Holzwarth, K. Schaffner, J. Mol. Struct. 349 (1995) 183]. On the basis of the chlorophyll–protein complex photosystem I (PSI), which has recently been characterized by single crystal crystallography [P. Jordan, P. Fromme, H.T. Witt, O. Klukas, W. Saenger, N. Krauß, Nature 411 (2001) 909], we find that chlorophyll a molecules are much more frequently bound by the protein matrix from one side ( anti) than the other one ( syn) in a ratio of 82:14, which corresponds to a significant ΔΔ G value of 4.3 kJ/mol. Syn and anti denote the orientation of the Mg–ligand with respect to the 17-propionic acid esterified by phytol. Furthermore, by parallel sequence analysis we find that the binding sites for both syn and anti chlorophylls have been strongly conserved during evolution—a fact which stresses the nonrandom manner in which chlorophylls are bound by the apoprotein in antenna complexes, in order to exert efficiently their light harvesting function and energy funnelling. Most remarkably, all the syn chlorophylls are part of the inner core antenna system. Results from semiempirical quantum mechanical and detailed exciton coupling calculations allow us to speculate on the functional relevance of the diasteretopicity for PSI functioning.

Photosystem I: both sides now

Light absorbed by the chlorophylls of photosystem (PS) I drives charge separation and electron transfer through a chain of cofactors arranged along one half of the quasi-symmetrical heterodimeric core of the reaction centre. The other branch of the reaction centre is switched off though the influence of the protein, as is the case with the PSII and purple bacterial reaction centres. At least, that is the way the textbooks see it. New data 1 Guergova-Kuras M. et al. Evidence for two active branches for electron transfer in photosystem I. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 4437-4442 Crossref PubMed Scopus (261) Google Scholar show that this view is probably wrong and that, in fact, electron transfer occurs on both sides of the reaction centre in PSI.

Biosynthesis of chlorophyll of photosystem II reaction centers in plant leaves in early stages of etiolation.

Spectral methods were used to study the sequences of chlorophyll biosynthesis reactions in etiolated pea, bean, and maize plants in early stages (3-4 days) of growth. For these juvenile plants, along with the reaction chain known for mature (7-9 day-old) plants, a new reaction chain was found which started with phototransformation of the long-wavelength form PChld 686/676 into PChld 653/648. (PChld 653/648 differs from the main known precursor form PChld 655/650). The subsequent photoreduction of PChld 653/648 leads to the formation of Chld 684/676, which is transformed into Chl 688/680 in the course of a dark reaction. After completion of this reaction, fast (20-30 sec) quenching of the fluorescence of the reaction product is observed with the formation of non-fluorescent Chl 680. The reaction accompanied by pigment fluorescence quenching is absent in pea mutants with depressed function of Photosystem II reaction centers. This suggests that the newly found reaction chain leads to the formation of chlorophyll of the Photosystem II reaction center.

Selective Raman Scattering from the Core Chlorophylls in Photosystem I via Preresonant Near-Infrared Excitation

The Raman scattering characteristics of photosystem I (PSI) over the 200−1700-cm-1 frequency range have been examined using near-infrared (NIR) excitation (λex = 800 nm). The salient features observed in the spectra are as follows: (1) The Raman spectra are characteristic of neutral, pentacoordinate chlorophyll a (Chl a) molecules regardless of the oxidation state of the primary electron donor, P700. No Raman bands are observed for the oxidized primary donor, P700+, despite the fact that the 800-nm excitation wavelength is coincident with a NIR absorption feature of the π-cation radical species. (2) The redox state of P700 has a strong influence on the temperature dependence of the preresonance Raman (PRR) scattering intensities of the neutral Chls in PSI (Qy absorption maxima 660−720 nm). When P700 is neutral, the PRR intensities of the neutral Chls decrease approximately 4-fold on going from 77 to 200 K. When P700 is chemically or photochemically oxidized, the PRR scattering intensities of the neutral ...

Spectral resolution of low-energy chlorophylls in Photosystem I of Synechocystis sp. PCC 6803 through direct excitation

We have measured fluorescence spectra from Photosystem I (PS I) on a PS II-less mutant of the cyanobacterium Synechocystis sp. PCC 6803 at room temperature as a function of excitation wavelength. Our data show a gradual enhancement of long-wavelength fluorescence at 710 nm as the excitation wavelength is increased from 695 to 720 nm. This verifies the presence of low-energy chlorophylls (LE Chls), antenna Chls with energy levels below that of the primary electron donor, P700. The change in fluorescence with excitation wavelength is attributed to the finite time it takes for equilibration of excitations between the bulk and LE Chls. The spectra were deconvoluted into the sum of two basis spectra, one an estimate for fluorescence from the majority or bulk Chls and the other, the LE Chls. The bulk Chl spectrum has a major peak at 688 nm and a lower amplitude vibrational band around 745 nm and is assumed independent of excitation wavelength. The LE Chl spectrum has a major peak at 710 nm, with shoulders at 725 and 760 nm. The relative amplitude of emission at the vibrational side bands increases slightly as the excitation wavelength increases. The ratio of the fluorescence yields from LE Chls to that from bulk Chls ranges from 0.3 to 1.3 for excitation wavelengths of 695 to 720 nm, respectively. These values are consistent with a model where the LE Chls are structurally close to P700 allowing for direct transfer of excitations from both the bulk and LE Chls to P700.

Biosynthesis of non-fluorescent chlorophyll of Photosystem II core in greening plant leaves. Effect of etiolated plants growing under heat shock conditions (38 °C)

Preliminary dark incubation of etiolated pea and maize plants at 38 °C allowed to observe a new dark reaction of Chl biosynthesis occuring after photoconversion of protochlorophyllide Pchld 655/650 into chlorophyllide Chld 684/676. This reaction was accompanied by chlorophyllide esterification and by the bathochromic shift of pigment spectra: Chld 684/676 → Chl 688/680. After completion of the reaction, a rapid (20–30 s at 26 °C) quenching of Chl 688/680 low-temperature fluorescence was observed. The reaction Chld 684/676 → Chl 688/680 was inhibited under anaerobic conditions as well as in the presence of KCN; the reaction accompanied by Chl fluorescence quenching was inhibited in the leaves of pea mutants with impaired function of Photosystem II reaction centers. The spectra position of newly formed Chl, effects of Chl fluorescence quenching allowed to assume that the new dark reaction is responsible for biosynthesis of P–680, the key pigment of Photosystem II reaction centres.