Antennas and Reaction Centers of Photosynthetic Bacteria: Structure, Interactions and Dynamics
Abstract
The workshop on "Antennas and Reaction Centers of Photosynthetic Bac teria" was held at Feldafing, Bavaria (F. R. G. )' March 23-25, 1985. This workshop focussed on primary processes with emphasis on structure, inter actions and dynamics. It assessed structural, spectroscopic and dynamic data which have accumulated recently, providing an overview of the mech anism of the acquisition, storage and useful disposal of energy in bacterial photosynthesis. This volume is a record of the invited papers presented at the workshop. The material was organized into five sections: I. Antennas: Structure and Energy Transfer II. Reaction Centers: Structure and Interactions III. Electron Transfer: Theory and Model Systems IV. Reaction Cen,ters: Structure and Dynamics V. Model Systems on Function of Antennas and Reaction Centers I would like to express my gratitude to all the participants in the work shop for their contributions, and to the authors for the timely preparation of their manuscripts. I am indebted to the members of the organizing committee, Professors Sighart F. Fischer and Hugo Scheer for their most valuable assistance and advice. The workshop would not have been so successful without the help of my secretary, Frau Petra KahlfuB, and my coworkers in its organization. I thank Frau KahlfuB particularly also for her assistance in the preparation of these proceedings. The workshop was organized under the auspices of the Technical Uni versity of Munich, the Max-Planck-Society and the University of Munich.
Chapters (28)
Cyanobacteria possess light-harvesting organelles attached to the outer surface of the photosynthetic membrane, which conduct the energy of light to the membrane-associated reaction centers, in which charge- separation across the membrane occurs (1). These organelles, the phycobilisomes, form rod-like substructures composed of different, but chemically and structurally related components. A main component is C-phycocyanin. Amino acid sequences of phycocyanins of different bacteria have been determined and compared with each other and with sequences of other phycobilisome components, phycoerythrin and allophycocyanin (2, 3). These show extensive homology indicating common three-dimensional foldings.
Photosynthetic organisms cover most of their energy needs with sunlight. They have consequently developed a variety of adaptation mechanisms to compete efficiently for it. In higher plants, a dominant mechanism is the growth towards the light. Aquatic and microorganisms adapt commonly by chromatic adaptation of the photosynthetic antenna. The chlorophylls a and b are rather inefficient in collecting green light, and several additional pigment systems have evolved to fill this hole in the action spectrum.
Of all light-harvesting biliproteins, C-Phycocyanin (C-PC) is certainly the most extensively studied one [1]. With respect to the investigation of energy-transfer between the phycocyanobilin chromophors, this biliprotein has the advantage that by now much is known about the structure of both the chromophor [1] and the protein [2, 3]. It was furthermore established that the fluorescence properties depend sensitively on temperature and the state of aggregation (see e.g. [4]–[6] and references there). PRIESTLE et al. [4] found that the fluorescence behaviour changes significantly if phycobiliproteins are crystallized. In the case of C-PC from the cyanophyte Agmenellum quadruplicatum, the absorbance peak of a suspension of small C-PC crystals was red-shifted by only 7nm relative to the solution with hexamers, while the single crystal fluorescence was red-shifted by about 60 nm. The spectral distribution of the crystal fluorescence, whose lifetime was determined as ∼0.5 ns, changed with excitation wavelength.
Blue-green alga (cyanobacteria) , red alga and cryptophyceae contain phycobiliproteins as major light-harvesting pigments which gather light in the wavelength region of low Chl absorption. The chromophores in these pigments are open chain tetrapyrroles which are bound covalently to apoproteins [1]. It has been shown that the energy absorbed by phycobiliproteins feeds the small pool of Chl in these algae [2]. In cyanobacteria and red alga the phycobiliproteins form large supramolecular antenna complexes, so-called phycobilisomes (PBS) [3, 4] which are located at the outer surface of the thylakoid membrane. PBS are made up of two or three different types of phycobiliproteins which occur predominantly in hexameric aggregation. The first picosecond measurements of phycobiliprotein containing algae and isolated PBS were carried out by Porter et al. [5, 6]. Phycobiliproteins and PBS are interesting objects for time-resolved studies for several reasons. Unlike the Chl protein complexes of higher plants, different phycobiliproteins have their absorption and emission spectra fairly well separated, which more easily allows the detailed sequence of energy-transfer steps to be explored. Furthermore, the single-step transfer times seem to be significantly longer than those of Chl complexes, which puts these processes in a time-range accessible to picosecond techniques.
Some of the recent findings on the structures of antenna pigment-protein complexes from a variety of photosynthetic organisms [1–7] invite a re-evaluation of the mechanism of electronic excited-state energy-transfer in these structures, and in the larger assemblies containing them. The clustering of pigments in these structures, and their overall rotational symmetries, have no obvious purpose in a Förster model of energy-transfer, but may play specific roles if the transfer mechanism is more coherent. The question of the role of these rotationally symmetric structures in the handling of electronic excitation energy is addressed here, in an admittedly somewhat speculative fashion, specifically for the antenna assemblies of purple bacterial membranes.
During the past few years, reverse-phase high-performance liquid chromatography (RP-HPLC) has emerged as one of the techniques best suited for the isolation and separation of low molecular weight proteins (Mr 5000) and polypeptide fragments generated by enzymic or chemical cleavage of proteins. (1, 2). The distinct advantages of RP-HPLC, speed, sensitivity, reproducibility and resolution, have contributed to great advances in protein structure analysis. Despite the success in isolating hydrophilic polypeptides of even considerable size (3), it was not until recently that RP-HPLC was employed as a final purification step in the isolation of hydrophobic membrane proteins. Examples are the separation of subunitsof chloroplast coupling factor (4), of mitochondrial cytochrome c-oxidase (5) and various structural envelope proteins of viral origin (sendai-, polio-, influenza-virus; 6, 7, 8).
Fluorescence-detected magnetic resonance (FDMR) measurements in zero-field have been performed on the antenna bacteriochlorophyll (BChl) complexes of the purple photosynthetic bacteria Rhodopseudomonas (Rps.) sphaeroides strains R 26.1 and R 26 S (original mutant) and Rps. capsulata A1a+. In every case, at least two distinct triplet states are observed.
Crude chromatophore membranes of R. sphaeroides 2.4.1 were purified by differential centrifugation and suspended in buffer (20 mM Tris-HCl, pH 8.0, 2mM Tris — EDTA) to a final absorbance A
8501cm. An equal volume of 1.6% (w/v) β;-D-octylglucopyranoside (βOG) in water was added at room temperature, and the resulting solubilizate centrifuged at ~2×105 g for 60 min. at 4° C. The supernatant was brought to room temperature, and 10% nonanoyl-N-methyl-glucamide (Nga) was added to a final concentration of 1%. The B800-850-complex was isolated by column chromatography on DEAE-cellulose in the presence of 1% Nga and buffer (10 mM Tris-HCl, pH 8.0, lmM EDTA-Tris, 1 mM β;-mercaptoethanol). The B800-850-complex was eluted with 200 mM NaCl in Nga and buffer. Further purification was accomplished by repeating the DEAE chromatography. For the crystallization, 1% βOG was substituted for Nga by repeated concentration and dilution in an Amicon concentration cell and a final dialysis.
The B800–850 light-harvesting complex of Rps. capsulata was crystallized in the presence of detergents. The crystals were obtained by vapour diffusion technique using polyethylene glycol 4000 (Merck, for gas chromatography). A technique to produce large and thick crystals suitable for X-ray diffraction is described elsewhere (1). For spectroscopic work, due to the high optical density of the crystals, thin extended crystalline plates are advantageous. The crystallization technique was therefore modified. The best yield of thin crystals was found when ß-D-octylglucopyranoside (1% w/v) or octyltetraoxyethylene (1%) plus heptanetriol (3%) were used as detergents and sodium chloride (0.5 M) was present. Other conditions were similar to the ones given in (1). Similar crystallization methods were applied by us to the B800–850 complex of Rps. palustris, and small crystals were obtained. Crystallization of the B800–850 complex of Rps. spaeroides has also been reported by Allen et al. (see J.Allen, R.Taylor and G.Feher, this volume). Thin crystalline plates of the B800–850 complex of Rps. capsulata, along the shortest dimension, can have optical densities as low as 0.3 at 850 nm. They have the form of parallelograms with edges being 0.05 to 0.2 mm in length. These thin crystals have similar geometry to the triclinic ones used for X-ray diffraction (1). We therefore assume that both lattices are identical. Spectra were obtained both from thin crystals grown together with large crystals for X-ray studies and from thin crystals grown with the modified procedure as described above. Crystals were carefully pipetted with some mother liquor into a microcell, formed by two glass windows and a spacer of 20 urn thickness. The crystal was thus aligned with its largest surface parallel to the microcuvette walls.
The recent crystallographic work of DEISENHOFER et al. [1] has provided a beautifully detailed map of the structure of a photosynthetic bacterial reaction center (RC). With such a map in hand, it now is possible to explore the connection between the structure and the RC’s complicated spectroscopic properties. An understanding of this connection is critical, if we are to have an accurate picture of how the RC captures the energy of light.
Recent measurements of absorbance and fluorescence spectra of reaction centers of photosynthetic bacteria indicate that lowering the temperature induces a significant narrowing and red shift of the long wavelength dimer band, whereas the positions of all the other absorption bands are unaffected. This phenomenon has been observed in absorbance /1, 2/ and emission /1/ spectra (300 K, 77 K) of reaction centers of Rp. sphaeroides. Reaction centers of Rp. viridis behave similarly when comparing absorbance spectra at various temperatures between 300 K and 2 K /3–6/, magnetooptical absorbance difference spectra at 285 K and 115 K /7/ and fluorescence spectra at 77 K /6/ and 2 K /6, 8/.
Reaction centers from purple photosynthetic bacteria (Rhodospirillales) appear to generally contain two identical sets of pigments. One of these sets is predominantly (but most probably not exclusively, ref. 1) involved in the primary electron-transfers. Recent X-ray crystallographic data on reaction centers (RCs) from Rhodopseudomonas viridis show that these two sets of pigments are symmetrically arranged within the proteic core along a C2 axis (2). The relative orientations and distances of molecules are very much the same in the two sets. Spectroscopic data indicate an identical situation in RCs from Rps. sphaeroides (3). Hence the very different electron-transfer properties of the two sets must largely result from differences in local environments, which may, in particular, result in different redox potentials of molecules in the two sets (4). These local environments still cannot be determined in a detailed fashion from X-ray data.
The △m = 1 triplet EPR spectra of bacterial RC’s exhibit a unique AEEAAE electron spin polarization (ESP) pattern (A = absorptive, E = emissive), which can be explained by invoking an intermediary radical pair state [1, 2]. We have observed that this pattern changes into AEAEAE at ∿v20K for reduced Rps. viridis RC’s.
The special pair model for the primary donor of bacterial photosynthesis which was originally proposed on the basis of EPR spectroscopy [1] has now been confirmed by x-ray structural data [2]. The initial interpretation of the EPR linewidth of the donor cation was based on a symmetrical sharing of the unpaired electron on a dimer of chlorophyll or bacteriochlorophyll. The ground state of the special pair as presently revealed from the x-ray study is indeed a highly symmetrical structure with approximate C2 symmetry.
The technique of absorbance-detected magnetic resonance (ADMR) of triplet states and that of ADMR-monitored triplet-minus-singlet (T — S) absorbance-difference spectroscopy is discussed. Applications of T — S and linear dichroic T — S spectroscopy on photosynthetic reaction centers of bacterial and plant origin are reviewed.
The structure of the primary donor P in bacterial photosynthesis is of considerable interest due to the unique role this species plays in the very first events of the charge-separation process in the reaction center (RC). It could be shown that P is a specialized bacteriochlorophyll (BChl) [l] which was proposed by NORRIS et al. [2] to be a BChl-dimer. This “special pair” model was a matter of considerable controversy in recent years. Different dimer models [3, 4, 5] and perturbed monomer models [6, 7] have been proposed for the primary donor. The existence of a dimer has recently been demonstrated, at least for Rhodopseudomonas (Rp.) viridis RC’s by an X-ray structure analysis of RC single crystals [8]. An alternative approach to the characterization of the primary donor in its cationic state is provided by EPR and electron nuclear doubleresonance (ENDOR) spectroscopy [4, 5, 9–12]. In this paper we summarize the most recent contributions of EPR and ENDOR in elucidating the electronic structure details of the primary donor cation radicals. In particular P+.865in RC’s of Rp. sphaeroides and Rhodospirillum (R.) rubrum, and P+.960 in RC’s of Rp. viridis are studied. The isotropic hyperfine coupling constants (hfc’s) extracted by ENDOR and TRIPLE resonance [13, 14] are compared with values from advanced MO calculations, thereby relating the experimental values to structural details of the dimers.
At the heart of photosynthesis are the electron-transfer reactions between the primary reactants. To understand these from first principles, a knowledge of both the spatial as well as the electronic structure of the reactants is essential. The exciting and beautiful work on the three-dimensional structure of reaction centers (RCs) in Rps. viridis by the Munich group will be discussed in a subsequent session. Here, we would like to discuss experiments designed to elucidate the electronic structure of the quinone acceptors, Q
A
and Q
B
in Rps. sphaeroides R-26.1. This work is still continuing; the presentation should, therefore, be taken in the spirit of a progress report.
A time domain of particular interest in the study of photosynthetic charge separation is from 5 to 100 ns after the initiation of primary photochemistry within the reaction center. This regime is especially significant to the magnetic resonance spectroscopist because, although charge separation has proceeded to a radical pair state, the electrons on the radicals remain close enough to interact with each other magnetically during this time. Such magnetic interactions largely govern the fate of the radical pair state when the charges recombine; however, the dynamic evolution of the state occurs too quickly to be directly observable with the EPR, ENDOR, or ESE (electronspin echo) techniques. By using optically detected magnetic resonance (ODMR) and reaction yield-detected magnetic resonance (RYDMR), however, the time resolution necessary to observe short-lived paramagnetic states of the bacterial reaction center can be achieved, starting at about 5 ns.
The optical detection of recombination dynamics and its dependence on external magnetic fields gives access to kinetic and structural features of short-lived radical pairs (RPs) In reaction centers (RCs) of purple bacteria this RP consists of the cation of the bacteriochlorophyll dimer, (BC)+2 and the anion of the bacteriopheophytin, BP−and is formed within a few picoseconds by electron-transfer from the singlet excited dimer state. In quinone-depleted RCs at room temperature the RP is stabilized for approximately 10 ns. The initially formed RP is singlet-phased as its precursor state and recombines via the rate ks to form the ground state species. This is illustrated in the following kinetic scheme: Hyperfine interaction HFI of the two electron spins with their different nuclear spin environments can change the multiplicity of the RP spin state. This leads to a RP triplet state,thus providing a new channel of recombination forming the dimer triplet state with the rate kT . Such singlet-triplet transitions,however, can be hindered by exchange and spin dipolar interaction (J and D) between the radical electrons as well as by the recombination rates, which are responsible for the lifetime broadening of the RP states.
Being a membrane-bound process photosynthesis depends on the functional interaction of membrane-bound or associated proteins [1] . For molecules of the photosynthetic membrane carrying out the primary process of energy and electron- transfer it is therefore essential to study protein/lipid and lipid mediated protein/protein interaction.
The centerpiece of this meeting is the exciting measurements of the structure of the photosynthetic reaction center of R. viridis by MICHEL, DEISENHOFER and co-workers [1–3]. To understand the marriage of structure to function in this beautiful machine, we need to understand the basic principles of several processes, one of which is electron transfer (ET) between molecules. Things we must know are how electron transfer rates depend on distance, energy, spatial orientation, polarity of the medium and molecular structure of all the material involved. Our work is aimed at learning these basic principles through experimental measurements of electron transfer between molecules. The molecules we use are not components of photosynthesis, but are usually aromatic molecules. To learn how ET rates depend on distance, it has been necessary to hold electron donors and acceptors at fixed distances. It turns out that fixed distances are very helpful, perhaps even necessary, to answer the other questions as well.
The primary electron-transfer event of photosynthesis involves oxidation of the lowest excited singlet state of a chlorophyll electron donor by a nearby electron acceptor [1]. The distance between the donor and the acceptor is restricted by the surrounding reaction center protein. The high quantum efficiency of photosynthetic charge-separation depends on favorable electron-transfer rates between electron donors and acceptors that are positioned in precise spatial relationships relative to one another, and that possess redox potentials which result in movement of an electron down a stepped potential gradient. We recently prepared a series of restricted distance porphyrin-quinone donor-acceptor molecules designed to study electron-transfer proceeding from the lowest excited singlet state of the porphyrin, Fig. 1 [2]. We have measured the dependence of photoinduced electron-transfer rates and subsequent dark charge recombination rates on the free energy of reaction in these molecules [3]. We find that the rate-constants for both radical ion pair formation and recombination in these molecules depend on the exothermicity of the respective electron-transfer reaction in the manner originally proposed by Marcus and later modified and extended by others [4].
The efficiency of photoinduced charge separation in the photosynthetic reaction center is achieved by a choice of suitable electron-transfer rates which are fast enough in the forward and slow enough in the backward direction. The separated charges seem to be paying a minimum price in terms of irreversible energy losses. The cardinal question is why the reaction center functions so efficiently. An answer to this question is tantamount to explaining which properties of the reaction center protein material are responsible for the realization of its transfer rates.
The reaction center (RC) of the photosynthetic bacterium Rhodopseudomonas sphaeroides consists of three small polypeptides that hold a cluster of four molecules of bacteriochlorophyll-a (BChl), two molecules of bacteriopheophytin-a (BPh), one nonheme iron atom, and two molecules of ubiquinone [1, 2]. The RCs can be purified readily, and they recently have been crystallized [3], Although the crystal structure has not yet been solved, RCs from Rps. sphaeroides are functionally very similar to those from Rps. viridis whose structure has been solved to 3 Å resolution [4]. In both species, two of the four BChls form a closely interacting pair (P) that undergoes oxidation to a radical cation (P+) when the RC is excited with light. The electron is transferred to a BPh, and from there to a quinone (Q) that is close to, but probably not bound to the Fe. (In Rps. vi ridis, Q is menaquinone instead of ubiquinone.) In Rps. sphaeroides RCs, the photochemical electron-transfer reaction from P to Q has a quantum yield of essentially 100% [5], The kinetics of electron-transfer from the intermediate BPh anion radical (BPh−) to Q have been measured under a variety of conditions, and are similar in the two species, in spite of the difference between the qui nones [6–10], It has been suggested that one of the two BChls that are not part of P mediates electron-transfer from the excited singlet state of P (P*) to the BPh [10–12]. However, the evidence for the formation of a P+ BChl− radical pair prior to the formation of P+ BPh− is inconclusive [13].
Knowledge on structure and dynamics of photosynthetic reaction centers (RCs) is crucial to understand their function i.e. the light-driven charge separation process. The recently obtained structure of Rhodopseudomonas viridis /1, 2/ is an important step towards this goal. It has been shown that steady state optical spectra are mainly determined by the static structural arrangement of the pigments /3–7/. Time-resolved spectra are related more to the dynamical aspects of RCs. However, structural information on RCs is also useful to understand time-dependent spectra.
The initial photochemistry of photosynthesis involves a highly efficient electron-transfer reaction that generates an anion-cation radical pair. This primary charge-separation is followed by a cascade of electron-transfer steps in which the electron and hole become increasingly spatially separated and stabilized with respect to recombination, allowing the formation of stable chemical products. At each step, the rate of the forward electron transfer reaction exceeds the rate of the charge-recombination reaction by several orders of magnitude. In this paper I discuss results related to two aspects of the initial events in photosynthesis: the relationship between the energetics and kinetics of charge- separation and recombination in bacterial photosynthesis, and the distance and orientation dependence of excitation energy-transfer in semi-synthetic chlorophyll-protein complexes.
There are four obvious forms of bacteriochlorophyll a (Bchla) in purple photosynthetic bacteria with Qy transitions at 800, 850, 860 and 870 nm [1, 2] and three spectral forms of Bchlb with Qy transitions at 830, 960 and 1020 nm [3]. The Bchl molecules participate in two types of protein-chromophore complexes, serving as antennae pigments in the light-harvesting devices and primary electron donors in the photoreaction centers. All in vivo forms have their lowermost transition (Qy) bathochromically shifted, intensified by a factor of 1.5 – 2.0 and optically active with respect to the Qy transition of the in vitro monomers [5].
ENDOR spectra have been recorded for the anion radicals of p-benzoquinone, 2-methyl-p-benzoquinone and ubiquinone in frozen isopropanol solution. Hyperfine interaction between solvent -OH protons hydrogen-bonded to the semiquinone oxygens and the unpaired electron of the radical occurs and is detected by selective deuteration of the isopropanol hydroxyl group. The hyperfine tensor of the hydrogen-bonded proton is axial with A11 positive and equal to approximately twice the negative of A1, which indicates that the interaction is essentially dipolar in nature. For UQ−, A11 =6.1 MHz and A1=−2.8 MHz. These results demonstrate the utility of ENDOR in studying interactions between quinone radicals and their local environments, and should be useful in characterizing quinone binding sites in reaction centers and other quinone binding proteins.
... Superexchange coupling If a molecular bridge B is located between D and A, the interaction between D and A can be drastically increased by mixing of B states into the donor-acceptor coupling. [36,37]. In this "superexchange" model, the states of B are only virtually occupied; therefore also energetically higher states can influence the exchange interaction. ...
... 42 Recently, Hartwich et al. reported a charge transfer quantum yield of 1 upon P + excitation. 43 Michel-Beyerle and co-workers 44 have also reported a fluorescence up-conversion study of B* f Penergy transfer. The Pfluorescence rise time does not directly measure the energy transfer rate: the analysis of depolarization data presented here indicates that energy is transferred from B to P + and that P + undergoes internal conversion to P -. ...
The energy transfer from the accessory bacteriochlorophylls (B) to the special pair (P) in the photosynthetic reaction center has been time resolved with pump-probe polarization anisotropy measurements using 20-25 fs duration pulses near 800 nm. The pump excitation corresponds to 1.4 x 10{sup 6} photons/{mu}m{sup 2}: the `saturation intensity` for the charge separation quantum yield is 3 x 10{sup 7} photons/{mu}m{sup 2}. The initial pump-probe anisotropy is 0.4 and decays with a nearly 80 fs time constant, which we attribute to dipole reorientation by electronic energy transfer. Simultaneous kinetic modeling of the parallel, perpendicular, and magic angle pump-probe transients using the reaction center structure and dipole orientations is consistent with energy transfer proceeding in two steps: nearly 80 fs electronic energy transfer from the accessory bacteriochlorophylls to the upper exciton component of the special pair (B {yields} P{sub +}) followed by a nearly 150 fs internal conversion from the upper exciton component to the lower exciton component of the special pair (P{sub +} {yields} P{sub -}). Finally, charge separation after electron transfer from P{sub -} to H causes an electrochromic (Stark) shift of B and produces the 2.8 ps bleach rise. The two-step energy transfer model is supported by the observation of weak quantum beat oscillations (125 cm{sup -1} and 227 cm{sup -1}) with near-zero anisotropy in the pump-probe signals. 86 refs., 13 figs., 4 tabs.
... Energy transport, in the following denoted as E.T., in organized molecular systems is one of the most important processes in photosynthesis, optoelectronic devices, artificial light harvesting systems, and solar cells. [1][2][3][4][5] The unique property of E.T. in photosynthetic complexes is its high efficiency even though this transfer occurs over relatively long distances. The reasons for this high efficiency are currently only partly understood. ...
An improved application is presented of the Monte Carlo method including simultaneous parameter fitting to analyze the experimental time-resolved fluorescence and fluorescence anisotropy decay of two organized molecular systems exhibiting a number of different, nonisotropic energy transfer processes. Using physical models and parameter fitting for these systems, the Monte Carlo simulations yield a final set of parameters, which characterize the energy transfer processes in the investigated systems. The advantages of such a simulation-based analysis for global parametric fitting are discussed. Using this approach, energy transfer processes have been analyzed for two porphyrin model systems, i.e., spin-coated films of zinc tetra(octylphenyl)-porphyrins (ZnTOPP) and the tetramer of zinc mono(4-pyridyl)triphenylporphyrin (ZnM(4-Py)TrPP). For the ZnTOPP film energy transfer rate constants of similar to1 x 10(12) s(-1) and similar to 80 x 10(9) s(-1) have been found, and are assigned to intra- and interstack transfer, respectively. For the tetramers, the transfer rate constants of 38 x 10(9) and 5 x 10(9) s(-1) correspond to energy transfer to nearest and next nearest neighbor molecules, respectively. The results are in agreement with a Forster type energy transfer mechanism.
... The primary photochemistry and structure of the photosynthetic reaction center of the photosynthetic purple bacterium Rhodopseudomonas (Rps.) viridis are extensively studied and thoroughly characterized (1)(2)(3)(4)(5). In sharp contrast to this stands the almost complete lack of knowledge of how the energy of light is captured by the light-harvesting antenna and transported to the reaction center. ...
By low intensity picosecond absorption spectroscopy it is shown that the exciton lifetime in the light-harvesting antenna of Rhodopseudomonas (Rps.) viridis membranes with photochemically active reaction centers at room temperature is 60 +/- 10 ps. This lifetime reflects the overall trapping rate of the excitation energy by the reaction center. With photochemically inactive reaction centers, in the presence of P+, the exciton lifetime increases to 150 +/- 15 ps. Prereducing the secondary electron acceptor QA does not prevent primary charge separation, but slows it down from 60 to 90 +/- 10 ps. Picosecond kinetics measured at 77 K with inactive reaction centers indicates that the light-harvesting antenna is spectrally homogeneous. Picosecond absorption anisotropy measurements show that energy transfer between identical Bchlb molecules occurs on the subpicosecond time scale. Using these experimental results as input to a random-walk model, results in strict requirements for the antenna-RC coupling. The model analysis prescribes fast trapping (approximately 1 ps) and an approximately 0.5 escape probability from the reaction center, which requires a more tightly coupled RC and antenna, as compared with the Bchla-containing bacteria Rhodospirillum (R.) rubrum and Rhodobacter (Rb.) sphaeroides.
... (i) The presence of ⌬ should be inhibitory, and ii) this rate constant is also very high (see e.g. Ref. 55), at least when a continuous network of proton carriers is involved, as is the case in the quinone binding pockets (see a review in Ref. 56). ...
We have investigated the effects of the light-induced thylakoid transmembrane potential on the turnover of theb
6
f complex in cells of the unicellular green alga Chlamydomonas reinhardtii. The reduction of the potential by either decreasing the light intensity or by adding increasing concentrations of the ionophore
carbonylcyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) revealed a marked inhibition of the cytochromeb
6 oxidation rate (10-fold) without substantial modifications of cytochrome f oxidation kinetics. Partial recovery of this inhibition could be obtained in the presence of ionophores provided that the
membrane potential was re-established by illumination with a train of actinic flashes fired at a frequency higher than its
decay. Measurements of isotopic effects on the kinetics of cytochrome b
6 oxidation revealed a synergy between the effects of ionophores and the H2O-D2O exchange. We propose therefore, that protonation events influence the kinetics of cytochromeb
6 oxidation at the Qi site and that these reactions are strongly influenced by the light-dependent generation of a transmembrane
potential.
A model of charge separation symmetry breaking (SB) in symmetric excited dyads and dimers are presented. The minimal model should include at least four basis electronic states due to a small energy gap between the locally excited and charge separated (zwitterionic) states of the chromophores. There are electronic couplings between all these states. The model includes the interactions: (i) the Coulomb interaction between charges on the chromophores of the dyad, (ii) the interaction of the dipole moment of the asymmetric dyad with the solvent polarization, and (iii) the electronic-vibrational interaction. SB becomes possible only if the intensity of these interactions exceeds a threshold value. The threshold vanishes if there is a degeneration of the levels. Unusual resonant dependencies of the dissymmetry degree on the model parameters are revealed. Resonances arise due to the degeneration of energy levels. The ranges of the parameters in which energy level crossings occur are established. The oddity lies in the dependence of the resonance shape on the parameters of the model. A variation in the electronic couplings and the energy gap between the locally excited and ionic states, which leads to a broadening of the resonance, simultaneously leads to an increase in the resonant height. This opens up wide possibilities for controlling the charge separation degree. The predictions of the theory agree with the available experimental data. The charge separation SB is predicted to accompany by SB in the excitation distribution on the branches of dyads.
Within the general subject of photoinduced electron and energy transfer, the study of unimolecular processes in covalently linked supramolecular systems constitutes a recent but very active research field. In the present report, the status of this field is briefly outlined. Particular attention is given to the evolution taking place from studies of fundamental interest on simple two-component systems (dyads) to studies of more oriented character involving increasingly complex systems (triads, tetrads, pentads, etc.). The development of this tendency towards the design of photochemical molecular devices is finally pointed out.
This paper is a general appendix to the three parts of ‘Celebrating the millennium — historical highlights of photosynthesis research’ (Photosynth Res, Vols 73 (2002), 76 (2003), 80 (2004)). The major part of this paper includes a comprehensive list of most of the edited volumes on research in the area of photosynthesis. However, I begin this paper by describing selected conferences, related to photosynthesis, held in USA, during 1935–1965, followed by the 1963 International Colloquim held in Gif-sur-Yvette, France (René Wurmser (1890–1993), President); and the 2nd Western European Congress (1965), held in the Netherlands, under the leadership of Jan B. Thomas (1907–1991), both these European conferences being the precursors of the first International Congress on Photosynthesis (1968), organized by Helmut Metzner (1925–1999) at Freudenstadt, Germany. A list of the subsequent international photosynthesis congresses is available in Govindjee and David Krogmann (this volume).
Light-induced electron transfer in the reaction center (RC) is coupled to the uptake of protons from the cytoplasm at the binding site of the secondary quinone (QB). With quinone reconstitution, a higher QB occupancy has been obtained in the Rhodopseudomonas (Rp.) viridis RC crystals, which, after X-ray analysis, has resulted in a well-defined QB-site model. On the basis of this new model and the structure of a stigmatellin-RC complex, a new understanding of QB binding, particularly concerning the role of Ser L223, has emerged. Furthermore, with high quality data collected on RC complexes with atrazine and two chiral atrazine derivatives, it has been possible to describe the exact nature of triazine binding and its effect on the structure of the protein. The new data are of sufficient quality to improve the original model also in other parts, e.g. regarding the asymmetry of binding of the primary quinone (QA), the structure of the carotenoid, and additional tightly-bound water molecules.
One outstanding question in bacterial photosynthesis research concerns the relevance of the dimeric nature of the primary donor P for the fast electron transfer step from the first excited singlet state 1P*H to the charge separated state P+H−, where H stands for the intermediate acceptor bacteriopheophytin. This problem can only be attacked by performing a detailed study of the electronic structure of the primary donor itself and of the electronic interactions between all involved cofactors in their various active states. This requires the combined effort of high resolution X-ray crystallography, powerful spectroscopic methods and sufficiently advanced quantum-chemical calculations.
As indicated by their traditional name of “complexes”, coordination compounds have a composite nature. Their relevant molecular orbitals are often predominantly localized either on the metal or on the ligands, which can thus be considered as electronically independent fragments. Because of this composite nature, coordination compounds of transition metals exhibit a remarkable variety of electronically excited state types. The common classification includes metal-centered (MC), ligand-centered (LC), or charge transfer (CT) states, with the possibility of metal-to-ligand (MLCT) or ligand-to-metal (LMCT) subtypes. A given type of excited state usually gives rise to a typical spectroscopic transition and (when it occurs as the lowest excited state of the system) to a specific type of photophysical and photochemical behavior [1–5]. The possibility to play with metal and ligands in a large number of combinations provides a remarkable degree of synthetic control on excited state properties [6–13]. This flexibility can be useful, e.g., in the design of redox photosensitizers [14] for various applications, including chemical conversion of light energy [15].
In the analysis of complex phenomena such as the biogenesis or photochemical function of reaction center proteins, the approach of a geneticist is to generate and characterize a large number of mutants affecting specific steps in the process. In some analyses, spontaneous mutants which possess selectable characteristics may be informative. For example, the role of specific amino acid residues in herbicide and quinone binding can be deduced from the analysis of spontaneous herbicide resistant mutants. However, the analysis of spontaneous mutations as a general approach to study structure-function relationships has serious limitations. Many mutations have phenotypes which possess unselectable characteristics. With the determination of the structure of the reaction center1, this class of mutations can be generated by replacing important amino acid residues adjacent to chromophores throughout the protein by site-directed mutagenesis.
Natural and artificial light harvesting systems often operate in a regime
where the flux of photons is relatively low. Besides absorbing as many photons
as possible it is therefore paramount to prevent excitons from annihilation via
photon re-emission until they have undergone an irreversible conversion
process. Taking inspiration from photosynthetic antenna structures, we here
consider ring-like systems and introduce a class of states we call ratchets:
excited states capable of absorbing but not emitting light. This allows our
antennae to absorb further photons whilst retaining the excitations from those
that have already been captured. Simulations for a ring of four sites reveal a
peak power enhancement of 35% under ambient conditions owing to a combination
of ratcheting and the prevention of emission through dark state population. In
the slow extraction limit the achievable current enhancement exceeds hundreds
of percent.
The current situation on experiment and theory as regards the early electron transfer steps in bacterial photosynthesis is discussed. Recent experiments have limited the mechanistic possibilities, while an internal consistency test has limited the values of some parameters in the superexchange mechanism. The partitioning method has provided a useful and unified way of treating superexchange and other properties in these systems. The alternative reaction mechanisms have a number of consequences, and various experimental tests are considered or suggested.
Two 4,13-diaza-18-crown-6 ethers with either two pyrenyl or two carbazolyl groups were synthesized. The two crown ethers can form complexes with methyl viologen in methanol solution. Photoirradiation of the complexes resulted in the electron transfer from the excited states of the chromophores to methyl viologen as demonstrated by the quenching of the chromophore fluorescence and the detection of the absorption spectrum of the generated viologen radical cation. The back electron transfer in these systems was inhibited by the electrostatic repulsion between the positively charged viologen radical cation and the generated chromophore radical cation. Long-lived charge separation states (up to tens of min) were observed.
The shifts of the special pair redox potential of the photosynthetic reaction center of Rhodobacter sphaeroides are evaluated for point mutations in the neighborhood of the special pair and compared with experimental data. The shifts are computed from electrostatic energies using DelPhi and CHARMM. With the CHARMM energy function contributions of Van der Waals interactions are also considered. The influence of water molecules on the computed values of the shift is investigated. Agreement of the calculated experimental values of the shift can be obtained with DelPhi and CHARMM. With CHARMM the agreement is generally better if electrostatic energies are considered only. Including crystal waters and filling the protein cavities with water molecules is also improving the results obtained with the CHARMM energy function.
Solar energy exploitation by photosynthetic water cleavage is of central relevance for the development and sustenance of all higher forms of living matter in the biosphere. The key steps of this process take place within an integral protein complex referred to as Photosystem II (PS II) which is anisotropically incorporated into the thylakoid membrane. This minireview concentrates on mechanistic questions related to i) the generation of strongly oxidizing equivalents (holes) at a special chlorophyll a complex (designated as P680) and ii) the cooperative reaction of four holes with two water molecules at a manganese containing unit WOC (water oxidizing complex) resulting in the release of molecular oxygen and four protons. The classical work of Pierre Joliot and Bessel Kok and their coworkers revealed that water oxidation occurs via a sequence of univalent oxidation steps including intermediary redox states Si (i = number of accumulated holes within the WOC). Based on our current stage of knowledge, an attempt is made a) to identify the nature of the redox states Si, b) to describe the structural arrangement of the (four) manganese centers and their presumed coordination and ligation within the protein matrix, and c) to propose a mechanism of photosynthetic water oxidation with special emphasis on the key step, i.e. oxygen-oxygen bond formation. It is assumed that there exists a dynamic equilibrium in S3 with one state attaining the nuclear geometry and electronic configuration of a complexed peroxide. This state is postulated to undergo direct oxidation to complexed dioxygen by univalent electron abstraction with YZox and simultaneous internal ligand to metal charge transfer.
Key questions on the mechanism will be raised. The still fragmentary answers to these questions not only reflect our limited knowledge but also illustrate the challenges for future research.
In this article, the three-dimensional structures of photosynthetic reaction centers (RCs) are presented mainly on the basis of the X-ray crystal structures of the RCs from the purple bacteria Rhodopseudomonas (Rp.) viridis and Rhodobacter (Rb.) sphaeroides. In contrast to earlier comparisons and on the basis of the best-defined Rb. sphaeroides structure, a number of the reported differences between the structures cannot be confirmed. However, there are small conformational differences which might provide a basis for the explanation of observed spectral and functional discrepancies between the two species.A particular focus in this review is on the binding site of the secondary quinone (QB), where electron transfer is coupled to the uptake of protons from the cytoplasm. For the discussion of the QB site, a number of newlydetermined coordinate sets of Rp. viridis RCs modified at the QB site have been included. In addition, chains of ordered water molecules are found leading from the cytoplasm to the QB site in the best-defined structures of both Rp. viridis and Rb. sphaeroides RCs.
Ein wasserlosliches, sieben Naphthoatchromophore tragendes P-Cyclodextrin bildet mit einem Merocyaninfarbstoff einen sehr stabilen 1 : 1-Einschluskomplex, der den Antennen-Effekt in Photosynthese-Einheiten nachahmt (Bild rechts). Der Energietransfer von den Naphthoatgruppen auf das Merocyanin findet mit einer Wirksamkeit von 100% statt.
Die Spaltung von Wasser durch Sonnenlicht in Sauerstoff und metabolisch gebundenen Wasserstoff bei der Photosynthese ist von zentraler Bedeutung für die Existenz höher organisierter Lebewesen auf der Erde. Die Realisierung dieses Prozesses in biologischen Organismen ermöglichte es nicht nur, das riesige Wasserreservoir unseres Planeten als Substrat für die Sonnenenergie zu nutzen, sondern führte zugleich auch zur Bildung einer aeroben Atmosphäre. Der so bereitgestellte Sauerstoff ist ein Reagens, das einen energetisch äußerst effizienten Nährstoffumsatz bewirkt. In den letzten Jahren sind beträchtliche Fortschritte im Verständnis der funktionellen und strukturellen Organisation der photosynthetischen Wasserspaltung erzielt worden. Dieser Beitrag versucht, einen Überblick über unsere heutigen Vorstellungen zu geben, wobei der Schwerpunkt auf Aspekten der Realisierung eines Teilschrittes der Wasserspaltung in biologischen Systemen liegt: der Wasseroxidation zu O2.
The cleavage of water by solar radiation into dioxygen and metabolically bound hydrogen during photosynthesis is of central importance for the existence of higher forms of life on earth. The realization of this process in biological organisms made possible the use of the earth's huge water reservoir for the exploitation of solar energy and, at the same time, led to the creation of an aerobic atmosphere. The dioxygen thereby formed is a powerful oxidant which permits an energetically highly efficient nutrient turnover. In recent years considerable progress has been made in understanding the functional and structural organization of photosynthetic water splitting. This article attempts to give a review of our current state of knowledge with special emphasis on the oxidation of water to O2 in biological systems.
The electronic mechanism and the origin of the unidirectionality of the electron transfer from photoexcited special pair to bacteriopheophytin in the photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps) Viridis are studied theoretically by using the SAC(symmetry adapted cluster)-CI (configuration interaction) method. The effects of the surrounding proteins are considered by using the point charge model. The L-branch selectivity of the electron transfer is explained by the asymmetry of the transfer integral, an electronic factor, which originates from a small structural asymmetry of the PSRC: the L-side chromophores are locally closer than the M-side ones, though the average separations are almost the same. The smallness of the charge recombination rate is attributed to the difference in the electron localization between the LUMO and HOMO of special pair. Protein effects on the unidirectionality are quite small as far as the electrostatic model is valid, though the proteins keep the three-dimensional arrangement of the chromophores in the PSRC. A mutation experiment for realizing M-side selectivity is suggested.
Hexakis(2-naphthyloxy)cyclotriphosphazene showed an interesting emission behavior between its monomer and excimer forms, the latter nearly completely dominating in water. Encapsulation studies with β-cyclodextrin in water partially revived the monomer emission.
A water-soluble β-cyclodextrin bearing seven naphthoate chromophores forms a very stable 1:1 inclusion complex (shown right) with a merocyanine dye, which mimics the antenna effect in photosynthetic units. The energy transfer from the naphthoate chromophores to the merocyanine is shown to take place with 100 % efficiency.
A temperature- and current-controlled diode laser at ∼780 nm was used to perform permanent hole-burning experiments on the S1 ← S0 0–0 (Qy) transition of bacteriochlorophyll a (BChl a) in amorphous hosts. Hole widths were obtained for the glass triethylamine (TEA) between 0.4 and 15 K, for micelles of the detergent lauryldimethylamine N-oxide (LDAO) between 1.2 and 4.2 K, and for wet and dry CCl4 at 1.2 K. The homogeneous linewidth, Γhom, of BChl a in TEA follows a T1.3±0.1 dependence at temperatures T < 7 K, whereas at T ≈ 7 K a crossover to an exponential dependence with an activation energy of 33 ± 3 cm⁻¹ is observed. The results, when compared with those obtained for free-base porphin in the same glass, suggest that a low-frequency mode of TEA is responsible for the dephasing. From the experiments on micelles conclusions are drawn about the molecular conformations of BChl a in the detergent.
We have reviewed a number of correlations between kinetic parameters which characterize optical and thermal electron transfer between two molecular sites. The correlations involve directly observables such as the molar absorptivity and emitted spectral distribution for optical and the rate constants for the thermal processes. We have also discussed the possible direct optical detection of the transition state configuration. This is feasible in principle by emission or Raman scattering from the first excited state in the adiabatic limit. The correlations can be extended both to optical and thermal electron transfer between metal or semiconductor electrodes and depolarizer molecules or molecular adsorbates, and to electron transfer in three-level, superexchange systems. In the latter case relations between superexchange rate constants and Raman scattering profiles can be pointed out.
The fluorescence of individual light-harvesting 2 complexes from Rhodopseudomonas acidophila has been observed by confocal microscopy in a temperature range between 300 and 7 K. Under ambient conditions, changes in the polarization of the fluorescence emission of single complexes on a time scale from milliseconds to seconds are found. In the temperature range between 250 and 100 K most complexes emit fluorescence with a temporally stable linear fluorescence polarization. At temperatures below 70 K, spectral diffusion is found to dominate the dynamics of the fluorescence intensity and polarization. The increase in photostability of single complexes at low temperature allows the detection of fluorescence emission spectra of single complexes. A marked variation in the shape and the position of the spectra is found. The results are discussed by considering static and dynamic disorder within the B850 aggregate of the light-harvesting complexes.
The excitation spectrum of the photosynthetic reaction center (PSRC) of Rhodopseudomonas (Rps.) viridis is assigned by using the SAC(symmetry adapted cluster)−CI(configuration interaction) method. All the chromophores included in the PSRC, bacteriochlorophyll b dimer (special pair, P), bacteriochlorophyll b in L- and M-branches (BL and BM), bacteriopheophytin b in L- and M-branches (HL and HM), menaquinone (MQ), ubiquinone (UQ), and four different hemes, c-552, c-554, c-556, and c-559 in c-type cytochrome subunit, were calculated within the environment of proteins, waters, and the other chromophores which were dealt with by the point-charge electrostatic model. We have assigned successfully all the peaks in the experimental spectrum in the energy range from 1.2 to 2.5 eV. The assignment was done by comparing the SAC−CI theoretical spectrum with the experimental one in excitation energy, oscillator strength, linear dichroism data (angle of transition moment), and other experimental information available. Almost all the peaks were red shifted due to the effect of proteins. The present assignment of the spectrum would give a basis for future photoexperimental studies of the PSRC.
Direct electrochemical evidence shows that photoinduced electron transfer from excited Mg−octaethylporphyrin (MgOEP) to a Mg (or Zn) complex of tetrakis(pentafluorophenyl)porphyrin (TFPP) in lipid bilayers occurs with a second-order rate constant ≥108 M-1 s-1. This reaction is 100 fold faster than the reported intermolecular rate between porphyrins, and occurs even under aerobic conditions. Electron transfer from the ground state of MgOEP to excited ZnTFPP is observable under anaerobic conditions, but the rate is 10-fold slower. Time-resolved photoconductivity of the lipid bilayer with the mixed metalloporphyrins suggests that the charge recombination times (τ) of the geminate ion pairs in the MgTFPP−MgOEP and ZnTFPP−MgOEP systems are 20 and 38 μs, respectively. The MgTFPP- anion reduces O2 with a second-order rate constant of 107 M-1 s-1, but the oxidation of ZnTFPP- anions by O2 is very slow. The differences between the two systems may arise from different redox potentials of ZnTFPP and MgTFPP. These data prove that, even containing Mg, the least electronegative element which can be stably chelated, a metalloporphyrin with poly electron-withdrawing groups is a good electron acceptor. Our results suggest that such electronegative porphyrins are useful molecular parts for assembling of porphyrin-based biomimetic energy conversion devices.
Excited states of free base chlorin (FBC), free base Bacteriochlorin (FBBC), pheophytin a (Pheo a), and chlorophyll a (Chlo a), which are derivatives of free base porphine (FBP), were calculated by the SAC (symmetry adapted cluster)/SAC−CI (configuration interaction) method. The results reproduced well the experimentally determined excitation energies. The reduction of the outer double bonds in the porphine ring in the order of FBP, FBC, and FBBC causes a breakdown of the symmetry and a narrowing of the HOMO−LUMO gap, which result in a red shift of the Qx band and an increase of its intensity. In the change from Pheo a to Chlo a, the Mg coordination reduces the quasidegeneracy in the Qx state and then increases the spectral intensity. The disappearance of the Qy humps from the absorption spectrum of Pheo a, compared with that of Chlo a, is due to the red shift of the Qy state.
Information about excited states of radical ions can enable their use as powerful oxidizing and reducing agents capable of driving chemical reactions. This article presents the first measurements of fluorescence from an excited state of the radical anion of 1,4-benzoquinone (BQ-), and the first report of a fluorescence quantum yield from any radical anion. BQ- was observed in both a 77 K 2-methyltetrahydrofuran matrix and a room temperature isooctane solution. The low fluorescence quantum yield, φf = 0.003, and the presence of a 0.5 eV red shift of the emission band edge (593 nm) from the absorption band edge (475 nm) imply that the lowest energy transition in BQ-, which is the source of the weak fluorescence, is formally forbidden. This conclusion is supported by both semiempirical and ab initio molecular orbital calculations. In addition, we determined the excited state lifetime of BQ- at 77 K to be 63 ns, with an excited state absorption spectrum peaking at 415 nm.
Oxygen-evolving photosystem II particles (DT 20) isolated from pea chloroplasts by digitonin-Triton X-100 fractionation were photoinhibited with 150 W. m(-2) white light, at 20°C under three conditions: aerobic, anaerobic and strongly reducing (E(h) poised to approx. -250 mV with dithionite). Hill reaction rate (H (20) + BQ)and variable fluorescence (Fv) declined in parallel in all three cases with shortening half times: 30, 10 and 2.5 min, respectively. Light-induced absorbance changes at 685 nm characteristic of reversible photo accumulation of reduced pheophytin (& z -2.50 mV) remained essentially unchanged. We conclude that the three types of photoinhibitory treatment do not impair the separation of charges between chlorophyll P-680 and pheophytin in the photosystem II reaction center.
Der Begriff molekulare Elektronik ist zum Stimulans für vielfältige Forschungsarbeiten geworden, obwohl es sie im eigentlichen Sinn noch kaum gibt. Ob das Ziel je erreicht wird, hängt vor allem von einer engen Zusammenarbeit zwischen Organikern und Festkörperphysikern ab.
Ab initio molecular orbital (MO) calculations of the chlorophyll dimer have been carried out, using the structure determined by Deisenhofer et al. for Rhodopseudomonas viridis. AMOSS, a new MO program package developed by the NEC quantum chemistry group for vector computers was used. The dimerization energy is estimated to be 16.1 kcal/mol, indicating that the dimerization of the chlorophylls at the present geometry seems to be unfavorable. Two possible reasons are discussed. One is the underestimation of π-π interactions between the chlorophylls. The other is the influences by the proteins surrounding the dimer.
The tunneling time for nonadiabatic electron transfer reactions described within the superexchange model is estimated using a Büttiker type internal clock: the electron is taken to possess two internal spin states that are weakly coupled on the bridge. By studying the transition probability between these channels during the tunneling process the traversal time through the bridge can be estimated. Like the Büttiker-Landauer result it is linear in the bridge length, but its dependence on the barrier energy U B approaches the Büttiker-Landauer form only in the limit of strong interstate coupling (broad band). In the "normal" superexchange (weak coupling) limit it is inversely proportional to the barrier energy.
The function of photosynthetic bacterial reaction centers (RCs) is closely related to their structure. In the last 15 years a wealth of structural data has been accumulated on bacterial RCs, mainly through X-ray structure analysis of three-dimensional RC crystals. In this chapter, the arrangement of protein subunits and cofactors in the RC complexes of the non-sulfur purple bacteria Rhodobacter (Rb.) sphaeroides and Rhodopseudomonas (Rp.) viridis are delineated. A prominent feature of the bacterial RCs is their location in the photosynthetic membrane. Inside the RC complex, a finely tuned arrangement of amino acid residues and cofactors maintains a highly ordered system. The positions and likely functions of hydrogen bonds are described, since they play a key role in protein-cofactor interactions. Special emphasis is placed on the symmetry relations in the RC and on the functional asymmetry of electron and proton transfer that contradicts the observed pseudo two-fold structural symmetry.
The structures of the RCs from Rb. sphaeroides and Rp. viridis show a striking identity, apart from the cytochrome-c subunit found only in the latter RC. The core regions around the bacteriochlorophylls and bacteriopheophytins, including the carotenoid, are particularly similar. New observations of water clusters close to the primary and secondary quinones are described and their impact on proton transfer processes is discussed. These findings help elucidate the intermeshed processes of electron-proton coupling in the RC.
Selectivity of product formation in activated chemical reactions can be accomplished by the control of energy acquisition,
storage and disposal in the molecular system. This goal can be achieved by ‘passive control’ via energy acquisition, i.e.,
selecting the initial conditions and letting the system evolve under its own Hamiltonian, and by the ‘active control’ of energy
storage and disposal, i.e., modifying the equations of motion by imposing external, additional terms in the Hamiltonian. We
discuss the possibility and conditions for non-statistical reaction dynamics in molecules, van der Waals molecules, clusters,
surfaces, condensed phases and biophysical systems, which result in selective chemistry.
The fluorescence emission of a naphthalene unit attached to a polyamine chain is quenched by intramolecular electron transfer from the deprotonated amines to the excited fluorophore. Measurements of the respective quenching rate constants as a function of the distance, reveal an exponential dependence with beta = 0.45 Angstrom(-1). Identical measurements carried out in deuterated water have shown a similar dependence with the distance beta = 0.49 Angstrom(-1) but an average reduction of the absolute values of the rate constants of ca. 1.2. The polyamine chains seem to constitute a bridge through which the electron can find a route to its movement, more efficiently than through space.
We study how Forster energy transfer from a semiconductor quantum dot to a metallic nanoparticle can be gated using quantum coherence in quantum dots. We show this allows us to use a laser field to open the flow of the energy transfer for a given period of time (on-state) before it is switched off to about zero. Utilizing such an energy gating process it is shown that quantum-dot-metallic-nanoparticle systems (meta-molecules) can act as functional nanoheaters capable of generating heat pulses with temporal widths determined by their environmental and physical parameters. We discuss the physics behind the energy nanogates using molecular states of such meta-molecules and the resonance fluorescence of the quantum dots.
The one-electron oxidation potentials [E(ox)(NHE)(H(2)Q)], pK(a) (pK(a1) and pK(a2)) values, and bond dissociation energies (BDE(1) and BDE(2)) of 118 important p- and o-dihydroquinones in DMSO were systematically predicted for the first time by using DFT method and the PCM cluster continuum model. The calculated results agree well with the available experimental determinations. The study shows that all the five thermodynamic parameters correlate well with the Hammett substituent parameters σ(p) (for p-H(2)Q, E(ox)(NHE)(H(2)Q(·+)/H(2)Q) = 1.66Σσ(p) + 0.54, pK(a1) = -5.69Σσ(p) + 16.54, pK(a2) = -5.19Σσ(p) + 23.91, BDE(1) = 3.43Σσ(p) + 82.29, BDE(2) = 4.64Σσ(p) + 67.70 and for o-H(2)Q, E(ox)(NHE)(H(2)Q(·+)/H(2)Q) = 1.85Σσ(p) + 0.46, pK(a1) = -5.53Σσ(p) + 13.28, pK(a2) = -5.24Σσ(p) + 26.70, BDE(1) = 3.54Σσ(p) + 82.08, BDE(2) = 3.82Σσ(p) + 75.93), which hints that we can get these thermodynamic parameters as long as the structure of the hydroquinones were known. The comparisons of the calculated five thermodynamic parameters between p-hydroquinones and o-hydroquinones and the number of the phenyl ring effects on these thermodynamic parameters were also studied. At last, intramolecular hydrogen bond energies in hydroquinones at neutral, radical cation, radical, anion different state were systematically calculated and analyzed. Combined with the papers published in our group before, we will have a systematic thermodynamic picture of the transfer details between different kinds of quinones and corresponding hydroquinones, which strongly promote the fast development of the understanding and applications of quinones.
This Thesis is devoted to the investigation of optical photophysical processes in organized porphyrin systems. These systems can serve as molecular antennas for organic solar cells, a field of research which recently has received increasing interest. Using a novel application of Monte Carlo computer simulation an improved analysis of the complex fluorescence and fluorescence anisotropy decay in the presence of energy transfer processes has been introduced.Self-organized [Zn(4-Py)TrPP] 4 tetramers in solution and in solid films as well as ZnTOPP domains in spin coated films have been studied experimentally by steady state and time-resolved spectroscopy. The results have been analyzed using the abovementioned Monte Carlo simu-lation, yielding the characteristic rate constants for energy transfer- and relaxation processes.The results of these Monte Carlo simulations are: for [Zn(4-Py)TrPP] 4 tetramers in solution the fluorescence lifetime is ~ 1.5 × 10 -9 s and nearest neighbor energy transfer rate constant is ~ 40 × 10 9 s -1 ; ZnTOPP forms parallel porphyrin stacks within one domain in the films, whereas in each stack the porphyrin planes are perpendicularly oriented with respect to the substrate and make an angle of 45˚ with the long stack axis. As follows from the fit of simulated decay curves to the experimental fluorescence- and fluorescence anisotropy decay curves the rate constants for intra-stack and inter-stack energy transfer are ~ 1 × 10 12 s -1 and ~ 80 × 10 9 s -1 , respectively, whereas the fluorescence lifetime is ~ 1.8 × 10 -9 s.
The atomic model of the photosynthetic reaction centre from the purple bacterium Rhodopseudomonas viridis has been refined to an R-value of 0.193 at 2.3 A resolution. The refined model contains 10,288 non-hydrogen atoms; 10,045 of these have well defined electron density. A Luzzati-plot indicates an average co-ordinate error of 0.26 A. During refinement, the positions of a partially ordered carotenoid, a unibiquinone in the partially occupied QB site, a detergent molecule, seven putative sulphate ions, and 201 water molecules were found. More than half of these waters are bound at interfaces between protein subunits and therefore contribute significantly to subunit interactions. Water molecules also play important structural and probably functional roles in the environment of some of the cofactors. Two water molecules form hydrogen bonds to the accessory bacteriochlorophylls and to the protein in the vicinity of the special pair of bacteriophylls, the primary electron donor. A group of about 10 water molecules is bound near the binding site of the secondary quinone QB. These waters are likely to participate in the transfer of protons to the doubly reduced QB.
This paper is a general appendix to the three parts of 'Celebrating the millennium - historical highlights of photosynthesis research' (Photosynthesis Research, Vols~73(2002), 76(2003), 80(2004)). The major part of this paper includes a comprehensive list of most of the edited volumes on research in the area of photosynthesis. However, I begin this paper by describing selected conferences, related to photosynthesis, held in USA, during 1935-1965, followed by the 1963 International Colloquim held in Gif-sur-Yvette, France (René Wurmser (1890-1993), President); and the 2nd Western European Congress (1965), held in the Netherlands, under the leadership of Jan B. Thomas (1907-1991), both these European conferences being the precursors of the first International Congress on Photosynthesis (1968), organized by Helmut Metzner (1925-1999) at Freudenstadt, Germany. A list of the subsequent international photosynthesis congresses is available in Govindjee and David Krogmann (this issue).
This paper introduces the third and final part of the 'millennium celebrations of historical highlights of photosynthesis research.' Part 1 (308 pages) was published in October 2002 as Vol. 73 of the journal Photosynthesis Research, and Part 2 (458 pages) was published in July 2003 as Vol. 76. Here, we recognize particularly the work of three major contributors to our understanding of photosynthesis: Roger Stanier (1916-1982); Germaine Cohen-Bazire (Stanier) (1920-2001); and William Arnold (1904-2001). We also introduce the historical papers contained in this volume; consider the legacy of Alfred Nobel (1833-1896); and identify Nobel prizes of special relevance to understanding the capture, conversion, and storage of light energy in both anoxygenic and oxygenic photosynthesis.
The study of excited state properties of chlorophyll a is a subject of foremost interest, given that it plays important roles in biological process and has also been proposed for applications in photonics. This work reports on the excited state absorption spectrum of chlorophyll a solution from 460 to 700 nm, obtained through the white-light continuum Z-scan technique. Saturation of absorption was observed due to the ground state depletion, induced by the white-light continuum region that is resonant with the Q band of chlorophyll a. The authors also observed reverse saturation of absorption related to the excitation from the first excited state to a higher energy level for wavelengths below 640 nm. An energy-level diagram, based on the electronic states of chlorophyll a, was employed to interpret their results, revealing that more states than the ones related to the Q and B bands participate in the excited state absorption of this molecule.
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