Mimicking Photosystem II with Manganese Model Complexes to Approach Artificial Photosynthesis

The work in thesis aims for artificial photosyntesis, which could and will mimic natural photosynthesis, the process that uses light to create energy rich compounds from H2O and CO2. Water is oxidized in Photosystem II (PSII) by the Water Oxidizing Complex (WOC), which is a catalytic site on the lumenal side of the thylakoid membrane. The chlorophyll complex P680 absorbs light and electrons are transferred to the acceptor side of PSII. The electron-hole on P680 is filled by electrons from the Mn-cluster, which in turn oxidizes water. This can be viewed as a D (donor)-PS (photosensitizer)-A (acceptor) system. Artificial photosynthesis builds on the same principles, and in this work we have been focusing on redox reactions in dinuclear manganese complexes, using RuII(bpy)3 as photosensitizer.WOC in PSII is built up by four manganeses in a cluster, with ?-oxo bridges between the manganeses. On at least one of the manganeses, there is an open site for water binding.In this thesis I have investigated oxidation reactions of three different dinuclear manganese complexes, denoted complex 1, 2, and 3. 1 is a complex with N3O3 ligands coordination to each manganese, and is synthesised in Mn2II/II valance state. We have showed that it is possible to oxidize this complex three times to Mn2III/IV, which means that it forms four stable oxidation states. 2 has a N2O4 ligands coordinating each manganese, and is synthesised in the Mn2III/III valence state. This complex can be oxidized to what we think is Mn2IV/IV and Mn2IV/IVL?. 2 has five stable oxidation states. From electro chemistry we know that 2 is stable in Mn2II/II and Mn2II/III, and this means that 2 can be oxidized by photosensitizer three steps. 3 is an unymmetric manganese complex, with a mixture of the ligands in 1 and 2. On one side the manganese is coordinated to N3O3 ligands, and the other manganese is coordinated to N2O4 ligands. 3 is synthesised in the Mn2II/III valence state, and it is possible to oxidize this complex to, what we think is a Mn2IV/IV and a Mn2IV/IVL? oxidation state. By electro chemistry we can reduce 3 to Mn2II/II. This means that 3 can be oxidized four times with the photosensitizer and have five stable oxidation states. All three manganese complexes have acetate groups as bridging ligands to the manganeses. EXAFS measurements and from electro chemistry, indicate that the acetate groups can detach from the manganese, so that water can access the site and be converted into ligands to the manganese. From the X-ray absorption spectroscopy (EXAFS and K-edge), we could se that there was a shortening in the Mn-Mn distance of 0.5 Å for 1 in the Mn2III/III and at Mn2III/III for 2, which is an indication that a ?-oxo or a ?-hydroxo bond is formed. This means that the manganese cluster changes its conformation during oxidation.To oxidize our model complexes beyond Mn2III/III, we use CoIII(NH3)5Cl as sacrificial electron acceptor. When CoIII is reduced by Ru*(bpy)3 to CoII, an EPR signal appears in the g=5 region which belongs to CoII(H2O)6. After some time of illumination a new EPR signal appears in the same region, which is narrower and more symmetric in its shape, resulting from a photoreduction process in CoIII(NH3)5Cl. It is known that UV light can photoreduce CoIII(NH3)5Cl to CoII(NH3)4 and Cl?, but we have found that the same reaction occurs at 532 nm. We have interpreted this signal as an intermediate, when cobalt changes the ligands from NH3 and Cl- to H2O

EPR studies of the oxygen-evolving complex reveal a light-adaption process in Photosystem II

Photosystem II utilizes solar energy to drive electrons from the Mn cluster at the lumenal side to the quinone at the stromal side of the thylakoid membrane. The source of electrons is H2O, which is split to oxygen by the OEC. The water-splitting process involves cycling of the Mn-cluster through four semi-stable oxidation states, termed the S0, S1, S2, and S3 states. The OEC can be trapped in the different S-states, and studied by EPR techniques. The S2 state is often obtained by exposing a dark-adapted (S1 state) sample to continuous illumination at low temperature, or to a single flash of light at room temperature followed by freezing. Both procedures capture the S2 state, as reached by a single oxidation of the OEC following a period of dark-adaptation. With a laser flash procedure, the S2 state can be obtained a second time after 5 flashes, when the S-cycle has been completed once. We have discovered differences between the S2 state formed after one flash and the S2 state formed after five flashes, in terms of the relaxation behavior of the S2 multiline EPR signal. These data indicate a change within the Mn cluster that builds up during the first turnovers after dark-adaptation. Pulsed field-swept spectra of samples given 0-5 flashes of light show a similar trend: the S1 state obtained by zero and four flashes differ significantly from one another. Based on the data presented and similar reports in the literature, we propose that the OEC undergoes a light-adaptation process during the first two turnovers after dark-adaptation, adjusting the system for efficient continuous water-splitting. We tentatively suggest that light-adaptation involves rearrangements of the proton network in the OEC, possibly as a means of setting up proton channels

Flash-induced relaxation changes of the EPR signals from the manganese cluster and YD reveal a light-adaptation process of Photosystem II

By exposing photosystem II (PSII) samples to an incrementing number of excitation flashes at room temperature, followed by freezing, we could compare the Mn-derived multiline EPR signal from the S-2 oxidation state as prepared by 1, 5, 10, and 25 flashes of light. While the S-2 multiline signals exhibited by these samples differed very little in spectral shape, a significant increase of the relaxation rate of the signal was detected in the multiflash samples as compared to the S-2-state produced by a single oxidation. A similar relaxation rate increase was observed for the EPR signal from Y-D(.). The temperature dependence of the multiline spin-lattice relaxation rate is similar after 1 and 5 flashes. These data are discussed together with previously reported phenomena in terms of a light-adaptation process of PSII, which commences on the third flash after dark-adaptation and is completed after 10 flashes. At room temperature, the fast-relaxing, light-adapted state falls back to the slow-relaxing, dark-adapted state with t(1/2) = 80 s. We speculate that light-adaptation involves changes necessary for efficient continuous water splitting. This would parallel activation processes found in many other large redox enzymes, such as Cytochrome c oxidase and Ni-Fe hydrogenase. Several mechanisms of light-adaptation are discussed, and we find that the data may be accounted for by a change of the PSII protein matrix or by the light-induced appearance of a paramagnetic center on the PSII donor side. At this time, no EPR signal has been detected that correlates with the increase of the relaxation rates, and the nature of such a new paramagnet remains unclear. However, the relaxation enhancement data could be used, in conjunction with the known Mn-Y-D distance, to estimate the position of such an unknown relaxer. If positioned between Y-D and the Mn cluster, it would be located 7-8 Angstrom from the spin center of the S-2 multiline signal

Optimizing Column Length and Particle Size in Preparative Batch Chromatography Using Enantiomeric Separations of Omeprazole and Etiracetam as Models : Feasibility of Taguchi Empirical Optimization

The overreaching purpose of this study is to evaluate new approaches for determining the optimal operational and column conditions in chromatography laboratories, i.e., how best to select a packing material of proper particle size and how to determine the proper length of the column bed after selecting particle size. As model compounds, we chose two chiral drugs for preparative separation: omeprazole and etiracetam. In each case, two maximum allowed pressure drops were assumed: 80 and 200 bar. The processes were numerically optimized (mechanistic modeling) with a general rate model using a global optimization method. The numerical predictions were experimentally verified at both analytical and pilot scales. The lower allowed pressure drop represents the use of standard equipment, while the higher allowed drop represents more modern equipment. For both compounds, maximum productivity was achieved using short columns packed with small-particle size packing materials. Increasing the allowed backpressure in the separation leads to an increased productivity and reduced solvent consumption. As advanced numerical calculations might not be available in the laboratory, we also investigated a statistically based approach, i.e., the Taguchi method (empirical modeling), for finding the optimal decision variables and compared it with advanced mechanistic modeling. The Taguchi method predicted that shorter columns packed with smaller particles would be preferred over longer columns packed with larger particles. We conclude that the simpler optimization tool, i.e., the Taguchi method, can be used to obtain “good enough” preparative separations, though for accurate processes, optimization, and to determine optimal operational conditions, classical numerical optimization is still necessaryFunding textAcknowledgements This work was supported by the Swedish Knowledge Foundation as part of the KKS SYNERGY project 2016 “BIO-QC: Quality Control and Purification for New Biological Drugs” (grant number 20170059) and by the Swedish Research Council (VR) as part of the project “Fundamental Studies on Molecular Interactions aimed at Preparative Separations and Biospecific Measurements” (Grant number 2015-04627). We also thank the National Science Centre, Poland for support via Grant number 2015/18/M/ST8/00349. We are grateful to AstraZeneca and UCB Pharma for kindly giving us omeprazole and etiracetam, respectively, and to Kromasil/Akzo Nobel Pulp and Performance Chemicals AB for giving us the AmyCoat columns.</p

Light-induced multistep oxidation of dinuclear manganese complexes for artificial photosynthesis

Two dinuclear manganese complexes, [Mn2BPMP(@m-OAc)2].ClO4 (1, where BPMP is the anion of 2,6-bis{[N,N-di(2-pyridinemethyl)amino]methyl}-4-methylphenol) and [Mn2L(@m-OAc)2].ClO4 (2, where L is the trianion of 2,6-bis{[N-(2-hydroxy-3,5-di-tert-butylbenzyl)-N-(2-pyridinemethyl)amino]met hyl}-4-methylphenol), undergo several oxidations by laser flash photolysis, using rutheniumII-tris-bipyridine (tris(2,2-bipyridyl)dichloro-ruthenium(II) hexahydrate) as photo-sensitizer and penta-amminechlorocobalt(III) chloride as external electron acceptor. In both complexes stepwise electron transfer was observed. In 1, four Mn-valence states from the initial Mn2II,II to the Mn2III,IV state are available. In 2, three oxidation steps are possible from the initial Mn2III,IIIstate. The last step is accomplished in the Mn2IV,IV state, which results in a phenolate radical.For the first time we provide firm spectral evidence for formation of the first intermediate state, Mn2II,III, in 1 during the stepwise light-induced oxidation. Observation of Mn2II,III is dependent on conditions that sustain the @m-acetato bridges in the complex, i.e., by forming Mn2II,III in dry acetonitrile, or by addition of high concentrations of acetate in aqueous solutions. We maintain that the presence of water is necessary for the transition to higher oxidation states, e.g., Mn2III,III and Mn2III,IV in 1, due to a bridging ligand exchange reaction which takes place in the Mn2II,III state in water solution. Water is also found to be necessary for reaching the Mn2IV,IV state in 2, which explains why this state was not reached by electrolysis in our earlier work (Eur. J. Inorg. Chem (2002) 2965).In 2, the extra coordinating oxygen atoms facilitate the stabilization of higher Mn valence states than in 1, resulting in formation of a stable Mn2IV,IV without disintegration of 2. In addition, further oxidation of 2, led to the formation of a phenolate radical (g=2.0046) due to ligand oxidation. Its spectral width (8 mT) and very fast relaxation at 15 K indicates that this radical is magnetically coupled to the Mn2IV,IV center

Multiscale Colloidal Assembly of Silica Nanoparticles into Microspheres with Tunable Mesopores

Colloidal assembly of silica (nano)particles is a powerful method to design functional materials across multiple length scales. Although this method has enabled the fabrication of a wide range of silica‐based materials, attempts to design and synthesize porous materials with a high level of tuneability and control over pore dimensions have remained relatively unsuccessful. Here, the colloidal assembly of silica nanoparticles into mesoporous silica microspheres (MSMs) is reported using a discrete set of silica sols within the confinement of a water‐in‐oil emulsion system. By studying the independent manipulation of different assembly parameters during the sol–gel process, a design strategy is outlined to synthesize MSMs with excellent reproducibility and independent control over pore size and overall porosity, which does not require additional ageing or post‐treatment steps to reach pore sizes as large as 50 nm. The strategy presented here can provide the necessary tools for the microstructural design of the next generation of tailor‐made silica microspheres for use in separation applications and beyond

Multiscale Colloidal Assembly of Silica Nanoparticles into Microspheres with Tunable Mesopores

Colloidal assembly of silica (nano)particles is a powerful method to design functional materials across multiple length scales. Although this method has enabled the fabrication of a wide range of silica-based materials, attempts to design and synthesize porous materials with a high level of tuneability and control over pore dimensions have remained relatively unsuccessful. Here, the colloidal assembly of silica nanoparticles into mesoporous silica microspheres (MSMs) is reported using a discrete set of silica sols within the confinement of a water-in-oil emulsion system. By studying the independent manipulation of different assembly parameters during the sol–gel process, a design strategy is outlined to synthesize MSMs with excellent reproducibility and independent control over pore size and overall porosity, which does not require additional ageing or post-treatment steps to reach pore sizes as large as 50 nm. The strategy presented here can provide the necessary tools for the microstructural design of the next generation of tailor-made silica microspheres for use in separation applications and beyond

Bridging-type changes facilitate successive oxidation steps at about 1 V in two binuclear manganese complexes - implications for photosynthetic water-oxidation

The redox behavior of two synthetic manganese complexes illustrates a mechanistic aspect of importance for light-driven water oxidation in Photosystem 11 (PSII) and design of biomimetic systems (artificial photosynthesis). The coupling between changes in oxidation state and structural changes was investigated for two binuclear manganese complexes (1 and 2), which differ in the set of first sphere ligands to Mn (N3O3 in 1, N2O4 in 2). Both complexes were studied by electron paramagnetic resonance (EPR) and X-ray absorption spectroscopy (XAS) in three oxidation states which had been previously prepared either electro- or photochemically. The following bridging-type changes are suggested. In 1: Mn-II-(mu-OR)(mu-OCO)(2)-Mn-II double left right arrow Mn-II-(mu-OR)(mu-OCO)(2)-Mn-III double right arrow Mn-III-(mu-OR)(mu-OCO)-(mu-O)-Mn-IV. In 2: Mn-II-(mu-OR)(mu-OCO)(2)-Mn-III double left right arrow Mn-III-(mu-OR)(mu-OCO)(2)-Mn-III double right arrow Mn-III-(mu-OR)([mu-OCO)(mu-O)-Mn-IV. In both complexes, the first one-electron oxidation proceeds without bridging-type change, but involves a redox-potential increase by 0.5-1 V. The second one-electron oxidation likely is coupled to mu-oxo-bridge (or mu-OH) formation which seems to counteract a further potential increase. In both complexes, mu-O(H) bridge formation is associated with a redox transition proceeding at similar to 1 V, but the mu-O(H) bridge is observed at the Mn-2(III,III) level in I and at the Mn-III,Mn-IV level in 2, demonstrating modulation of the redox behavior by the terminal ligands. It is proposed that also in PSII bridging-type changes facilitate successive oxidation steps at approximately the same potential. (c) 2006 Elsevier Inc. All rights reserved

DTPA-Functionalized Silica Nano- and Microparticles for Adsorption and Chromatographic Separation of Rare Earth Elements

Silica nanoparticles and porous microparticles have been successfully functionalized with a monolayer of DTPA-derived ligands. The ligand grafting is chemically robust and does not appreciably influence the morphology or the structure of the material. The produced particles exhibit quick kinetics and high capacity for REE adsorption. The feasibility of using the DTPA-functionalized microparticles for chromatographic separation of rare earth elements has been investigated for different sample concentrations, elution modes, eluent concentrations, eluent flow rates, and column temperatures. Good separation of the La(III), Ce(III), Pr(III), Nd(III), and Dy(III) ions was achieved using HNO3 as eluent using a linear concentration gradient from 0 to 0.15 M over 55 min. The long-term performance of the functionalized column has been verified, with very little deterioration recorded over more than 50 experiments. The results of this study demonstrate the potential for using DTPA-functionalized silica particles in a chromatographic process for separating these valuable elements from waste sources, as an environmentally preferable alternative to standard solvent-intensive processes.QC 20180509</p

Local quantification of mesoporous silica microspheres using multiscale electron tomography and lattice Boltzmann simulations

The multiscale pore structure of mesoporous silica microspheres plays an important role for tuning mass transfer kinetics in technological applications such as liquid chromatography. While local analysis of a pore network in such materials has been previously achieved, multiscale quantification of microspheres down to the nanometer scale pore level is still lacking. Here we demonstrate for the first time, by combining low convergence angle scanning transmission electron microscopy tomography (LC-STEM tomography) with image analysis and lattice Boltzmann simulations, that the multiscale pore network of commercial mesoporous silica microspheres can be quantified. This includes comparing the local tortuosity and intraparticle diffusion coefficients between different regions within the same microsphere. The results, spanning more than two orders of magnitude between nanostructures and entire object, are in good agreement with bulk characterization techniques such as nitrogen gas physisorption and add valuable local information for tuning mass transfer behavior (in liquid chromatography or catalysis) on the single microsphere level