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Introduction: Photosynthetic water oxidation

Photosynthesis is the only sustainable energy input into our ecosystem and thereby forms the basis for the life on earth. It has the potential to reveal the blue prints for artificial systems for solar hydrogen and oxygen production from water; a process that is likely to be essential in the near future for an environmentally safe energy production. Of special interest in this regard are the structural and functional details of photosynthetic water splitting. Figure 1 shows the overall architecture of photosystem II that was revealed by recent X-ray crystallography studies (1,2,3).

Figure 1: Schematic view of the photosystem II (PSII) complex in the thylakoid membrane that is based on the 3.0 Å crystal structure of Loll et al.(Nature 2005, 438, 1040-1044). For clarity the inner antenna proteins CP43 and CP47, which are involved in harvesting the light energy, and the cytochrome b559 subunits are not shown. The other core proteins of PSII are shown in grey with corresponding labels. The cofactors of the D1, D2, cyt c550 and cytb559 proteins are placed in color on top of the proteins. The arrows indicate the electron transfer reactions and the water splitting chemistry that are induced by light-driven charge separations between P680, a chlorophyll tetramer, and the pheophytin molecule (Pheo) of the D1 protein.

Structure of Mn 4 O 5 Ca cluster

Figure 2: S 2   state, of the OEC, structutal model. Adopted from [DOI: 10.1021/ja2041805 ]

Mechanism of photosynthetic water splitting

Figure 3 : Flash-induced oxygen evolution pattern (FIOP) of dark-adapted spinach thylakoids (left) and Kok model (right). FIOPs were first measured by Joliot et al. 1969 (Photochem. Photobiol., 10, 309-329). 

References

1-Architecture of the Photosynthetic Oxygen-Evolving Center. Ferreira, et al., Science 2004, DOI:10.1126/science.1093087

2-Towards complete cofactor arrangement in the 3.0 angstrom resolution structure of photosystem II. Loll et al., Nature 2005, doi: 10.1038/nature04224

3- Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9Umena et al. Nature 2011, Doi:10.1038/nature09913

4-The Mn Cluster in the S0 State of the Oxygen-Evolving Complex of Photosystem II Studied by EXAFS Spectroscopy:
Are There Three Di-oxo-bridged Mn Moieties in the Tetranuclear Mn Complex? Robbllee et al., J. Am. Chem. Soc. 2002, DOI: 10.1021/ja011621a

5-Where Water Is Oxidized to Dioxygen: Structure of the Photosynthetic Mn4Ca Cluster. Yano, et al., Science 2006, DOI:10.1126/science.1128186

6-Structure and Orientation of the Mn4Ca Cluster in Plant Photosystem II Membranes Studied by Polarized Range-extended X-ray Absorption Spectroscopy. Pushkar et al., THE JOURNAL OF BIOLOGICAL CHEMISTRY 2007, Doi: 10.1074/jbc.M610505200

7-Structural changes in the Mn4Ca cluster and the mechanism of photosynthetic water splitting. Pushkar et al., PNAS 2008,
Doi: 10.1073/pnas.0707092105

8-Theoretical Evaluation of Structural Models of the S(2) State in the Oxygen Evolving Complex of  Photosystem II: Protonation States and Magnetic Interactions.Ames et al., J Am Chem Soc. 2011, Doi: 10.1021/ja2041805

9- Detection of one slowly exchanging substrate water molecule in the S3 state
of photosystem II. Messinger et al., Proc. Natl. Acad. Sci. 1995, PMID: 11607525 [PubMed]

10- Is Mn-Bound Substrate Water Protonated in the S2 State of Photosystem II?Su et al., Appl. Magn. Reson. 2010 , DOI: 10.1007/s00723-009-0051-1

11-Electronic structure of the Mn4OxCa cluster in the S0 and S2 states of the oxygen-evolving complex of photosystem II based on pulse 55Mn-ENDOR and EPR spectroscopy.Kulik et al., J. Am. Chem. Soc. 2007, DOI: 10.1021/ja071487f

12-Structures and Energetics for O2 Formation in Photosystem II. Siegbahn., Acc. Chem. Res., 2009, DOI: 10.1021/ar900117k

13-FTIR detection of water reactions in the oxygen-evolving centre of photosystem II. T.Noguchi , Phil.  Trans. R. Soc. B 2008 , Doi: 10.1098/rstb.2007.2214.

14-  Mechanism of photosynthetic oxygen production. In Photosystem II. The light-driven water:plastoquinone oxidoreductase, Hillier W, Messinger JAdvances in Photosynthesis and Respiration Vol. 22, 2005 (Wydrzynski T, Satoh K, Eds.), Springer, Netherlands, pp. 567-608

Artificial photosynthesis projects

The artificial leaf transforms solar energy into a chemical fuel such as H 2 . The vision is that this wireless device (Fig 4) consists of a printable membrane comprising light-absorbing pigments and electron donors/acceptors for vectorial charge transport across the membrane. The membrane provides three-dimensional orientation and protects sensitive components from the surrounding water and oxygen. It is coated with transparent layers of a water-splitting catalyst on one side, and a proton-reducing catalyst on the other side. It also contains proton-conducting channels/materials that are impermeable to gas. Furthermore, a stacked arrangement of several membranes containing different pigments, or a combination of two or more pigments acting in series within one membrane, will be utilized so that the amount of harvested solar radiation can be maximized. The great advantages of such an integrated design are the opportunity for low-cost fabrication via printing, the ease of up-scaling, and the simplicity of use. The design also leads to an immediate separation of the produced O 2 and H 2 , which reduces the costs for gas separation/purification. An important goal is that the artificial leaf will be built from earth-abundant elements. We will investigate both purpose designed molecular assemblies oriented within a membrane, and "conventional" (organic) solar cells decorated with a catalyst pair.

Figure 5: The two half-reactions of biological water-splitting into molecular hydrogen (H 2 ) and oxygen (O 2 ) that is catalyzed by photosystem II and hydrogenases (middle) in certain green algae and cyanobacteria. In the bottom a scheme for an artificial, solar-light driven water-splitting device is shown.

The fact that this reaction does occur under specific conditions in cyanobacteria shows that a detailed understanding of the underlying reactions can reveal crucial information for the development of artificial systems.

Artificial leaf project (ALP)

We have the following subprojects:

Project A: Light-harvesting and charge separation in artificial devices
PIs: Bertil Eliasson, Ludvig Edman, Thomas Wågberg, Mattias Marklund

Project B: Water-binding and water-exchange at the Mn 4 O 5 Ca cluster in photosystem II and artificial catalysts
PIs: Johannes Messinger, Göran Samuelson, Jyri-Pekka Mikkola

Project C: Artificial catalysts for water-oxidation into O 2 and H +
PIs: Jyri Pekka Mikkola, Thomas Wågberg, Ludvig Edman, Johannes Messinger

Project D: Electron transfer and H + reduction to H 2
PIs: Thomas Wågberg, Jyri-Pekka Mikkola, Ludvig Edman, Johannes Messinger

Project E: Proton-coupled electron transfer and H + release into the bulk
PIs: Mattias Marklund, Göran Samuelson, Johannes Messinger

Project F: Design and manufacturing of the artificial leaf
PIs: Ludvig Edman, Thomas Wågberg, Johannes Messinger, Jyri-Pekka Mikkol


Figure 4: Schematic representation of a wireless and printable artificial leaf device for solar-driven water-splitting into O 2 and H 2

Solar Fuels Environment (SFE)

Profile of the environment

The environment links teams of scientists from a broad spectrum of disciplines with the aim to combine their special expertise to approach questions that cannot be answered by any of the individual disciplines or teams by themselves. Emphasis is placed on novel cross-over collaborations that bring together scientist from biology, chemistry, physics, and mathematics to focus on all aspects of photon harvesting, photon transformation and photon usage for the future benefit of mankind. The research will be focused on Natural and Artificial Photosynthesis. In this multidisciplinary project we bring together experimentalists with theoreticians, which act in surface chemistry, device physics, nanophysics, biophysics, structural biology, plant biochemistry/molecular biology and mathematical/computational research, with the clear aim to boost cross-fertilization between these disciplines.

Strengths of the environment

o SFE is a multidisciplinary composed research environment.
o The involved research groups cover a broad competence from mathematics/theoretical physics to experimental biology, each PI contributing with excellence in their research field.
o The involved research groups are located within close reach of each other and will have full access to KBC´s platforms. This allows the use of state-of-the-art equipment and enables efficient research.
o The researchers in the proposed environment have shown national and international excellence. SFE includes several young researchers that were granted Young Researcher Awards, and several have received external prestigious grants and/or positions, such as the ERC Starting Independent Researcher grant and KVA Research Fellow positions.
o The environment will have projects with a clear focus and exceptional timeliness.

The primary aims are to reveal in detail those processes that are of importance to understand how light is absorbed and converted into energy forms that can be used by man. We will both explore natural systems as well as create devices in a model scale, partly mimicking or even improving the natural photosynthetic machineries, for the production of usable energy. For achieving both aims mathematical and physical modelling in combination with experimental data is required. Our ambition is to move from a detailed knowledge in relatively small-scale models to the complete understanding of the processes in complex photosynthetic systems. We will use the knowledge derived from the natural mechanisms to construct and optimize artificial photosynthetic devices.

Natural photosynthesis

Using experimental and theoretical tools we will in particular focus on resolving the following questions:
o Mechanisms and driving forces for proton transport away from the water splitting centre.
o Geometric and electronic structure of the water-splitting MnOCa cluster.
o Binding of substrate water to the water-splitting cluster.
o Molecular dynamics of PS II donor side.
o Structural water and PS II proteins.
o Regulation of assembly of the photosynthetic apparatus.
o Signalling.
o Regulatory proteins and post-translational regulations.
o Structure of photosynthetic proteins.
o Light harvesting and charge separation.

Artificial photosynthesis

The following separate research projects have been identified that will develop the knowledge and know-how required for constructing the ‘artificial leaf’:
o Study of water-binding to inorganic model complexes and of the properties of water at different (catalytic) surfaces.
o Synthesis, functionalization and detailed characterization of carbon nanotubes
o Design and fabrication of flexible and efficient organic solar cells, preferably in a manner consistent with low-cost and high-throughput roll-to-roll fabrication.
o Theoretical description of light-harvesting, charge separation, electron/hole transfer and catalysis in the artificial devices.

Modelling:

Qualitative models:
 a) Structural models, such as networks, with simple discrete interactions
 b) Structural models with continuous interactions
 c) Fully dynamical models.

Quantitative models:
 a) Exact data of potential structure of the molecular environment of charge transport
 b) Back-reaction on the molecular structures
 c) Quantum dynamics of the charge transport



The research in my group concentrates on the mechanism of water-oxidation in photosynthesis and artificial systems. We employ a large range of biophysical techniques such as polarographic oxygen measurements, time-resolved mass spectrometry, EPR and NMR relaxation. In natural photosynthesis, we study the structure of Mn 4 CaO 5 cluster using advanced EPR (cooperation with Nicholas Cox and Wolfgang Lubitz, Mülheim), X-ray spectroscopy (cooperation with Juko Yano and Vittal Yachandra, Berkeley) and DFT calculations (cooperation with Frank Neese, Mülheim). We have also a strong focus on substrate binding to the Mn 4 CaO 5 cluster. Our primary technique here is time-resolved membrane inlet mass spectrometry. More recently we also started using NMR relaxation to study this process.
The goal of the artificial photosynthesis projects is the development of an artificial leaf (see below) that split water into O 2 and H 2 when exposed to sunlight. My group is responsible for developing and testing water splitting catalysts that are made of abundant and environmentally safe materials. Critical aspects are the catalytic rate and long term stability. We test this using time resolved membrane inlet mass spectrometry in combination with electrochemical methods. Below a brief introduction into photosynthetic water splitting is given and some recent results and projects are briefly summarized.

During light-driven catalysis the OEC cycles through five different oxidation states, which are denoted as S 0 , S 1 , S 2 , S 3 and S 4 states (Kok model; Figure 3). This was first revealed about 40 years ago by flash-induced oxygen evolution patterns of well dark-adapted PSII preparations. The most striking features of flash induced oxygen evolution patterns are the period four oscillation that corresponds to the four electrons that need to be removed from two water molecules for O 2 formation and the first maximum after the 3 rd flash. On that basis Kok et al. proposed the S i state cycle (i = 0, 1, 2, 3, 4 stored oxidizing equivalents). The small oxygen yield after the 2 nd flash and the damping of the oscillation are explained in this model by the assumption that each flash-induced turnover is coupled to a double hit probability and a miss hit probability. The pronounced maximum after the 3 rd flash indicates that almost all centers are in the dark-stable S 1 state after sufficiently long dark-adaptation. The damping of the oscillation is caused by "misses" and "double hits". For more details see for example reference (14)

Our research is guided by the principles learned from the natural systems. Figure 5 below shows that the combination of PSII and hydrogenase can in principle bypass the latter reaction and lead to solar water splitting into O 2 and H 2 .

Water-splitting in photosystem II leads to the formation of molecular oxygen, four protons and four metabolically bound electrons of high potential that are eventually used for CO 2 reduction.
Water splitting is carried out in photosystem II by the oxygen evolving complex (OEC). The heart of this complex is a cluster of four manganese ions, one calcium ion and at least five connecting oxygen bridges (Mn 4 O 5 Ca cluster). This inorganic core is surrounded by a functionally important protein matrix that (i) supports structural changes, (ii) allows efficient coupling of electron and proton transfer reactions and (iii) regulates the access of water to the catalytic site. In addition chloride is discussed as a cofactor for water splitting, but is not a direct ligand. The structure of the Mn 4 O 5 Ca cluster was intensively studied by EPR and EXAFS spectroscopy, X-ray crystallography and DFT calculations. The latest crystallographic model was reported at 1.9 A by Umena et al. (2011) requires adjustments to compensate for radiation induced lengthening of the Mn-O and Mn-Mn, Mn-Ca distances. DFT calculations for optimizing, an energy minimized structure that is consistent with (polarized) EXAFS data (4,5,6,7) and spectroscopic information obtained by advanced EPR for the S2 state (8).
Figure 2 below shows our most recent proposal that gives a detailed suggestion for the protonation state. At present, it is not clear which (if any) of the terminal water/hydroxo molecules in this structure are the substrate molecules, or if for example the oxo-bridge between the Mn A -Mn B and the Ca is the slow substrate " water" (9,10,11). It has also been proposed that the fast exchanging substrate binds during the S 2 to S 3 transition (12,13)

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