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Electronic Structure and Substrate Binding of the Water-Oxidizing Electronic Structure and Substrate Binding of the Water- Oxidizing Manganese Cluster in Photosystem II as Studied by Pulse EPR and ENDOR Spectroscopy vorgelegt von Master of Science Thomas Frank Lohmiller geb. in Herrenberg von der Fakultät II - Mathematik und Naturwissenschaften der Technischen Universität Berlin zur Erlangung des akademischen Grades Doktor der Naturwissenschaften Dr. rer. nat. genehmigte Dissertation Promotionsausschuss: Vorsitzender: Prof. Dr. Andreas Grohmann Berichter/Gutachter: Prof. Dr. Wolfgang Lubitz Berichter/Gutachter: Prof. Dr. Peter Hildebrandt Tag der wissenschaftlichen Aussprache: 13. Juni 2014 Berlin 2016 D 83 Abstract Oxygenic photosynthesis is the biochemical process by which plants, algae and cyanobacteria convert solar into chemical energy, fixing carbon from CO2 in the form of carbohydrates. As such, it produces almost all biomass and biological primary energy on Earth and is the origin of our oxygen atmosphere, essential for all aerobic life. Oxygen is generated from light-driven oxidation of water by the oxygen-evolving complex (OEC), a Mn4O5Ca cofactor of photosys- tem II (PSII). Understanding this catalytic reaction is not only of great interest in a biological and ecological context, but also with respect to sustainable energy production. In the present work, an array of multifrequency EPR methods was employed to characterize the OEC in defined states of the catalytic Si cycle (i = 0-4). This includes the identification of the oxidation states of the individual Mn ions and sites and modes of substrate binding, as well as of the effects of structural perturbations and the function of the Ca2+ ion, specifically: (i) The current state of research on the OEC and oligonuclear Mn model compounds is reviewed focusing on concepts and potentials of and recent advances from (pulsed) EPR experiments and their combination with theoretical (DFT) calculations. (ii) An EPR/55Mn-ENDOR-based investigation of the OEC inhibited by Ca2+-depletion has been conducted. It is shown that the Ca2+ ion is not crucial for its electronic and spatial structure, but instead is possibly essential for proton-coupled electron transfer from the Mn ions to the nearby tyrosine YZ and/or substrate binding and activation. (iii) A combined EPR/DFT study of the electronic structures and ligand binding results in a 2+ 2+ common general model for native, Ca /Sr -exchanged and NH3-modified S2 state variants. On this basis, perturbation of the interactions with 17O ligands allows for an unambiguous assignment of the only exchangeable µ-oxo bridge to a specific oxygen in the cluster, next to the Ca2+/Sr2+ site and probably the early binding substrate. (iv) An EPR/55Mn-ENDOR analysis characterizes the regenerated catalyst immediately III IV after O2 release (S0 state). It represents a Mn 3Mn complex, in which the position of the MnIV is assigned via the 14N coupling of the histidine ligand of the OEC and DFT modelling. 17O and 2H spectra identify a single, exchangeable µ-hydroxo bridge, con- sistent with the DFT model and the S2 state, and thus the early binding substrate water. The combined outcome is a conclusive mechanistic description of the first half of the reaction cycle of photosynthetic water oxidation, starting from catalyst reactivation after oxygen re- lease and including Mn oxidation state changes, substrate binding and deprotonation events. iii Abstract iv Zusammenfassung Mittels der oxygenen Photosynthese wandeln Pflanzen, Algen und Cyanobakterien Sonnen- energie in chemische Energie um und speichern Kohlenstoff aus CO2 in Form von Kohlen- hydraten. Sie erzeugt fast die gesamte Biomasse und biologische Primärenergie auf der Erde sowie die für aerobes Leben essentielle Sauerstoffatmosphäre. O2 entsteht bei der lichtgetrie- benen Oxidation von Wasser durch den Mn4O5Ca-Kofaktor OEC des Enzyms Photosystem II (PSII). Diese katalytische Reaktion ist daher im biologischen und ökologischen Kontext wie auch für eine nachhaltige Energieerzeugung von größtem Interesse. In der vorliegenden Arbeit wurden EPR-Methoden zur Charakterisierung des OEC in mehre- ren Zuständen des Si-Katalysezyklus (i = 0-4) verwendet. Dies umfasst die Bestimmung der Oxidationszustände der einzelnen Mn-Ionen, der Bindungsstellen und –formen der Wasser- Substrate, der Auswirkungen struktureller Modifikationen und der Funktion des Ca2+-Ions. (i) Der derzeitige Forschungsstand zum OEC und oligonuklearen Mn-Modellverbindun- gen wird zusammengefasst, mit Schwerpunkt auf der EPR-Methodik, kombiniert mit DFT-Rechnungen. Konzepte, Potential sowie neueste Erkenntnisse werden vorgestellt. (ii) Eine EPR/55Mn-ENDOR-basierte Untersuchung des durch Entfernung des Ca2+-Ions inhibierten OEC zeigt, dass das Ca2+ nicht entscheidend ist für die elektronische und räumliche Struktur, sondern wahrscheinlich für den Protonen-gekoppelten Elektronen- transfer zum benachbarten Tyrosin YZ und/oder für Substratbindung und –aktivierung. (iii) Eine EPR/DFT-Studie ihrer Strukturen und Ligandenbindungen führt zu einem allge- 2+ 2+ meinen Modell für den nativen und die Ca /Sr - und NH3-modifizierten S2-Zustände. Dadurch ermöglicht die Variation von 17O-Ligandensignalen die Zuordnung der einzi- gen austauschbaren µ-Oxo-Brücke zu einem Sauerstoff, der an das Ca2+/Sr2+-Ion ko- ordiniert. Er resultiert wahrscheinlich vom zuerst bindenden Substratwasser. 55 (iv) Eine EPR/ Mn-ENDOR-Analyse charakterisiert den Katalysator direkt nach der O2- III IV IV Freisetzung (S0-Zustand) als Mn 3Mn -Komplex. Das Mn -Ion lässt sich über die 14N-Kopplung eines Histidins im OEC und DFT-Modellierung zuordnen. 17O- und 2H- Spektren identifizieren eine einzelne µ-Hydroxo-Brücke, übereinstimmend mit DFT- Modell und S2-Zustand, als austauschbar und somit als das zuerst bindende Substrat. Zusammen führt dies zu einer mechanistischen Beschreibung der ersten Hälfte des katalyti- schen Zyklus der Wasseroxidation, beginnend mit der Reaktivierung des OEC durch Substrat- bindung, einschließlich der Änderungen der Mn-Oxidationszustände und Deprotonierungen. v Zusammenfassung vi Table of Contents Chapter 1 Motivation: Chemical Energy Conversion and Storage and Nature’s Water-Oxidizing Complex ....... 1 1.1 References ................................................................................................................................................. 4 Chapter 2 Photosynthesis and Photosytem II .......................................................................................... 7 2.1 Overview ................................................................................................................................................... 7 2.2 The Light Reactions in Oxygenic Photosynthesis .................................................................................. 9 2.3 Photosystem II ........................................................................................................................................ 12 2.3.1 Structure .............................................................................................................................................. 12 2.3.2 Charge Separation and Electron Transfer ............................................................................................ 13 2.3.3 The Tyrosines YZ and YD .................................................................................................................... 15 2.3.4 The Oxygen-Evolving Complex (OEC) .............................................................................................. 15 2.4 References ............................................................................................................................................... 17 Chapter 3 Electron Paramagnetic Resonance ....................................................................................... 21 3.1 The Spin Hamiltonian ............................................................................................................................ 21 3.1.1 The Electron and Nuclear Zeeman Interactions ................................................................................... 22 3.1.2 The Fine Structure or Zero-Field Splitting .......................................................................................... 23 3.1.3 The Electron Exchange Interaction...................................................................................................... 24 3.1.4 The Hyperfine Interaction.................................................................................................................... 25 3.1.5 The Nuclear Quadrupole Interaction ................................................................................................... 26 3.2 EPR Spectroscopy .................................................................................................................................. 27 3.2.1 The Continuous-Wave (CW) EPR Experiment ................................................................................... 28 3.2.2 Pulse EPR Experiments ....................................................................................................................... 29 3.2.2.1 Electron Spin Echo (ESE)-detected EPR .................................................................................... 30 3.2.2.2 Two- and Three-Pulse ESE Envelope Modulation (ESEEM) and Hyperfine Sublevel Correlation (HYSCORE) ................................................................................................................................................. 32 3.2.2.3 Multi-Resonance Spin-Polarization-Transfer
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