| 1. |
- Cox, Nicholas, et al.
(author)
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The S-1 split signal of photosystem II : a tyrosine-manganese coupled interaction
- 2009
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In: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650. ; 1787:7, s. 882-889
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Journal article (peer-reviewed)abstract
- Detailed optical and EPR analyses of states induced in dark-adapted PS II membranes by cryogenic illumination permit characterization and quantification of all pigment derived donors and acceptors, as well as optically silent (in the visible, near infrared) species which are EPR active. Near complete turnover formation of Q(A)(-) is seen in all centers, but with variable efficiency, depending on the donor species. In minimally detergent-exposed PS II membranes, negligible (<5%) oxidation of chlorophyll or carotenoid centers occurs for illumination temperatures 5-20 K. An optically silent electron donor to P680(+) is observed with the same decay kinetics as the S-1 split signal. Cryogenic donors to P680(+) seen are: (i) transient (t(1/2)similar to 150 s) tyrosine related species, including 'split signals' (similar to 15% total centers), (ii) reduced cytochrome b(559) (similar to 30-50% centers), and (iii) an organic donor, possibly an amino acid side chain, (similar to 30% centers).
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| 2. |
- Han, Guangye, et al.
(author)
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Direct quantification of the four individual S states in Photosystem II using EPR spectroscopy
- 2008
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In: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650. ; 1777:6, s. 496-503
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Journal article (peer-reviewed)abstract
- Car, carotenoid; Chl, chlorophyll; ChlZ, secondary chlorophyll electron donor to P680+; Cytb559, cytochrome b559; EPR, electron paramagnetic resonance; DMSO, dimethylsulfoxide; MES, 2-(N-morpholino) ethanesulfonic acid; NIR, near-infrared; OEC, oxygen evolving complex; P680, primary electron donor chlorophylls in PSII; PpBQ, phenyl-p-benzoquinone; PSII, Photosystem II; QA and QB, primary and secondary plastoquinone acceptors of Photosystem II; YD, tyrosine 161 of the PSII D2 polypeptide; YZ, tyrosine 161 of the PSII D1 polypeptide
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| 3. |
- Havelius, Kajsa G.V. 1977-
(author)
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EPR Studies of Photosystem II : Characterizing Water Oxidizing Intermediates at Cryogenic Temperatures
- 2009
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Doctoral thesis (other academic/artistic)abstract
- The principles of natures own light-driven water splitting catalyst, Photosystem II (PSII), can in the future inspire us to use water as electron and proton source to generate light-driven H2 production. To mimic this challenging step, it is important to understand how the enzyme system can oxidize water. The mechanism of light-driven water oxidation in PSII is in this thesis addressed by EPR spectroscopy. P680+ is a strong oxidant formed by light-oxidation of the chlorophyll species P680 positioned in the center of PSII. The redox active tyrosine-Z (YZ) can reduce P680+ and the YZ• radical is formed. This transient radical is further reduced by the CaMn4-cluster, which is the binding site of the substrate water molecules. In a cyclic process called the S-cycle, this catalytic cluster accumulates four oxidizing equivalents to evolve one molecule of O2 and to oxidize two molecules of water. We can induce the YZ• radical at cryogenic temperatures in the different oxidation states of the catalytic S-cycle and observe this in metalloradical EPR signals. These metalloradical EPR signals are here characterized and used to deduce mechanistic information from the intact PSII. The "double nature" of these spin-spin interaction signals, so called split EPR signals, makes them excellent probes to both YZ oxidation and, when YZ• is present, also to the S-states of the CaMn4-cluster. The metalloradical EPR signals presented here, form a way to study the transient YZ• radical in active PSII that has not been depleted of the catalytic metal cluster. This depleting method that has often been used in the past to study YZ is not representing studies of a mechanistically relevant material. The previously suggested disorder around YZ and accessibility to the bulk can be artifactual properties induced in the mechanistically defect PSII. On the contrary, our observation that proton coupled electron transfer from YZ to the light induced P680+ can occur in a high yield at cryogenic temperatures, suggests a well ordered catalytic site in the protein positioned for optimal performance. The optimized positioning of the redox components found in PSII might be a feature also important to build in an efficient water oxidizing catalyst.
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| 4. |
- Havelius, Kajsa G. V., et al.
(author)
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Metalloradical EPR Signals from the Y-Z center dot S-State Intermediates in Photosystem II
- 2010
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In: Applied Magnetic Resonance. - : Springer Science and Business Media LLC. - 0937-9347 .- 1613-7507. ; 37:1-4, s. 151-176
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Research review (peer-reviewed)abstract
- The redox-active tyrosine residue (Y-Z) plays a crucial role in the mechanism of the water oxidation. Metalloradical electron paramagnetic resonance (EPR) signals reflecting the light-induced Y-Z center dot in magnetic interaction with the CaMn4-cluster in the particular S-state, Y-Z center dot S-X intermediates, have been found in intact photosystem II. These so-called split EPR signals are induced by illumination at cryogenic temperatures and provide means to both study the otherwise transient Y-Z center dot and to probe the S-states with EPR spectroscopy. The illumination used for signal induction grouped the observed split EPR signals in two categories: (i) Y-Z in the lower S-states was oxidized by P680(+) formed via charge separation, while (ii) Y-Z in the higher S-states was oxidized by an excited, highly oxidizing Mn species. Applied mechanistic studies of the Y-Z center dot S-X intermediates in the different S-states are reviewed and compared to investigations in photosystem II at physiological temperature. Addition of methanol induced S-state characteristic changes in the split signals' formation which reflect changes in the magnetic coupling within the CaMn4-cluster due to methanol binding. The pH titration of the split EPR signals, on the other hand, could probe the proton-coupled electron transfer properties of the Y-Z oxidation. The apparent pK (a)s found for decreased split signal induction were interpreted in the fate of the phenol proton.
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| 5. |
- Havelius, Kajsa G. V., et al.
(author)
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pH Dependent Competition between YZ and YD in Photosystem II Probed by Illumination at 5 K
- 2007
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In: Biochemistry. - : American Chemical Society (ACS). - 0006-2960 .- 1520-4995. ; 46:26, s. 7865-7874
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Journal article (peer-reviewed)abstract
- The photosystem II (PSII) reaction center contains two redox active tyrosines, YZ and YD, situated on the D1 and D2 proteins, respectively. By illumination at 5 K, oxidation of YZ in oxygen-evolving PSII can be observed as induction of the Split S1 EPR signal from YZ* in magnetic interaction with the CaMn4 cluster, whereas oxidation of YD can be observed as the formation of the free radical EPR signal from YD*. We have followed the light induced induction at 5 K of the Split S1 signal between pH 4-8.5. The formation of the signal, that is, the oxidation of YZ, is pH independent and efficient between pH 5.5 and 8.5. At low pH, the split signal formation decreases with pKa approximately 4.7-4.9. In samples with chemically pre-reduced YD, the pH dependent competition between YZ and YD was studied. Only YZ was oxidized below pH 7.2, but at pH above 7.2, the oxidation of YD became possible, and the formation of the Split S1 signal diminished. The onset of YD oxidation occurred with pKa approximately 8.0, while the Split S1 signal decreased with pKa approximately 7.9 demonstrating that the two tyrosines compete in this pH interval. The results reflect the formation and breaking of hydrogen bonds between YZ and D1-His190 (HisZ) and YD and D2-His190 (HisD), respectively. The oxidation of respective tyrosine at 5 K demands that the hydrogen bond is well-defined; otherwise, the low-temperature oxidation is not possible. The results are discussed in the framework of recent literature data and with respect to the different oxidation kinetics of YZ and YD.
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| 6. |
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| 7. |
- Havelius, Kajsa G. V., et al.
(author)
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The formation of the split EPR signal from the S-3 state of Photosystem II does not involve primary charge separation
- 2011
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In: Biochimica et Biophysica Acta - Bioenergetics. - : Elsevier BV. - 0005-2728 .- 1879-2650 .- 0006-3002 .- 1878-2434. ; 1807:1, s. 11-21
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Journal article (peer-reviewed)abstract
- Metalloradical EPR signals have been found in intact Photosystem II at cryogenic temperatures. They reflect the light-driven formation of the tyrosine Z radical (Y-z(center dot)) in magnetic interaction with the CaMn4 cluster in a particular S state. These so-called split EPR signals, induced at cryogenic temperatures, provide means to study the otherwise transient Y-z(center dot) and to probe the S states with EPR spectroscopy. In the S-0 and S-1 states, the respective split signals are induced by illumination of the sample in the visible light range only. In the S-3 state the split EPR signal is induced irrespective of illumination wavelength within the entire 415-900 nm range (visible and near-IR region) [Su, J. H., Havelius, K. G. V., Ho, F. M., Han, G., Mamedov, F., and Styring, S. (2007) Biochemistry 46. 10703-10712]. An important question is whether a single mechanism can explain the induction of the Split S-3 signal across the entire wavelength range or whether wavelength-dependent mechanisms are required. In this paper we confirm that the Y-z(center dot) radical formation in the S-1 state, reflected in the Split S-1 signal, is driven by P680-centered charge separation. The situation in the S-3 state is different. In Photosystem II centers with pre-reduced quinone A (Q(A)), where the P680-centered charge separation is blocked, the Split S-3 EPR signal could still be induced in the majority of the Photosystem II centers using both visible and NIR (830 nm) light. This shows that P680-centered charge separation is not involved. The amount of oxidized electron donors and reduced electron acceptors (Q(A)(-)) was well correlated after visible light illumination at cryogenic temperatures in the S-1 state. This was not the case in the S-3 state, where the Split S-3 EPR signal was formed in the majority of the centers in a pathway other than P680-centered charge separation. Instead, we propose that one mechanism exists over the entire wavelength interval to drive the formation of the Split S-3 signal. The origin for this, probably involving excitation of one of the Mn ions in the CaMn4 cluster in Photosystem II, is discussed.
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| 8. |
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| 9. |
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| 10. |
- Leidel, Nils, et al.
(author)
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Electronic Structure of an [FeFe] Hydrogenase Model Complex in Solution Revealed by X-ray Absorption Spectroscopy Using Narrow-Band Emission Detection
- 2012
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 134:34, s. 14142-14157
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Journal article (peer-reviewed)abstract
- High-resolution X-ray absorption spectroscopy with narrow-band X-ray emission detection, supported by density functional theory calculations (XAES-DFT), was used to study a model complex, ([Fe-2(mu-adt)(CO)(4)(PMe3)(2)] (1, adt = S-CH2-(NCH2Ph)-CH2-S), of the [FeFe] hydrogenase active site. For 1 in powder material (1(powder)), in MeCN solution (1'), and in its three protonated states (1H, 1Hy, 1HHy; H denotes protonation at the adt-N and Hy protonation of the Fe-Fe bond to form a bridging metal hydride), relations between the molecular structures and the electronic configurations were determined. EXAFS analysis and DFT geometry optimization suggested prevailing rotational isomers in MeCN, which were similar to the crystal structure or exhibited rotation of the (CO) ligands at Fe1 (1(CO), 1Hy(CO)) and in addition of the phenyl ring (1H(CO,ph), 1HHy(CO,ph)), leading to an elongated solvent-exposed Fe-Fe bond. Isomer formation, adt-N protonation, and hydride binding caused spectral changes of core-to-valence (pre-edge of the Fe K-shell absorption) and of valence-to-core (K beta(2,5) emission) electronic transitions, and of K alpha RIXS data, which were quantitatively reproduced by DFT. The study reveals (1) the composition of molecular orbitals, for example, with dominant Fe-d character, showing variations in symmetry and apparent oxidation state at the two Fe ions and a drop in MO energies by similar to 1 eV upon each protonation step, (2) the HOMO-LUMO energy gaps, of similar to 2.3 eV for 1(powder) and similar to 2.0 eV for 1', and (3) the splitting between iron d(z(2)) and d(x(2-)y(2)) levels of similar to 0.5 eV for the nonhydride and similar to 0.9 eV for the hydride states. Good correlations of reduction potentials to LUMO energies and oxidation potentials to HOMO energies were obtained. Two routes of facilitated bridging hydride binding thereby are suggested, involving ligand rotation at Fe1 for 1Hy(CO) or adt-N protonation for 1HHy(CO,ph). XAES-DFT thus enables verification of the effects of ligand substitutions in solution for guided improvement of [FeFe] catalysts.
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