Search

In the present contribution, we report a theoretical investigation of the magnetic properties of the dihydrogen-evolving enzyme [FeFe]-hydrogenase, based on both DFT models of the active site (the H-cluster, a Fe6S 6 assembly including a binuclear portion directly involved in substrates binding), and QM/MM models of the whole enzyme. Antiferromagnetic coupling within the H-cluster has been treated using the broken-symmetry approach, along with the use of different density functionals. Results of g value calculations turned out to vary as a function of the level of theory and of the extension of the model. The choice of the broken-symmetry coupling scheme also had a significant influence on the calculated g values, for both the active-ready (Hox) and the CO-inhibited (Hox-CO) enzyme forms. However, hyperfine coupling-constant calculations were found to provide more consistent results. This allowed us to show that the experimentally detected delocalization of an unpaired electron at the binuclear subcluster in Desulfovibrio desulfuricans Hox is compatible with a weak interaction between the catalytic centre and a low-weight exogenous ligand like a water molecule. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA.

Transient electron paramagnetic resonance and pulsed electron-nuclear double resonance (ENDOR) spectra of the state P(700)(center dot+)A(1) in photosystem I containing a series of non-native naphthoquinones (NQs) are presented. Previous studies have shown that quinones bind to the A(1) site with only one of their carbonyl groups H-bonded to the protein and that the asymmetric H-bond produces an odd alternant distribution of the spin density within the quinone. It is known that the native phylloquinone binds with its methyl group meta and its phytyl tail ortho to the H-bonded carbonyl. Monosubstituted NQs with short alkyl chains have been found to bind preferentially with their alkyl side groups meta to the H-bonded carbonyl. The selectivity of the binding site toward methyl and short chain substituents is studied by incorporating disubstituted NQs that have a methyl group at the 2-position and a short chain at the 3-position of the quinone ring. The hyperfine couplings (hfcs) of the methyl group protons are sensitive to the spin density distribution on the quinone and are used to deduce the position of the methyl group relative to the H-bonded carbonyl. The measured methyl proton hfcs indicate that the disubstituted quinones bind exclusively with their methyl group in the meta position relative to the H-bonded carbonyl and no evidence for binding with the methyl group in the ortho position is found. The disubstituted quinones have also been chosen to study the effect of electron withdrawing substituents on the spin density distribution. When the short chain contains electronegative atoms such as sulfur or chlorine, the methyl proton hfcs of the quinone in the A(1) binding site are found to be significantly larger than those of 2-methyl-1,4-naphthoquinone and phylloquinone in the same environment. Solution ENDOR measurements of the quinone radical anions in isopropanol and density functional theory (DFT) calculations in vacuo show that this increase in the hfcs is mo

Beautifully presented, this publication reveals the knowledge in several layers appealing to a wide audience. Parts of the publication are so easy to read and so well illustrated that every reader will be able to get a glimpse of how key inventions from the past feed science and technology today and what they will bring us in the future. Business people and professionals will be able to discover opportunities revealed through innovative scientific research. Students will get a wider perspective on what science is about and finally scientists themselves will discover how seemingly unrelated discoveries may have a profound impact on their own research. Now it's your turn to discover "The Treasures of Eureka" created to entertain and challenge people from different walks of life, including you.The Treasures of Eureka" series starts from the publication devoted to a recent invention, which in just a short 70 years became one of the key methods of matter research and diagnostic and is widely used from control of food processing to medical treatment, from discovery of new superconductors to building elements of quantum computers.It might not be common knowledge that all these are referring to the discovery of the Electron Paramagnetic Resonance in 1944 by E.K. Zavoisky in Kazan, Tatarstan, Russia. An international Zavoisky Award has been established in 1991 to commemorate the significance of EPR theoretical and practical applications. Since 1991 the Zavoisky Award is awarded in Kazan, Russia, to recognize outstanding applications or developments of electron paramagnetic resonance in any field of science. This publication is a celebration of the EPR discovery, scientists working in this area, Zavoisky Awardees and technology applications which enhance quality of everyday living for every human on the planet today. Over 100 of the world's leading EPR experts contributed articles to introduce the marvels of EPR theoretical research and its practical applications to a wider a, THE TREASURES OF EUREKAELECTRON PARAMAGNETIC RESONANCE, From Fundamental Research to Pioneering Applications & Zavoisky Award

Pulse EPR, Mn-55-ENDOR and ELDOR-detected NMR experiments were performed on the S-2-state of the oxygen-evolving complex from spinach Photosystem II. The novel technique of random acquisition in ENDOR was used to suppress heating artefacts. Our data unambiguously shows that four Mn ions have significant hyperfine coupling constants. Numerical simulation of the Mn-55-ENDOR spectrum allowed the determination of the principal values of the hyperfine interaction tensors for all four Mn ions of the oxygen-evolving complex. The results of our Mn-55-ENDOR experiments are in good agreement with previously published data [Peloquin JM et al. (2000) J Am Chem Soc 122: 10926-10942]. For the first time ELDOR-detected NMR was applied to the S-2-state and revealed a broad peak that can be simulated numerically with the same parameters that were used for the simulation of the Mn-55-ENDOR spectrum. This provides strong independent support for the assigned hyperfine parameters.

The temperature dependence of the electron spin-lattice relaxation time T, was measured for the So state of the oxygen-evolving complex (OEC) in photosystem II and for two dinuclear manganese model complexes by pulse EPR using the inversion-recovery method. For [Mn(III)Mn(IV)(mu-O)(2)bipy(4)]ClO4, the Raman relaxation process dominates at temperatures below 50 K. In contrast, Orbach type relaxation was found for [Mn(II)Mn(III)(mu-OH)(mu-piv)(2)(Me(3)tacn)(2)](ClO4)(2) between 4.3 and 9 K. For the latter complex, an energy separation of 24.7-28.0 cm(-1) between the ground and the first excited electronic state was determined. In the So state of photosystem II, the T-1 relaxation times were measured in the range of 4.3-6.5 K. A comparison with the relaxation data (rate and pre-exponential factor) of the two model complexes and of the S-2 state of photosystem II indicates that the Orbach relaxation process is dominant for the So state and that its first excited state lies 21.7 +/- 0.4 cm(-1) above its ground state. The results are discussed with respect to the structure of the OEC in photosystem II.

In spinach photosystem II (PSII) membranes, the tetranuclear manganese cluster of the oxygen-evolving complex (OEC) can be reduced by incubation with nitric oxide at -30 degreesC to a state which is characterized by an Mn-2(II, III) EPR multiline signal [Sarrou, J., Ioannidis, N., Deligiannakis, Y., and Petrouleas, V. (1998) Biochemistry 37, 3581-3587]. This state was recently assigned to the S-2 state of the OEC [Schansker, G., Goussias, C., Petrouleas, V., and Rutherford, A. W. (2002) Biochemistry 41, 3057-3064]. On the basis of EPR spectroscopy and flash-induced oxygen evolution patterns, we show that a similar reduction process takes place in PSII samples of the thermophilic cyanobacterium Synechococcus elongatus at both -30 and 0 degreesC. An EPR multiline signal, very similar but not identical to that of the S-2 state in spinach, was obtained with monomeric and dimeric PSII core complexes from S. elongatus only after incubation at -30 degreesC. The assignment of this EPR multiline signal to the S-2 state is corroborated by measurements of flash-induced oxygen evolution patterns and detailed fits using extended Kok models. The small reproducible shifts of several low-field peak positions of the S-2 EPR multiline signal in S. elongatus compared to spinach suggest that slight differences in the coordination geometry and/or the ligands of the manganese cluster exist between thermophilic cyanobacteria and higher plants.

Oxidation of histidine by OH* radicals has been studied at room temperature over a large range of pH values in a Ti3+/EDTA/H2O2-Fenton system using a special EPR continuous-flow setup. At pH 7 to 8, during fast flow, an EPR spectrum from a new transient h, Addresses: Lassmann G, Tech Univ Berlin, Max Volmer Inst Biophys & Biochem, D-10623 Berlin, Germany. Tech Univ Berlin, Max Volmer Inst Biophys & Biochem, D-10623 Berlin, Germany. Uppsala Univ, Dept Quantum Chem, S-75120 Uppsala, Sweden.

Transient histidine radicals formed in aqueous solutions by oxidation of histidine with a Ti3+/H2O2 Fenton system at pH 2.0 have been studied by EPR using a fast continuous-flow setup and a dielectric ring resonator equipped with a mixing chamber. A histi, Lassmann G, Tech Univ Berlin, Max Volmer Inst Biophys Chem & Biochem, D-10623 Berlin, Germany. Tech Univ Berlin, Max Volmer Inst Biophys Chem & Biochem, D-10623 Berlin, Germany. Univ Uppsala, Dept Quantum Chem, S-75120 Uppsala, Sweden. Univ Stockholm, Dep