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Salunke-Gawali S.,University of Pune | Kathawate L.,University of Pune | Shinde Y.,University of Pune | Puranik V.G.,CSIR - National Chemical Laboratory | Weyhermuller T.,MPI fur Bioanorganische Chemie
Journal of Molecular Structure | Year: 2012

2-hydroxy-1,4-naphthoquinone; Lawsone (Lw) is a natural compound found in henna leaves. The reaction of lawsone with 'Na' metal (Lw-1), CH 3COONa (Lw-2), NaOH (Lw-3), KOH (Lw-4), K 2CO 3 (Lw-5) and Tris(hydroxymethyl)aminomethane (Lw-6) were studied. Red orange solids obtained for Lw-1 to Lw-6 are characterized by Elemental Analysis, FTIR, 1HNMR and EPR studies. The results reveal the coordination of alkali metals 'Na' and 'K' to lawsone anion. The single crystal X-ray structure of Lw-6 was solved and it crystallizes in triclinic space group P-1 with extensive hydrogen bonding network of CH⋯O, NH⋯O and OH⋯O between cations and anions. Polycrystalline powder X-band EPR spectra of Lw-1 to Lw-5 shows signals ∼2.004 at 133 K, while Lw-6 is EPR silent. The naphthosemiquinone (NSQ -) radical formed in Lw-2 to Lw-5, is due to disproportion reaction of catechol and naphthoquinone. © 2011 Elsevier B.V. All rights reserved.

Sedoud A.,CEA Saclay Nuclear Research Center | Kastner L.,CEA Saclay Nuclear Research Center | Cox N.,MPI fur Bioanorganische Chemie | El-Alaoui S.,CEA Saclay Nuclear Research Center | And 2 more authors.
Biochimica et Biophysica Acta - Bioenergetics | Year: 2011

EPR was used to study the influence of formate on the electron acceptor side of photosystem II (PSII) from Thermosynechococcus elongatus. Two new EPR signals were found and characterized. The first is assigned to the semiquinone form of QB interacting magnetically with a high spin, non-heme-iron (Fe2+, S = 2) when the native bicarbonate/carbonate ligand is replaced by formate. This assignment is based on several experimental observations, the most important of which were: (i) its presence in the dark in a significant fraction of centers, and (ii) the period-of-two variations in the concentration expected for QB •- when PSII underwent a series of single-electron turnovers. This signal is similar but not identical to the well-know formate-modified EPR signal observed for the QA •-Fe2+ complex (W.F.J. Vermaas and A.W. Rutherford, FEBS Lett. 175 (1984) 243-248). The formate-modified signals from Q A •-Fe2+ and QB •-Fe2+ are also similar to native semiquinone-iron signals (QA •-Fe2+/QB •-Fe2+) seen in purple bacterial reaction centers where a glutamate provides the carboxylate ligand to the iron. The second new signal was formed when QA •- was generated in formate-inhibited PSII when the secondary acceptor was reduced by two electrons. While the signal is reminiscent of the formate-modified semiquinone-iron signals, it is broader and its main turning point has a major sub-peak at higher field. This new signal is attributed to the QA •-Fe2+ with formate bound but which is perturbed when QB is fully reduced, most likely as QBH2 (or possibly QBH•- or QB 2•-). Flash experiments on formate-inhibited PSII monitoring these new EPR signals indicate that the outcome of charge separation on the first two flashes is not greatly modified by formate. However on the third flash and subsequent flashes, the modified QA •-Fe2+QBH 2 signal is trapped in the EPR experiment and there is a marked decrease in the quantum yield of formation of stable charge pairs. The main effect of formate then appears to be on QBH2 exchange and this agrees with earlier studies using different methods. © 2010 Elsevier B.V. All rights reserved.

Cardona T.,CEA Saclay Nuclear Research Center | Sedoud A.,CEA Saclay Nuclear Research Center | Sedoud A.,Imperial College London | Cox N.,MPI fur Bioanorganische Chemie | And 2 more authors.
Biochimica et Biophysica Acta - Bioenergetics | Year: 2012

Our current understanding of the PSII reaction centre owes a great deal to comparisons to the simpler and better understood, purple bacterial reaction centre. Here we provide an overview of the similarities with a focus on charge separation and the electron acceptors. We go on to discuss some of the main differences between the two kinds of reaction centres that have been highlighted by the improving knowledge of PSII. We attempt to relate these differences to functional requirements of water splitting. Some are directly associated with that function, e.g. high oxidation potentials, while others are associated with regulation and protection against photodamage. The protective and regulatory functions are associated with the harsh chemistry performed during its normal function but also with requirements of the enzyme while it is undergoing assembly and repair. Key aspects of PSII reaction centre evolution are also addressed. This article is part of a Special Issue entitled: Photosystem II. © 2011 Elsevier B.V. All rights reserved.

Birk T.,Copenhagen University | Schau-Magnussen M.,Copenhagen University | Weyhermuller T.,MPI fur Bioanorganische Chemie | Bendix J.,Copenhagen University
Acta Crystallographica Section E: Structure Reports Online | Year: 2011

In the title compound, [Cr 2Nd 2F 4(NO 2) 8(C 12H 8N 2) 4]·4CH 3OH·H 2O, two cis-difluoridobis(1,10-phenanthroline)chromium(III) fragments containing octa-hedrally coordinated chromium(III) bridge via fluoride ions to two tetra-nitratoneodymate(III) fragments, forming an uncharged tetra-nuclear square-like core. The fluoride bridges are fairly linear, with Cr - F - Nd angles of 168.74 (8)°. Cr - F bond lengths are 1.8815 (15) Å, slightly elongated compared to those of the parent chromium(III) complex, which has bond lengths ranging from 1.8444 (10) to 1.8621 (10) Å. The tetra-nuclear complex is centered at a fourfold rotoinversion axis, with the Cr and Nd atoms situated on two perpendicular twofold rotation axes. The uncoordinated water molecule resides on a fourfold rotation axis. The four methanol solvent molecules are located around this axis, forming a cyclic hydrogen-bonded arrangement. The title compound is the first structurally characterized example of unsupported fluoride bridges between lanthanide and transition metal ions. © Birk et al. 2011.

Woertink J.S.,Stanford University | Tian L.,Stanford University | Maiti D.,Johns Hopkins University | Lucas H.R.,Johns Hopkins University | And 10 more authors.
Inorganic Chemistry | Year: 2010

A variety of techniques including absorption, magnetic circular dichroism (MCD), variable-temperature, variable-field MCD (VTVH-MCD), and resonance Raman (rR) spectroscopies are combined with density functional theory (DFT) calculations to elucidate the electronic structure of the end-on (η1) bound superoxo-Cu(II) complex [TMG3trenCuO 2]+ (where TMG3tren is 1,1,1-tris[2-[N 2-(1,1,3,3-tetramethylguanidino)]ethyl]amine). The spectral features of [TMG3-trenCuO2]+ are assigned, including the first definitive assignment of a superoxo intraligand transition in a metalsuperoxo complex, and a detailed description of end-on superoxo-Cu(II) bonding is developed. The lack of overlap between the two magnetic orbitals of [TMG3trenCuO2]+ eliminates antiferromagnetic coupling between the copper(II) and the superoxide, while the significant superoxo π* δ character of the copper dz2 orbital leads to its ferromagnetically coupled, triplet, ground state. © 2010 American Chemical Society.

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