Structural and Functional Models for the Dinuclear Copper Active Site in Catechol Oxidases: Syntheses, X-ray Crystal Structures, Magnetic and Spectral Properties, and X-ray Absorption Spectroscopic Studies in Solid State and in Solution

Two novel tridentate dinucleating ligands containing benzimidazole were prepared, 1,3-bis(2-benzimidazolyl)-2-propanol (Hbbp, 1) and 1,5-bis(2-benzimidazolyl)-3-pentanol (Hbbpen, 2). Their complexing properties toward copper were studied in order to obtain structural and functional models for catechol oxidases. Syntheses and crystal structures of dinuclear Cu(II) complexes derived from these ligands are reported. [Cu(2)bbp(2)](ClO(4))(2).2MeOH, 3, crystallizes in the triclinic space group P&onemacr; with the following unit cell parameters: a = 7.702(3) ?, b = 10.973(6) ?, c = 12.396(6) ?, alpha = 100.59(4) degrees, beta = 99.02(4) degrees, gamma = 98.90(4) degrees, V = 998.7(8) ?(3), and Z = 1. [Cu(2)bbpen(2)](ClO(4))(2).3MeOH, 4, crystallizes in the orthorhombic space group Pccn, with the following unit cell parameters: a = 17.478(9) ?, b = 18.795(8) ?, c = 13.888(6) ?, V = 4562.2(4) ?(3), and Z = 4. Magnetic susceptibility measurements in the temperature ranges 4.6-459 K (3) and 4.6-425 K (4) indicate an antiferromagnetic coupling between the Cu(II) centers of both complexes. In order to determine the structures of the complexes in solution, XAS spectra (EXAFS and XANES) were recorded in the solid state and in solution. The interpretation of these data, including multiple scattering calculations, together with UV-vis titrations, shows that the complexes have the same structure in the crystalline state as well as in methanolic solution. Complex 4 is able to oxidize 3,5-di-tert-butylcatechol (3,5-DTBC) to the quinone (catecholase activity). This reaction was also studied by XAS and UV-vis spectroscopy. These measurements reveal the reduction of Cu(II) to Cu(I) accompanied by a decrease of the coordination number.     

Metal/insulator/metal junctions for electrochemical surface science

We describe the preparation and characterization of Al-AlO_x-Ag tunnel junctions and calculate the energy distribution of the tunneling hot electrons in the range 0–2.5 eV above the Fermi level of silver. Because the mean free path of the hot electrons is of the order of the thickness of the silver film of the junction, which is at the same time the electrode in contact with an electrolyte, new surface effects can be studied. Hot electrons can be injected into the nonhydrated electron band in water. Hot electrons also cause hydrogen evolution at electrode potentials more positive than the ones needed in common electrochemistry. We observed the emission of hot electrons into silver during transients of hydrogen oxidation at silver and during oxidation of overpotential hydrogen on platinum clusters deposited on the silver electrode. The tunnel current at constant tunnel voltage can be changed by faradaic reactions, but surprisingly also by nonfaradaic reactions; this is assigned to a mesoscopic quantum phenomenon.     

The structure of lipid A of the lipopolysaccharide from Burkholderia caryophylli with a 4-amino-4-deoxy-L-arabinopyranose 1-phosphate residue exclusively in glycosidic linkage

From the lipopolysaccharides (LPSs) of the plant-pathogenic bacterium Burkholderia caryophylli, the complete structure of lipid A has been characterized. For the first time, a 4-amino-4-deoxy-L-arabinopyranose 1-phosphate residue was proven to be exclusively linked to the reducing end of lipid A from a wild-type LPS. The LPSs of B. caryophylli were degraded by mild acetate buffer hydrolysis at pH 4.4. The obtained lipid A was analyzed as such, and also after de-O-acylation or dephosphorylation. The structure of lipid A was identified mainly by means of matrix-assisted laser desorption/ionisation mass spectrometry, and by various 1D and 2D (1)H and (13)C NMR spectroscopic measurements.     

Electronic excitation spectrum of an antiferromagnetic semiconductor (EuTe)

We study the electronic quasiparticle spectrum of an antiferromagnetic semiconductor, with a special application to EuTe. By use of the commonly accepteds-f model it is shown that the conduction band splits into two quasiparticle subbands, where this splitting persists in the paramagnetic region (T>T _n ), too. Shape, width and degree of overlapping of the two subbands are strongly temperature dependent and quite different for different types off-spin arrangements.—We present furthermore a critical classification of mean field treatments (Zener model), which neglect all spin exchange processes between conduction electrons and localized moments. Within our many body approach the lower conduction band edge of EuTe does not shift significantly with temperature, in agreement with the experiment, while the Zener model predicts a drastic red shift upon cooling belowT _n .     

Studying nanomagnets and magnetic heterostructures with X-ray PEEM at the Swiss Light Source

Polarization dependent X-ray absorption spectroscopy and microscopy enables the element selective investigation of magnetic systems at the nanoscale. At the Swiss Light Source a photoemission electron microscope is used for the study of a broad variety of systems. Here, a review of recent activities is presented with a focus on instrumental and analytical developments. A new procedure for the 3 dimensional determination of the magnetization vector has been developed, and is demonstrated for GdFeCo microstructures displaying in-plane and out-of-plane domains, and sub-20 nm Fe nanoparticles. The recent progress for measurements in applied magnetic fields is presented and a new set-up for time-resolved measurements employing femtosecond laser pulses is described.     

Cu cap layer on Ni8/Cu(001): reorientation and TC-shift

Adding a non-magnetic Cu overlayer on Ni₈/Cu(001) is known to induce the reorientation of the magnetic easy axis from in-plane to out-of-plane and to reduce the Curie temperature. In this paper both effects are described on the same footing using an effective Heisenberg trilayer. The model takes into account thermal fluctuations of the magnetization and allows to separate explicitly between two possible mechanisms behind the experimental observations, namely between a reduction of the magnetic moment by hybridization and a lattice relaxation at the Ni/Cu interface. Ferromagnetic resonance data for the reorientation and the decreased Curie temperature due to the Cu cap layer are best reproduced by assuming a reduction of the magnetic moment at the interface by ≈1/3.     

Pseudoequivalent nitrogen atoms in aqueous imidazole distinguished by chemical shifts in photoelectron spectroscopy

The photoelectron spectra of aqueous imidazole are presented, and the N 1s and C 1s binding energies are assigned aided by density functional theory calculations. The chemical equivalency of the two nitrogens of the cationic form is directly identified by the occurrence of a single N 1s photoelectron peak, which results from the delocalization of the positive charge over the molecule as a consequence of the Cv symmetry of the system. In contrast to NMR measurements, the photoemission process is faster than the rapid proton exchange in the aqueous environment, making the pseudoequivalent nitrogens of the neutral state clearly distinguishable with a N 1s binding energy shift of 1.7 eV.     

Ionization of imidazole in the gas phase, microhydrated environments, and in aqueous solution

Hydration of neutral and cationic imidazole is studied by means of ab initio and molecular dynamics calculations, and by photoelectron spectroscopy of the neutral species in a liquid microjet. The calculations show the importance of long range solvent polarization and of the difference between the structure of water molecules in the first shell around the neutral vs cationic species for determining vertical and adiabatic ionization potentials. The vertical ionization potential of neutral imidazole of 8.06 eV calculated using a nonequilibrium polarizable continuum model agrees well with the value of 8.26 eV obtained experimentally for an aqueous solution at pH 10.6.     

Hubbard Hamiltonian in the dimer representation Large‐U case

The nonperturbative approach, presented in this paper, can, in principle, be applied to clusters of any size, as well as, to another type of model Hamiltonians.