Sergiu V. Nistor

Nitric oxide binding properties of neuroglobin: A characterization by EPR and flash photolysis

Neuroglobin is a recently discovered member of the globin superfamily. Combined electron paramagnetic resonance and optical measurements show that, in Escherichia colicell cultures with low O2 concentration overexpressing wild-type mouse recombinant neuroglobin, the heme protein is mainly in a hexacoordinated deoxy ferrous form (F8His-Fe2+-E7His), whereby for a small fraction of the protein the endogenous protein ligand is replaced by NO. Analogous studies for mutated neuroglobin (mutation of E7-His to Leu, Val, or Gln) reveal the predominant presence of the nitrosyl ferrous form. After sonication of the cells wild-type neuroglobin oxidizes rapidly to the hexacoordinated ferric form, whereas NO ligation initially protects the mutants from oxidation. Flash photolysis studies of wild-type neuroglobin and its E7 mutants show high recombination rates (k on) and low dissociation rates (k off) for NO, indicating a high intrinsic affinity for this ligand similar to that of other hemoglobins. Since the rate-limiting step in ligand combination with the deoxy-hexacoordinated wild-type form involves the dissociation of the protein ligand, NO binding is slower than for the related mutants. Structural and kinetic characteristics of neuroglobin and its mutants are analyzed. NO production in rapidly growing E. coli cell cultures is discussed.

Nitrogen and hydrogen in thick diamond films grown by microwave plasma enhanced chemical vapor deposition at variable H2 flow rates

The presence and concentration of nitrogen and hydrogen impurities in thick diamond films grown by microwave plasma chemical vapor deposition at various H2 gas flow rates, keeping a constant [CH4]:[H2]=2.5% concentration ratio, have been determined by electron spin resonance and optical absorption spectroscopy. The relative concentration of both impurities, present as paramagnetic atomic species with different relaxation properties, has been found by ESR measurements to decrease exponentially with the increase in the H2 gas flow rate. Moreover, the resulting values were proportional to the content of substitutional nitrogen and CHx groups obtained from infrared and ultraviolet-visible optical absorption measurements, respectively. The decrease in the concentration of both impurities with an increase in the quality of the studied diamond films, early observed from high resolution electron microscopy studies on the same samples, strongly suggests that the incorporation of both impurities, as paramagnetic at...

EPR-spectroscopic evidence of a dominant His-FeIII-His coordination in ferric neuroglobin

Abstract The ferric form of the wild-type mouse neuroglobin (Ngb), a newly discovered heme protein which is primarily expressed in the brain of mammals, has been characterized by electron paramagnetic resonance (EPR) spectroscopy. The study reveals the simultaneous presence of two related structural forms in a wide range of pH values. The dominant low-spin form (>90%) with g -tensor principal values 3.15, 2.16 and 1.34 can be attributed to a His–Fe III –His configuration. The high-spin form with g ⊥ =5.97 and g ∥ ∼2, can be ascribed either to a hexacoordinated His–Fe III –H 2 O form or to a pentacoordinated His–Fe III . The high-spin to low-spin ratio is found to decrease with increasing pH values.

EPR of trigonal Fe3+ centers in chlorinated SrCl2: Fe single crystals

Abstract An EPR study of SrCl2: Fe2+ crystals grown in chlorine reveals the presence after X-ray irradiation, besides the trapped-hole Fecub3+ center with cubic symmetry previously observed in crystals grown under argon, of a new Fetrig3+ center with axial 〈1 1 1〉 symmetry. The quantitative analysis of the strongly anisotropic X- and Q-band spectra resulted in a g = 2.0141 value and zero-field-splitting parameters B20 = 800.6, B40 = −0.44, B4−3 = −10.5 and B43 = 0.33 (at T = 12 K, in 10−4 cm−1 units). The very large B20 value is attributed to the presence of a charged defect replacing one of the eight nearest neighbor Cl− ligands accompanied by an off-center displacement of the Fe3+ ion towards it.

The periodic character of the microtwin bandwidth distribution in YBa2Cu3O7−x

A statistical study of the microtwin domains in a standard ceramic sample of YBa2Cu3O7−x has been performed on transmission electron microscopy images. This study has evidenced a periodic character of the width distribution of twin lamellae, with maximum periodicity of about 30 nm. The presence of these maxima has been attributed to a local structural mechanism, independent of grain size, which operates in the tetragonal‐orthorhombic transformation, beside the normal stress relaxation mechanism.

On the agent role of Mn2+ in redirecting the synthesis of Zn(OH)2 towards nano-ZnO with variable morphology

One of the simplest routes to prepare polycrystalline Zn(OH)2 is by coprecipitation, with zinc nitrate as a cation source. However, the addition of even minute amounts of manganese nitrate to the precursors used to prepare pure Zn(OH)2 results in Mn2+ doped nanostructured ZnO. The comparison with other Mn2+ doped metal hydroxides prepared by the same coprecipitation method, involving metal nitrates precursors, shows that this behavior is unique, pertaining only to Zn(OH)2. A systematic study of the samples prepared without and with variable amounts of Mn2+ ions, in the 1 to 5000 ppm nominal concentrations range showed that the re-routing of the reaction takes place even for the lowest nominal dopant concentration of 1 ppm. According to X-ray diffraction, transmission electron microscopy and Fourier transform infrared spectroscopy investigations, both crystallite size and morphology of the resulting nanostructured ZnO samples varied with the Mn2+ nominal concentration. Moreover, quantitative electron paramagnetic resonance investigations showed that the incorporation rate of the Mn2+ ions at different sites in the nanostructured ZnO depended on the nominal Mn2+ concentration. The results are discussed in terms of the coordination properties of the Mn2+ and Zn2+ ions and the nature of the reaction precursors.

Revealing the Cu(2+) ions localization at low symmetry Bi sites in photorefractive Bi12GeO20 crystals doped with Cu and V by high frequency EPR.

The sites of incorporation of Cu(2+) impurity ions in Bi12GeO20 single crystals co-doped with copper and vanadium have been investigated by electron paramagnetic resonance (EPR). While the X-band EPR spectra consist of a simple broad (ΔB ∼50 mT) line with anisotropic lineshape, the W-band EPR spectra exhibit well resolved, strongly anisotropic lines, due to transitions within the 3d(9)-(2)D ground manifold of the Cu(2+) ions. The most intense group of lines, attributed to the dominant Cu(2+)(I) center, displays a characteristic four components hyperfine structure for magnetic field orientations close to a 〈110〉 direction. The g and A tensor main axes are very close to one of the 12 possible sets of orthogonal 〈1-10〉, 〈00-1〉 and 〈110〉 crystal directions. Several less intense lines, with unresolved hyperfine structure and similar symmetry properties, mostly overlapped by the Cu(2+)(I) spectrum, were attributed to Cu(2+)(II) centers. The two paramagnetic centers are identified as substitutional Cu(2+) ions at Bi(3+) sites with low C1 symmetry, very likely resulting from different configurations of neighboring charge compensating defects.

Experimental techniques for defect characterization of highly irradiated materials and structures

There are several applications where solid devices are exposed to irradiation. Depending on the operational conditions (type of the particles, temperature, fluence) the physical properties of the exposed device degrades differently, reaching the point of electrical failure in very harsh enviroments. The radiation damage, starting already under low irradiation fluences, get more complex with increasing fluences due to the generation of various type of irradiation induced, electrically active, defects. Accordingly, the defect characterization becomes a more difficult and costly task, requiring several complementary techniques to understand the detailed relation between the “microscopic” reasons as based on defect analysis and their “macroscopic” consequences for device performance. In this respect, we present the most powerful techniques employed and developed within the CERN RD50 Collaboration for investigating highly irradiated materials/structures: (i) Thermally Stimulated Current and Thermally Dielectric Relaxation Current techniques used for electrical characterization of bulk and interface defect states. With the obtained defect parameters several electrical characteristics of the devices could quantitatively explained; (ii) High Resolution Transmission Electron Microscopy and Electron Paramagnetic Resonance allowing the structural and chemical identification of the radiation induced defects.

EPR study of the low temperature ferroelectric phase transition in Cu2+ doped Rb2ZnCl4 single crystals

Abstract Temperature dependent EPR measurements on copper doped Rb2ZnCl4 single crystals allowed us to evidence and study the P21cn↔C1c1 structural phase transition that takes place in this compound at 74.6 K. From the two types of Cu2+ centers localized at different anionic sites, called Cu2+(I) and Cu2+(II), which are formed in this compound, only the Cu2+(II) centers exhibit observable changes in their EPR spectra, attributable to the symmetry lowering. The observed changes have been related to the soft-mode responsible for the structural phase transition.