Atomic-scale structure of biogenic materials by total X-ray diffraction: a study of bacterial and fungal MnOx.

Biogenic materials are produced by microorganisms and are typically found in a nanophase state. As such, they are difficult to characterize structurally. In this report, we demonstrate how high-energy X-ray diffraction and atomic pair distribution function analysis can be used to determine the atomic-scale structures of MnO(x) produced by bacteria and fungi. These structures are well-defined, periodic, and species-specific, built of Mn-O(6) octahedra forming birnessite-type layers and todorokite-type tunnels, respectively. The inherent structural diversity of biogenic material may offer opportunities for practical applications.

Selective deintercalation of apex over face-shared oxide ions in the topotactic reduction of Sr7Mn4O15 to Sr7Mn4O12.

Sodium hydride selectively deintercalates the apex rather than face-shared oxide ions within the structure of Sr(7)Mn(4)O(15) leading to the formation of the structurally related reduced phase Sr(7)Mn(4)O(12).

The hydride anion in an extended transition metal oxide array: LaSrCoO3H0.7.

We present the synthesis and structural characterization of a transition metal oxide hydride, LaSrCoO3H0.7, which adopts an unprecedented structure in which oxide chains are bridged by hydride anions to form a two-dimensional extended network. The metal centers are strongly coupled by their bonding with both oxide and hydride ligands to produce magnetic ordering at temperatures up to at least 350 kelvin. The synthetic route is sufficiently general to allow the prediction of a new class of transition metal--containing electronic and magnetic materials.

(13)C NMR investigation of the superconductor MgCNi(3) up to 800 K.

We report (13)C NMR characterization of the new superconductor MgCNi(3) [T. He et al., Nature (London) 411, 54 (2001)]. We found that both the uniform spin susceptibility and the spin fluctuations show a strong enhancement with decreasing temperature, and saturate below approximately 50 K and approximately 20 K, respectively. The nuclear spin-lattice relaxation rate 1/(13)T(1) exhibits typical behavior for isotropic s-wave superconductivity with a coherence peak below T(c) = 7.0 K that grows with decreasing magnetic field.

Giant anharmonicity and nonlinear electron-phonon coupling in MgB2: a combined first-principles calculation and neutron scattering study.

First-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB (2) are carried out and found to be in excellent agreement with our inelastic neutron scattering measurements. The numerical results reveal that the E(2g) in-plane boron phonons near the zone center are very anharmonic and strongly coupled to the planar B sigma bands near the Fermi level. This giant anharmonicity and nonlinear electron-phonon coupling is key to quantitatively explaining the observed high T(c) and boron isotope effect in MgB (2).

High critical current density and enhanced irreversibility field in superconducting MgB2 thin films.

The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.

Superconductivity in the non-oxide perovskite MgCNi3.

The interplay of magnetic interactions, the dimensionality of the crystal structure and electronic correlations in producing superconductivity is one of the dominant themes in the study of the electronic properties of complex materials. Although magnetic interactions and two-dimensional structures were long thought to be detrimental to the formation of a superconducting state, they are actually common features of both the high transition-temperature (Tc) copper oxides and low-Tc material Sr2RuO4, where they appear to be essential contributors to the exotic electronic states of these materials. Here we report that the perovskite-structured compound MgCNi3 is superconducting with a critical temperature of 8 K. This material is the three-dimensional analogue of the LnNi2B2C family of superconductors, which have critical temperatures up to 16 K (ref. 2). The itinerant electrons in both families of materials arise from the partial filling of the nickel d-states, which generally leads to ferromagnetism as is the case in metallic Ni. The high relative proportion of Ni in MgCNi3 suggests that magnetic interactions are important, and the lower Tc of this three-dimensional compound-when compared to the LnNi2B2C family-contrasts with conventional ideas regarding the origins of superconductivity.

Pressure dependence of the superconducting transition temperature of magnesium diboride.

We studied the pressure and temperature dependence of the electrical resistivity of the superconducting compound magnesium diboride (MgB(2)). The superconducting transition temperature decreases monotonically with pressure, being parabolic or linear, depending on samples. The rate of decrease under pressure is higher than in conventional superconductors. We discuss our results in terms of the semimetallic character of the electronic band structure of MgB(2).

Loss of superconductivity with the addition of Al to MgB2 and a structural transition in Mg1-x AlxB2.

The basic magnetic and electronic properties of most binary compounds have been well known for decades. The recent discovery of superconductivity at 39 K in the simple binary ceramic compound magnesium diboride, MgB2, was therefore surprising. Indeed, this material has been known and structurally characterized since the mid 1950s (ref. 2), and is readily available from chemical suppliers (it is commonly used as a starting material for chemical metathesis reactions). Here we show that the addition of electrons to MgB2, through partial substitution of Al for Mg, results in the loss of superconductivity. Associated with the Al substitution is a subtle but distinct structural transition, reflected in the partial collapse of the spacing between boron layers near an Al content of 10 per cent. This indicates that superconducting MgB2 is poised very near a structural instability at slightly higher electron concentrations.

Strongly linked current flow in polycrystalline forms of the superconductor MgB2.

The discovery of superconductivity at 39 K in magnesium diboride, MgB2, raises many issues, a critical one being whether this material resembles a high-temperature copper oxide superconductor or a low-temperature metallic superconductor in terms of its behaviour in strong magnetic fields. Although the copper oxides exhibit very high transition temperatures, their in-field performance is compromized by their large anisotropy, the result of which is to restrict high bulk current densities to a region much less than the full magnetic-field-temperature (H-T) space over which superconductivity is found. Moreover, the weak coupling across grain boundaries makes transport current densities in untextured polycrystalline samples low and strongly sensitive to magnetic field. Here we report that, despite the multiphase, untextured, microscale, subdivided nature of our MgB2 samples, supercurrents flow throughout the material without exhibiting strong sensitivity to weak magnetic fields. Our combined magnetization, magneto-optical, microscopy and X-ray investigations show that the supercurrent density is mostly determined by flux pinning, rather than by the grain boundary connectivity. Our results therefore suggest that this new superconductor class is not compromized by weak-link problems, a conclusion of significance for practical applications if higher temperature analogues of this compound can be discovered.

A spectrophotometric method for the direct detection and quantitation of nitric oxide, nitrite, and nitrate in cell culture media.

A method for the spectrophotometric determination of nitric oxide, nitrite, and nitrate in tissue culture media is presented. The method is based on the nitric oxide-mediated nitrosative modification of sulfanilic acid that reacts with N-(1-naphthyl)ethylenediamine dihydrochloride forming an orange-colored product absorbing at 496 nm. Nitric oxide levels were determined in culture media from this absorbance measurement using chemiluminescence standardization. Extinction coefficients of 5400 and 6600 M(-1) cm(-1) were determined for the nitric oxide product in assay solutions containing 0.1 or 100 mM KPO4 buffer (pH 7.4), respectively, with a limit of detection of 1 microM. Acidification of these reactions (pH 2.4) generated a pink-colored product absorbing at 540 nm allowing for quantitation of total nitric oxide/nitrite levels using extinction coefficients of 38,000 and 36,900 M(-1) cm(-1), for the assay solutions described. The limit of detection of this assay was approximately 300 nM. Using the 100 mM KPO4 buffer system, nitrate levels were determined following reduction to nitrite using a copper-coated cadmium reagent with an extinction coefficient of 29,500 M(-1) cm(-1) and a detection limit of 0.5 microM. The utility of these assays was demonstrated in the standardization of nitric oxide-saturated cell culture media, and the release of nitric oxide by the NONOate compound DEA/NO.

Characterization of 5-oxo-L-prolinase in normal and tumor tissues of humans and rats: a potential new target for biochemical modulation of glutathione.

5-Oxo-L-prolinase (5-OPase) is an enzyme of the gamma-glutamyl cycle involved in the synthesis and metabolism of glutathione (GSH), which is known to protect cells from the cytotoxic effects of chemotherapy and radiation. Previous studies on rats have shown that administration of the cysteine prodrug L-2-oxothiazolidine-4-carboxylate, a 5-oxo-L-proline analogue that is metabolized by 5-OPase, preferentially increases the GSH content of normal tissues while paradoxically decreasing it in the tumor and results in an enhanced in vivo tumor response to the anticancer drug melphalan. These observations initiated the present study of 5-OPase in experimental models and clinical specimens to investigate the potential role of this enzyme in the selective modulation of GSH in normal and tumor tissues. First, 5-OPase activity was measured in tissues of tumor-bearing rats, in the peripheral mononuclear cells of normal human subjects, and in surgically resected tumor and the adjacent normal tissues from patients. We found that the activity of 5-OPase in human kidney, liver, and lung is significantly lower than that found in rats. Second, we have raised a polyclonal IgG anti-5-OPase antibody by immunizing rabbits with purified 5-OPase from rat kidney. This antibody has very high affinity (shown by immunoprecipitation) and specificity (shown by Western blot) and cross-reacts with human 5-OPase (shown by Western blot and immunohistochemistry). It was then used to examine the distribution of 5-OPase in paired normal and neoplastic human specimens using Western blot and immunohistochemistry. Examination of paired normal and neoplastic tissues of stomach and lung revealed a significantly lower level of 5-OPase in tumor tissues than in the paired normal tissues. In colon tissues, there is no significant difference in 5-OPase level between the normal and tumor tissues. These findings could have implications for both carcinogenesis and therapy.

Pulmonary oxygen toxicity in mice is characterized by alterations in ascorbate redox status.

Pulmonary oxygen toxicity results from disruption of the usual antioxidant defenses of the body. We therefore investigated whether mice that suffer from oxygen toxicity show significant alterations in the redox status of ascorbate, an important antioxidant, as reflected by changes in the relative amounts of its oxidized and reduced forms. Mice were exposed to air or hyperoxia (> 97% O2, 760 mmHg). After 5 days, plasma and saline-perfused lungs were removed and levels of ascorbate (AA), oxidized ascorbate [dehydroascorbate (DHAA)], and total ascorbate species ([AA+DHAA]) were determined by a sensitive and specific high-performance liquid chromatography assay; lungs were also assayed for total glutathione and glutathione disulfide (GSSG), an established marker of oxidative stress. We found that with hyperoxic exposure plasma AA increased by 32%, plasma DHAA increased substantially from previously undetectable levels, and the DHAA-to-[AA+DHAA] ratio increased. In contrast, in lung, [AA+DHAA] decreased by 41%. Plasma AA, DHAA, and [AA+DHAA] each correlated inversely with lung [AA+DHAA] and directly with lung GSSC. We conclude that alterations in plasma ascorbate redox status reflect pulmonary oxygen toxicity in mice. Our results suggest that further investigations are warranted to determine whether similar findings occur in humans and have clinical utility.

S-alkyl-L-thiocitrullines. Potent stereoselective inhibitors of nitric oxide synthase with strong pressor activity in vivo.

Nitric oxide synthase catalyzes the oxidation of a guanidino nitrogen of L-arginine to nitric oxide with concomitant formation of citrulline. Enzyme activity is inhibited by a variety of N omega-monosubstituted L-arginine analogs including N omega-alkyl-, N omega-amino-, and N omega-nitro-L-arginine derivatives. We report here that both constitutive and inducible isoforms of nitric oxide synthase are strongly inhibited by S-alkyl-L-thiocitrullines (N delta-(S-alkyl)isothioureido-L-ornithines) with n-alkyl groups of one to three carbons. These compounds represent a novel class of inhibitors and are the most potent nitric oxide synthase-inhibiting amino acids described to date. Inhibition is reversible, stereoselective, and competitive with L-arginine. Spectral studies show no direct interaction of inhibitor sulfur with heme iron, a result in contrast to that seen previously with the parent compound, L-thiocitrulline. The S-alkyl-L-thiocitrullines have strong pressor activity in normotensive control rats; S-methyl-L-thiocitrulline reverses hypotension in a rat model of septic peritonitis and in dogs administered endotoxin. These latter findings suggest that the inhibitors may have therapeutic utility in treating hypotension due to the overproduction of nitric oxide.

Phosphoglucomutase-1 (PGM1) W23 and W26 variants detected in the Taiwanese Chinese population.

Phosphoglucomutase-1 (PGM1) phenotyping among Taiwanese Chinese was carried out on thin layer agarose gel using isoelectric focusing techniques. During routine paternity testing, two new PGM1 variants not previously observed in Taiwanese Chinese were detected. These are PGM1 W23 and PGM1 W26.

Phosphoglucomutase-1 polymorphism among Chinese in Taiwan.

Phosphoglucomutase-1 (PGM1) phenotyping of 1,128 Chinese blood donors was performed by thin-layer isoelectric focusing on agarose. The PGM1 gene frequencies were: 1A, 0.6005; 1B, 0.1500; 2A, 0.1510; 2B, 0.0973, and rare variants, 0.0058. The rare variants found in this series were PGM1 W21, W2, W3, W6 and W9 (or W10) with PGM1 W21 being the most common variant among Chinese with a phenotype frequency of 0.8%.

Phosphoglucomutase phenotyping among Chinese in Taiwan.

Phosphoglucomutase 1 (PGM1) phenotyping was performed in 1,128 Chinese blood donors by thin-layer isoelectric focusing on agarose. The PGM1 gene frequencies were as follows: 1A, 0.6005; 1B, 0.1500; 2A, 0.1510, 2B, 0.0973; variants, 0.0058, with W21, 0.0040. The variants found in this series were PGM1 W21, W2, W3, W6 and W9 (or W10) with PGM1 W21 being the most common variant among Chinese, having a phenotype frequency of 0.8%.

The effect of conjugated equine estrogens on ovariectomy-induced osteopenia in the rat.

The effect of estrogen replacement on ovariectomy-induced bone loss was evaluated in mature Sprague-Dawley rats. Undecalcified tibia of ovariectomized rats were processed for quantitative histologic assessment of cancellous bone in longitudinal sections from the primary and secondary spongiosa of the proximal metaphysis. Bone content in tissue specimens was quantified as the parameter B. Ar, two-dimensional bone mineral area. Estrogen, supplied as orally administered conjugated equine estrogens, prevented bone loss through 6 weeks of treatment. The effect of conjugated equine estrogens was dose-dependent, with significant protection against bone loss observed at doses of 10 micrograms/kg/day and higher.

Prostaglandins in inflammatory bone pathology: mechanism and therapeutic benefit of etodolac.

To investigate the role of PGE2 in the development of bone and joint pathology in rat adjuvant arthritis, hindlimb paws were evaluated by calcified tissue histologic techniques focusing on histochemical visualization of cartilage and bone lesions. Case studies of hindlimbs from normal, adjuvant arthritic, and etodolac-treated arthritic rats demonstrated the association of disease severity with inflammation, chondromalacia, replacement of adipose bone marrow with a fibroid marrow, osteoclastic bone resorption, synovial cysts, and pannus formation within the joints. Extensive periosteal intramembranous bone formation was temporally associated with joint destruction and medullary tissue pathology. In vivo data were correlated with in vitro effects of inflammatory mediators (IL-1, PGE2) on bone resorption. Etodolac blocked bone explant PGE2 accumulation at concentrations of 10(-7) M and higher, and inhibited bone resorption at concentrations of 10(-5) M and higher. The data indicate that in vitro and in vivo models of bone metabolism are well correlated regarding prostaglandin synthesis; that the inflammatory mediator PGE2 is largely responsible for the involvement of skeletal tissue in the adjuvant arthritis model; and that the effects of etodolac are specifically mediated by its ability to inhibit PGE2 accumulation in vivo.