Third-generation synchrotron x-ray diffraction of 6-μm crystal of raite, ≈Na3Mn3Ti0.25Si8O20(OH)2⋅10H2O, opens up new chemistry and physics of low-temperature minerals

The crystal structure of raite was solved and refined from data collected at Beamline Insertion Device 13 at the European Synchrotron Radiation Facility, using a 3 × 3 × 65 μm single crystal. The refined lattice constants of the monoclinic unit cell are a = 15.1(1) Å; b = 17.6(1) Å; c = 5.290(4) Å; β = 100.5(2)°; space group C2/m. The structure, including all reflections, refined to a final R = 0.07. Raite occurs in hyperalkaline rocks from the Kola peninsula, Russia. The structure consists of alternating layers of a hexagonal chicken-wire pattern of 6-membered SiO4 rings. Tetrahedral apices of a chain of Si six-rings, parallel to the c-axis, alternate in pointing up and down. Two six-ring Si layers are connected by edge-sharing octahedral bands of Na+ and Mn3+ also parallel to c. The band consists of the alternation of finite Mn–Mn and Na–Mn–Na chains. As a consequence of the misfit between octahedral and tetrahedral elements, regions of the Si–O layers are arched and form one-dimensional channels bounded by 12 Si tetrahedra and 2 Na octahedra. The channels along the short c-axis in raite are filled by isolated Na(OH,H2O)6 octahedra. The distorted octahedrally coordinated Ti4+ also resides in the channel and provides the weak linkage of these isolated Na octahedra and the mixed octahedral tetrahedral framework. Raite is structurally related to intersilite, palygorskite, sepiolite, and amphibole.

Subacute neuropathological effects of microplanar beams of x-rays from a synchrotron wiggler.

Microplanar beam radiation therapy has been proposed to treat brain tumors by using a series of rapid exposures to an array of parallel x-ray beams, each beam having uniform microscopic thickness and macroscopic breadth (i.e., microplanar). Thirty-six rats were exposed head-on either to an upright 4-mm-high, 20- or 37-microns-wide beam or to a horizontal 7-mm-wide, 42-microns-high beam of mostly 32- to 126-keV, minimally divergent x-rays from the X17 wiggler at the National Synchrotron Light Source at Brookhaven National Laboratory. Parallel slices of the head, separated at either 75 or 200 microns on center, were exposed sequentially at 310-650 grays (Gy) per second until each skin-entrance absorbed dose reached 312, 625, 1250, 2500, 5000, or 10,000 Gy. The rats were euthanized 2 weeks or 1 month later. Two rats with 10,000-Gy-entrance slices developed brain tissue necrosis. All the other 10,000- and 5000-Gy-entrance slices and some of the 2500- and 1250-Gy-entrance slices showed loss of neuronal and astrocytic nuclei and their perikarya. No other kind of brain damage was evident histologically in any rat with entrance absorbed doses < or = 5000 Gy. Brain tissues in and between all the 312- and 625-Gy-entrance slices appeared normal. This unusual resistance to necrosis is central to the rationale of microplanar beam radiation therapy for brain tumors.

Structure at 2.7 Å resolution of the Paracoccus denitrificans two-subunit cytochrome c oxidase complexed with an antibody FV fragment

The aa3 type cytochrome c oxidase consisting of the core subunits I and II only was isolated from the soil bacterium Paracoccus denitrificans and crystallized as complex with a monoclonal antibody Fv fragment. Crystals could be grown in the presence of a number of different nonionic detergents. However, only undecyl-β-d-maltoside and cyclohexyl-hexyl-β-d-maltoside yielded well-ordered crystals suitable for high resolution x-ray crystallographic studies. The crystals belong to space group P212121 and diffract x-rays to at least 2.5 Å (1 Å = 0.1 nm) resolution using synchrotron radiation. The structure was determined to a resolution of 2.7 Å using molecular replacement and refined to a crystallographic R-factor of 20.5% (Rfree = 25.9%). The refined model includes subunits I and II and the 2 chains of the Fv fragment, 2 heme A molecules, 3 copper atoms, and 1 Mg/Mn atom, a new metal (Ca) binding site, 52 tentatively identified water molecules, and 9 detergent molecules. Only four of the water molecules are located in the cytoplasmic half of cytochrome c oxidase. Most of them are near the interface of subunits I and II. Several waters form a hydrogen-bonded cluster, including the heme propionates and the Mg/Mn binding site. The Fv fragment binds to the periplasmic polar domain of subunit II and is critically involved in the formation of the crystal lattice. The crystallization procedure is well reproducible and will allow for the analysis of the structures of mechanistically interesting mutant cytochrome c oxidases.

Mineral surfaces and bioavailability of heavy metals: A molecular-scale perspective

There is a continual influx of heavy metal contaminants and pollutants into the biosphere from both natural and anthropogenic sources. A complex variety of abiotic and biotic processes affects their speciation and distribution, including adsorption onto and desorption from mineral surfaces, incorporation in precipitates or coprecipitates, release through the dissolution of minerals, and interactions with plants and microbes. Some of these processes can effectively isolate heavy metals from the biosphere, whereas others cause their release or transformation to different species that may be more (or less) bioavailable and/or toxic to organisms. Here we focus on abiotic adsorption and precipitation or coprecipitation processes involving the common heavy metal contaminant lead and the metalloids arsenic and selenium in mine tailings and contaminated soils. We have used extremely intense x-rays from synchrotron sources and a structure-sensitive method known as x-ray absorption fine structure (XAFS) spectroscopy to determine the molecular-level speciation of these elements at concentrations of 50 to several thousand ppm in the contaminated environmental samples as well as in synthetic sorption samples. Our XAFS studies of As and Pb in the mine tailings show that up to 50% of these contaminants in the samples studied may be present as adsorbed species on mineral surfaces, which makes them potentially more bioavailable than when present in sparingly soluble solid phases. Our XAFS studies of Se(VI) sorption on Fe2+-containing sulfates show that this element undergoes redox reactions that transform it into less bioavailable and less toxic species. This type of information on molecular-level speciation of heavy metal and metalloid contaminants in various environmental settings is needed to prioritize remediation efforts and to assess their potential hazard to humans and other organisms.