Layered growth of crayfish gastrolith: about the stability of amorphous calcium carbonate and role of additives.

Previous studies on pre-molt gastroliths have shown a typical onion-like morphology of layers of amorphous mineral (mostly calcium carbonate) and chitin, resulting from the continuous deposition and densification of amorphous mineral spheres on a chitin-matrix during time. To investigate the consequences of this layered growth on the local structure and composition of the gastrolith, we performed spatially-resolved Raman, X-ray and SEM-EDS analysis on complete pre-molt gastrolith cross-sections. Results show that especially the abundance of inorganic phosphate, phosphoenolpyruvate (PEP)/citrate and proteins is not uniform throughout the organ but changes from layer to layer. Based on these results we can conclude that ACC stabilization in the gastrolith takes place by more than one compound and not by only one of these additives.

Measuring the distribution of cellulose microfibril angles in primary cell walls by small angle X-ray scattering.

BACKGROUND: X-ray scattering is a well-established method for measuring cellulose microfibril angles in secondary cell walls. However, little data is available on the much thinner primary cell walls. Here, we show that microfibril orientation distributions can be determined by small angle X-ray scattering (SAXS) even in primary cell walls. The technique offers a number of advantages: samples can be analyzed in the native hydrated state without any preparation which minimizes the risk of artifacts and allows for fast data acquisition. The method provides data averaged over a specimen region, determined by the size of the used X-ray beam and, thus, yields the microfibril orientation distribution within this region. RESULTS: Cellulose microfibril orientation distributions were obtained for single cells of the alga Chara corallina, as well as for the multicellular hypocotyl of Arabidopsis thaliana. In both, Chara and Arabidopsis, distributions with a broad scattering around mean microfibril angles of approximately 0° and 90° towards the longitudinal axis of the cells were found. CONCLUSIONS: With SAXS, the structure of primary cell walls can be analysed in their native state and new insights into the cellulose microfibril orientation of primary cell walls can be gained. The data shows that SAXS can serve as a valuable tool for the analysis of cellulose microfibril orientation in primary cell walls and, in consequence, add to the understanding of its mechanical behaviour and the intriguing mechanisms behind cell growth.

Hierarchical structuring of liquid crystal polymer-Laponite hybrid materials.

Biomimetic organic-inorganic composite materials were fabricated via one-step self-organization on three hierarchical levels. The organic component was a polyoxazoline with pendent cholesteryl and carboxyl (N-Boc-protected amino acid) side chains that was able to form a chiral nematic lyotropic phase and bind to positively charged inorganic faces of Laponite. The Laponite particles formed a mesocrystalline arrangement within the liquid-crystal (LC) polymer phase upon shearing a viscous dispersion of Laponite nanoparticles and LC polymer in DMF. Complementary analytical and mechanical characterization techniques (AUC, POM, TEM, SEM, SAXS, µCT, and nanoindentation) covering the millimeter, micrometer, and nanometer length scales reveal the hierarchical structures and properties of the composite materials consisting of different ratios of Laponite nanoparticles and liquid-crystalline polymer.

Ion-association complexes unite classical and non-classical theories for the biomimetic nucleation of calcium phosphate.

Despite its importance in many industrial, geological and biological processes, the mechanism of crystallization from supersaturated solutions remains a matter of debate. Recent discoveries show that in many solution systems nanometre-sized structural units are already present before nucleation. Still little is known about the structure and role of these so-called pre-nucleation clusters. Here we present a combination of in situ investigations, which show that for the crystallization of calcium phosphate these nanometre-sized units are in fact calcium triphosphate complexes. Under conditions in which apatite forms from an amorphous calcium phosphate precursor, these complexes aggregate and take up an extra calcium ion to form amorphous calcium phosphate, which is a fractal of Ca(2)(HPO(4))(3)(2-) clusters. The calcium triphosphate complex also forms the basis of the crystal structure of octacalcium phosphate and apatite. Finally, we demonstrate how the existence of these complexes lowers the energy barrier to nucleation and unites classical and non-classical nucleation theories.

Plant cystoliths: a complex functional biocomposite of four distinct silica and amorphous calcium carbonate phases.

Plant cystoliths are mineralized objects that are formed by specialized cells in the leaves of certain plants. The main mineral component of cystoliths by volume is amorphous calcium carbonate (ACC) and the minor component is silica. We show that the silica stalk is formed first and is essential for ACC formation. Furthermore, the cystolith is shown to be composed of four distinct mineral phases with different chemical properties: an almost pure silica phase grades into a Mg-rich silica phase. This Mg-rich silica is overlaid by a relatively stable ACC phase. A bulky and less stable ACC phase encapsulates the first ACC phase. This architecture poses interesting questions about the role of Mg in the silica phase and suggests a strategy for ACC stabilization that takes advantage of a precise regulation of the mineral-growth microenvironment.

Enamel-like apatite crown covering amorphous mineral in a crayfish mandible.

Carbonated hydroxyapatite is the mineral found in vertebrate bones and teeth, whereas invertebrates utilize calcium carbonate in their mineralized organs. In particular, stable amorphous calcium carbonate is found in many crustaceans. Here we report on an unusual, crystalline enamel-like apatite layer found in the mandibles of the arthropod Cherax quadricarinatus (freshwater crayfish). Despite their very different thermodynamic stabilities, amorphous calcium carbonate, amorphous calcium phosphate, calcite and fluorapatite coexist in well-defined functional layers in close proximity within the mandible. The softer amorphous minerals are found primarily in the bulk of the mandible whereas apatite, the harder and less soluble mineral, forms a wear-resistant, enamel-like coating of the molar tooth. Our findings suggest a unique case of convergent evolution, where similar functional challenges of mastication led to independent developments of structurally and mechanically similar, apatite-based layers in the teeth of genetically remote phyla: vertebrates and crustaceans.

Structural purity of magnetite nanoparticles in magnetotactic bacteria.

Magnetosome biomineralization and chain formation in magnetotactic bacteria are two processes that are highly controlled at the cellular level in order to form cellular magnetic dipoles. However, even if the magnetosome chains are well characterized, controversial results about the microstructure of magnetosomes were obtained and its possible influence in the formation of the magnetic dipole is to be specified. For the first time, the microstructure of intracellular magnetosomes was investigated using high-resolution synchrotron X-ray diffraction. Significant differences in the lattice parameter were found between intracellular magnetosomes from cultured magnetotactic bacteria and isolated ones. Through comparison with abiotic control materials of similar size, we show that this difference can be associated with different oxidation states and that the biogenic nanomagnetite is stoichiometric, i.e. structurally pure whereas isolated magnetosomes are slightly oxidized. The hierarchical structuring of the magnetosome chain thus starts with the formation of structurally pure magnetite nanoparticles that in turn might influence the magnetic property of the magnetosome chains.

Sonication-assisted synthesis of large, high-quality mercury thiolate single crystals directly from liquid mercury.

The synthetic formation of mercury thiolates has been known for almost 200 years. These compounds are usually formed by a slow reaction of mercury salts with thiolates or disulfides to produce small (up to 1 µm), plate-like crystals of Hg(S-R)(2). Herein we show that such mercury thiolates can be formed directly from liquid mercury via sonication with neat thiols. The process not only produces crystals very rapidly (within seconds) but also leads to the formation of large crystals (up to hundreds of micrometers). The high quality of these crystals enabled their detailed structural characterization, which showed that the crystals are composed of ordered Hg(thiol)(2) stacks. We extended the experimental procedure to form and characterize a range of Hg thiolate crystals with various chain lengths. We propose a new self-assembly mechanism that can explain how sonication--which is usually used to break chemical bonds, to disperse materials, and to form nanosized crystallites--may lead to the growth of large, high-quality crystals.

Mapping amorphous calcium phosphate transformation into crystalline mineral from the cell to the bone in zebrafish fin rays.

The continuously forming fin bony rays of zebrafish represent a simple bone model system in which mineralization is temporally and spatially resolved. The mineralized collagen fibrils of the fin bones are identical in structure to those found in all known bone materials. We study the continuous mineralization process within the tissue by using synchrotron microbeam x-ray diffraction and small-angle scattering, combined with cryo-scanning electron microscopy. The former provides information on the mineral phase and the mineral particles size and shape, whereas the latter allows high-resolution imaging of native hydrated tissues. The integration of the two techniques demonstrates that new mineral is delivered and deposited as packages of amorphous calcium phosphate nanospheres, which transform into platelets of crystalline apatite within the collagen matrix.

Amelogenin nanoparticles in suspension: deviations from spherical shape and pH-dependent aggregation.

It is well-known that amelogenin self-assembles to form nanoparticles, usually referred to as amelogenin nanospheres, despite the fact that not much is known about their actual shape in solution. In the current paper, we combine SAXS and DLS to study the three-dimensional shape of the recombinant amelogenins rP172 and rM179. Our results show for the first time that amelogenins build oblate nanoparticles in suspension using experimental approaches that do not require the proteins to be in contact with a support material surface. The SAXS studies give evidence for the existence of isolated amelogenin nano-oblates with aspect ratios in the range of 0.45-0.5 at pH values higher than pH 7.2 and show an aggregation of these nano-oblates at lower pH values. The role of the observed oblate shape in the formation of chain-like structures at physiological conditions is discussed as a key factor in the biomineralization of dental enamel.

The grinding tip of the sea urchin tooth exhibits exquisite control over calcite crystal orientation and Mg distribution.

The sea urchin tooth is a remarkable grinding tool. Even though the tooth is composed almost entirely of calcite, it is used to grind holes into a rocky substrate itself often composed of calcite. Here, we use 3 complementary high-resolution tools to probe aspects of the structure of the grinding tip: X-ray photoelectron emission spectromicroscopy (X-PEEM), X-ray microdiffraction, and NanoSIMS. We confirm that the needles and plates are aligned and show here that even the high Mg polycrystalline matrix constituents are aligned with the other 2 structural elements when imaged at 20-nm resolution. Furthermore, we show that the entire tooth is composed of 2 cooriented polycrystalline blocks that differ in their orientations by only a few degrees. A unique feature of the grinding tip is that the structural elements from each coaligned block interdigitate. This interdigitation may influence the fracture process by creating a corrugated grinding surface. We also show that the overall Mg content of the tooth structural elements increases toward the grinding tip. This probably contributes to the increasing hardness of the tooth from the periphery to the tip. Clearly the formation of the tooth, and the tooth tip in particular, is amazingly well controlled. The improved understanding of these structural features could lead to the design of better mechanical grinding and cutting tools.

Surface-directed spinodal decomposition on a macroscopic scale in a nitrogen and carbon alloyed steel.

Interactions with the macroscopic specimen surface can profoundly modify phase-separation processes. This has previously been observed in liquids and polymer films and is theoretically described by the theory of surface-directed spinodal decomposition (SDSD). Here we report first observations of SDSD in a metallic alloy on a macroscopic scale. The influence of the surface leads to the development of concentric domains extending over the whole 10 mm thick cylindrical steel specimen, due to long-range interactions via elastic stresses and long-range diffusion of the interstitial elements nitrogen and carbon.