Rhombohedral calcite

The improvement of enzyme like MIP is currently another area of intense research. Beside the use of the MIP themselves as catalysts, they may also be applied as enhancer of product yield in bio-transformation processes. In an exemplary condensation of Z-L-aspartic acid with L-phenylalanine methyl ester to Z-aspartame, the enzyme thermolysin was used as catalyst. In order to shift the equilibrium towards product formation, a product imprinted MIP was added. By adsorbing specifically the freshly generated product from the reaction mixture, the MIP helped to increase product formation by 40% [130]. MIP can also be used to support a physical process. Copolymers of 6-methacrylamidohexanoic acid and DVB generated in the presence of calcite were investigated with respect to promotion of the nucleation of calcite. Figure 19 (left) shows the polymer surface with imprints from the calcite crystals. When employing these polymers in an aqueous solution of Ca2+ and CO2 the enhanced formation of rhombohedral calcite crystals was observed see Fig. 19 (right) [131]. 

Of the structures with stoichiometry ABO3 that may be derived from that of perovskite (Sect. 2.2), the calcite structure is the simplest in the sense that it requires the fewest parameters to specify it. We earlier described this structure in terms of the parameters obtained with regular BOe octahedra that are rotated (tilted) about their 3 axes from the positions they have in (cubic) perovskite. The coordination of A goes from 12 in the cubic structure to 3 in the rhombohedral calcite structure. 

The two polymorphs of CaC03, e. g. the rhombohedral calcite (space group R 3c) and the orthorhombic aragonite (space group Pmen) are most common in organisms. 

Fig. 15. Bonding of a cyclic Si3-siloxane molecule on an a-quartz (001) surface (a, c, e) and bonding of a cyclic Sis-siloxane molecule on a rhombohedral calcite (101) surface (b, d, f). a detail of Fig. ISc, c manual matching on the X-ray structure, e after energy minimization with DISCOVER/COMPASS (white lines mark the computational boxes), b detail of Fig. 15d, d manual matching on the X-ray structure, f energy minimization with DISCOVER/COMPASS, g calcite with characteristic Ca-Ca distances (green spheres) and calcium carbonate anions (C atoms gray, O atoms redX h as Fig. 15f, side view along the y axis (two rings removed for fi ee view).

Figure 2.14. Different crystalline forms of calcium carbonate. Courtesy of Omya/Pliiss-Staufer AG (micrographs of crystals), Solvay, GmbH, Rheinberg, Germany (crystal stmcture and micrographs of Socal trigonal-scalenohedral calcite), and ECC International Ltd., St. Austell, UK (rhombohedral calcite and aragonite).

Crystallization experiments were then carried out in supersaturated solutions of calcium carbonate. Under these conditions, rhombohedral calcite crystals were observed in the recognition sites of the calcite-imprinted polymer. Very few crystals... 

Surface-fimctionaUzed vaterite particles with Au-Tiopronin were formed by increasing the concentration of Au-Tiopronin compared to the product described above. The template mineraUzation was carried out in the presence of the surface-fimctionalized vaterite particles. The caldiun reactants were injected via syringe into an aqueous solution in the presence of the surface-functionalized vaterite particles with the additional Au-Tiopronin. The product was isolated after incubation for 1 day. The sea urchin-shaped CaCOs with a little rhombohedral calcite was observed by SEM (Fig. 16). From the SEM images, needle-shaped crystals on the surface of the vaterite particles were elongated which increasing the amount of the calcium reactants. This process helps with understanding how to develop the new biomimetic materials and with the template mineralization mechanism. 

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