2-Ammoniumethanethiolate, (-)SCH(2)CH(2)NH(3)(+), the first structurally characterized zwitterionic ammoniumthiolate, is the stable form of cysteamine (HL) in the solid state and in aqueous solution. Reactions of ZnCl(2), Cd(Oac)(2), and HgCl(2) with cysteamine and NaOH in a 1:2:2 ratio, respectively, lead to the homoleptic complexes ML(2). Their single-crystal X-ray structures demonstrate basic differences in the coordination chemistry of Zn(II), Cd(II), and Hg(II). While chelating N,S-coordination modes are found for all metal ions, Zn(II) forms a mononuclear complex with a distorted tetrahedral Zn(N(2)S(2)) coordination mode, whereas Hg(II) displays a dimer with Hg(N(2)S(2)) coordinated monomers being connected by two long Hg...S contacts. Solid-state (199)Hg NMR spectra of HgL(2) and [Hg(HL)(2)]Cl(2) reveal a low-field shift of the signals with increasing coordination number. Strong and nearly symmetric Cd-S-Cd bridges in solid CdL(2) lead to a chain structure, Cd(II) displaying a distorted square pyramidal Cd(N(2)S(3)) coordination mode. The ab initio [MP2/LANL2DZ(d,f)] structures of isolated ML(2) show a change from a distorted tetrahedral to bisphenoidal coordination mode in the sequence Zn(II)-Cd(II)-Hg(II). A natural bond orbital analysis showed a high ionic character for the M-S bonds and suggests that the S-M-S fragment is best described by a 3c4e bond. The strength of the M...N interactions and the stability of ML(2) toward decomposition to M and L-L decreases in the sequence Zn > Cd > Hg. Ab initio calculations further suggest that a tetrahedral S-M-S angle stabilizes Zn(II) against substitution by Cd(II) and Hg(II) in a M(N(2)S(2)) environment. Such geometry is provided in zinc-finger proteins, as was found by a database survey.
On the phase of it: The phase selection of calcium carbonate (spheres: C gray, Ca green, O red) is determined by chiral amino acids (stick models) present during the crystallization. The interplay of composition and chirality of the crystal surfaces and additives leads to enantiospecific adsorption of the D and L amino acids on chiral surface steps. The resulting surface passivation creates a kinetic barrier, which controls the phase selection.
ZUSAMMENFASSUNG :Es wird die Darstellung von Cyclo{ tris[(2-hydroxy-5-methyl-1,3-phenylen)methylen]-(2hydroxy-5-tert-butyl-1,3-phenylen)methylen) (4a), Cyclo{bis[(2-hydroxy-5-methyl-1,3phenylen)methylen]-bis[(2-hydroxy-5-tert-butyI-1,3-phenylen)methyIen]} (4b) und Cyclo-{pentakis[(2-hydroxy-5-methyl-l,3-phenylen)methylen]) (4c) beschrieben. Die IR-Spektren zeigen, da8 die phenolischen Hydroxygruppen der Vierringverbindungen 4a und 4b intramolekulare, vom LOsungs(CCl4)-oder Zerteilungsmittel (KBr) weitgehend unabhangige Wasserstoffbrucken bilden, die bei der pentameren Ringverbindung 4c schwacher sind. Anhand der 'H-NMR-Spektren la& sich bei diesen Verbindungen eine Pseudorotation nachweisen. Die Massenspektren von 4a und 4b sind fur verhdtnisma-Dig hoch bestandige Verbindungen charakteristisch, wahrend die pentamere Verbindung 4c leicht einen Grundbaustein abspaltet und so wahrscheinlich eine Ringverengung eingeht. SUMMARY: The preparation of cyclo{tris[(2-hydroxy-5-methyl-1,3-phenylene)methylene]-(2-hydroxy-5-tert-butyl-l,3-phenylene)methylene~ (4a), cyclo{bis[(2-hydroxy-5-methyl-1,3phenylene)methylene)bis[(2-hydroxy-5-f~rf-butyl-l,3-pheny~ene)methylene]~ (4b), and cyclo(pentakis[(2-hydroxy-5-methyl-1,3-phenylene)methylene]} (4c) is described. TheIR spectra show that the tetranuclear compounds 4a and 4b exhibit intramolecular hydrogen bonding formed by the phenolic hydroxy groups almost independent on solvent (CCI,) and dispersive medium (KBr) being weaker in the case of the pentameric compound 4c. The existence of pseudorotation is proved by means of 'H-NMR spectra of these compounds. The mass spectra of 4a and 4b are characteristic for the high stability of these molecules, whereas the pentamer 4c easily splits off a basic unit, thus, probably undergoing ring concentration. 3317 G. Happel, B. Mathiasch und H. KanimererEinleitung
The sponge-restricted enzyme silicatein-α catalyzes in vivo silica formation from monomeric silicon compounds from sea water (i.e. silicic acid) and plays the pivotal role during synthesis of the siliceous sponge spicules. Recombinant silicatein-α, which was cloned from the demosponge Suberites domuncula (phylum Porifera), is shown to catalyze in vitro condensation of alkoxy silanes during a phase transfer reaction at neutral pH and ambient temperature to yield silicones like the straight-chained polydimethylsiloxane (PDMS). The reported condensation reaction is considered to be the first description of an enzymatically enhanced organometallic condensation reaction.
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