Model studies of colloidal silica precipitation using biosilica extracts from<Emphasis Type="Italic"> Equisetum telmateia</Emphasis>

Structural materials containing silicon are produced in single celled organisms through to higher plants and animals. Hydrated amorphous silica is a colloidal mineral of infinite functionality that is formed into structures with microscopic and macroscopic form. Proteins and proteoglycans are suggested to play a critical role in the catalysis of silica polycondensation and in structure direction during the formation of these magnificent structures. This article extends knowledge on the effect of protein containing biosilica extracts from Equisetum telmateia on the kinetics of silica formation and structure regulation. Utilising potassium silicon catecholate as the source of soluble silicon, bioextracts obtained from plant silica by dissolution of the siliceous phase with aqueous HF following extensive acid digestion of the plant cell wall were found to modify the kinetic rate constants for the formation of small silicic acid oligomers under circumneutral pH conditions and to modify the solubility of silicic acid in solution. Addition of the bioextracts at ca. 1 wt% to the reaction medium reduced the sizes and range of sizes of the fundamental silica particles formed and led to the formation of crystalline polymorphs of silica under conditions of ca. neutral pH, room temperature and in the absence of multiply charged cations, conditions assumed to be relevant to the biological mineralization environment. The ability of biological organisms to regulate the formation of silica structures with prevention of crystallinity is discussed as are the implications of this study in terms of the generation of new materials with specific form and function for industrial application.     

Metal Ions in Biological Systems,Volume 4. Metal Ions as Probes; : edited by Helmut Sigel,Marcel Dekker,New York, 1974, xii + 261 pages, $24.75.

Trends in ³¹P NMR coordination shifts for the complexes M(CO)₃BrL₂, [M(CO)₃L₂(NCMe)]⁺, MeC₅H₄Mn(CO)L₂ and [MeC₅H₄Mn(CO)₂]₂L₂ (M = Mn and Re;L₂ = Ph₂PCH₂PPh₂, Ph₂PCH₂CH₂PPh₂ and Ph₂PCH₂CH₂AsPh₂) are discussed.     

Syntheses, structure, magnetism, and optical properties of the interlanthanide sulfides δ-Ln2−xLuxS3 (Ln=Ce, Pr, Nd)

δ-Ln_2−xLu_xS₃ (Ln=Ce, Pr, Nd; x=0.67-0.71) compounds have been synthesized through the reaction of elemental rare-earth metals and S using a Sb₂S₃ flux at 1000 °C. These compounds are isotypic with CeTmS₃, which has a complex three-dimensional structure. It includes four larger Ln³⁺ sites in eight- and nine-coordinate environments, two disordered seven-coordinate Ln³⁺/Lu³⁺ positions, and two six-coordinate Lu³⁺ ions. The structure is constructed from one-dimensional chains of LnS_n (n=6-9) polyhedra that extend along the b-axis. These polyhedra share faces or edges with two neighbors within the chains, while in the [ac] plane they share edges and corners with other chains. Least square refinements gave rise to the formulas of δ-Ce_1.30Lu_0.70S₃, δ-Pr_1.29Lu_0.71S₃ and δ-Nd_1.33Lu_0.67S₃, which are consistent with the EDX analysis and magnetic susceptibility data. δ-Ln_2−xLu_xS₃ (Ln=Ce, Pr, Nd; x=0.67-0.71) show no evidence of magnetic ordering down to 5 K. Optical properties measurements show that the band gaps for δ-Ce_1.30Lu_0.70S₃, δ-Pr_1.29Lu_0.71S₃, and δ-Nd_1.33Lu_0.67S₃ are 1.25, 1.38, and 1.50 eV, respectively. Crystallographic data: δ-Ce_1.30Lu_0.70S₃, monoclinic, space group P2₁/m, a=11.0186(7), b=3.9796(3), c=21.6562(15) Å, β=101.6860(10), V=929.93(11), Z=8; δ-Pr_1.29Lu_0.71S₃, monoclinic, space group P2₁/m, a=10.9623(10), b=3.9497(4), c=21.5165(19) Å, β=101.579(2), V=912.66(15), Z=8; δ-Nd_1.33Lu_0.67S₃, monoclinic, space group P2₁/m, a=10.9553(7), b=3.9419(3), c=21.4920(15) Å, β=101.5080(10), V=909.47(11), Z=8.     

Chiral building blocks: enantioselective syntheses of benzyloxymethyl phenyl propionic acids

The synthesis of (2S)-2-benzyloxymethyl-3-(2-fluoro-4-methoxyphenyl)- propionic acid, (2S)-2-benzyloxymethyl-3-(2-fluoro-4-methylphenyl)propionic acid and (2S)-2-benzyl-oxymethyl-3-(2,4-dimethylphenyl)propionic acid has been achieved by TiCl4 mediated alkylation of the corresponding (4R)-4-benzyl-3-[3-(2-fluoro-4-methoxyphenyl-, 2-fluoro-4-methylphenyl-, 2,4- dimethylphenyl-)propionyl]-2-oxazolidinones, followed by hydrolysis of the chiral auxiliary. The stereochemistry of the alkylation reaction was confirmed by an X-ray crystal structure of (4R)-4-benzyl-3-[(2S)-2-benzyloxymethyl-3-(2- fluoro-4-methylphenyl)propionyl]-2-oxazolidinone.     

Editorial

NMR techniques are employed to determine the relative fractions of boron atoms in Na₂OB₂O₃SiO₂ glasses of high soda content. The data show that if enough Na₂O is added, four-coordinated borons are destroyed and borons with one or two non-bridging oxygens are created, but that both the beginning point and the rate of these processes depend strongly on the amount of silica present. These findings are shown to be quantitatively inconsistent with structural models previously suggested in the literature. Utilizing the concept of proportionate atomic sharing of the additional Na₂O, a new structural model is proposed for K K = mol% SiO₂/mol% B₂O₃) which is consistent with all the data including previously reported data for glasses in the region of relatively low soda content. Using R = mol% Na₂O/mol% B₂O₃, the new model states that for , all the additional Na₂O is employed in forming non-bridging oxygens on the silica tetrahedra; then for R_D1 R R_D3 = 2 + K, the fraction $\text{(K +}\text{1}\text{4}\text{K)/(2 + K)}$ of the additional Na₂O destroys reedmernerite groups and forms pyroborate units plus silica tetrahedra with two non-bridging oxygens per Si atom, while the fraction $\text{(2 ?}\text{1}\text{4}\text{K)/(2 + K)}$ of the additional Na₂O destroys diborate groups and forms additional pyroborate units.     

The relative performance of the local density approximation and gradient-corrected density functional theory for computing metal–ligand distances in Werner-type and organometallic complexes

Optimized metal-ligand (M-L) bond lengths for 17 classical Werner-type transition-metal complexes were calculated using the local density approximation (LDA) and a gradient-corrected (GC) extension. GCs lengthen the bonds by between 0.02 and 0.09 Å relative to the LDA results. The latter range from 0.02 Å shorter than observed to 0.05 Å longer, while the GC data range from exact agreement with experiment to some 0.12 Å too long. The LDA rms deviation is 0.025 Å compared to the GC error of 0.070 Å. In contrast, data from the literature for organometallic species show that the LDA gives systematically too short M-L distances and GCs lead to a better agreement with experiment. The relative performance of LDA and GC functionals reflects the qualitatively different chemistries of organometallic and Werner-type complexes. The magnitude of the GC bond-length expansion for the latter correlates with the ionicity of the M-L interaction. © 1997 John Wiley & Sons, Inc.     

Quadrupole parameters of 11B in crystalline CaO.B2O3.

The quadrupole coupling constant (Qcc) and asymmetry parameter (eta) of 11B in crystalline CaO.B2O3 have been measured employing three different NQR and NMR methods: (1) 11B and 10B NQR; (2) 11B NQR and NMR; and (3) the 11B Zeeman NQR powder pattern. It is found that Qcc = 2594.3 +/- 0.5 kHz and eta = 0.515 +/- 0.001 at 77 K, and Qcc = 2573.5 +/- 0.5 kHz and eta = 0.511 +/- 0.002 at 300 K. These values are in agreement with, but far more accurate than, values obtained from a fourth procedure: measurement of the second-order quadrupolar effects evident in the m = + 1/2<-->m = - 1/2 transition of the 11B NMR spectrum.     

14N NQR study of some local anesthetics

Nitrogen-14 nuclear quadrupole resonance (NQR) spectra for eleven local anesthetics in the solid state are reported and analyzed using the Townes and Dailey approach. The changes in the electron distributions at various nitrogen sites, produced by protonating the tertiary amino nitrogen, are discussed and shown to be in general agreement with expectations based on the increased electrophilic character of the protonated amino group.