The effect of nitrobenzene and aminobenzoic acids on the hydration of tricalcium silicate: a conduction calorimetric study

The hydration of tricalcium silicate (C₃S) in the presence of 0.004, 0.008 and 0.016 mol.% (with respect to 100 g of the silicate) of benzole acid, o-, m- and p-nitrobenzoic and aminobenzoic acids, was followed by conduction calorimetry. Benzoic acid at 0.004 and 0.008 moles behaved as a delayed accelerator of hydration, whereas at a dosage of 0.016 moles it performed as an accelerator by decreasing the onset of the induction period and promoting the earlier appearance of the main exothermic peak. The m- and p-nitrobenzoic acids accelerated the hydration of C₃S, whereas o-nitrobenzoic acid acted as a retarder. Both m-and p-aminobenzoic acids retarded the hydration by delaying the appearance of the main exothermal peak. o-Aminobenzoic acid showed a similar effect to that of the reference at early times by not affecting the induction period and the maximum rate-of-heat peak. However, it increased slightly the amplitude of the main exothermic peak. The compounds that promoted the appearance of a heat peak at periods of 1 h or earlier exhibited an acceleration effect. In the presence of retarders this peak did not appear.     

Thermal decomposition of the biguanide complexes of the 3d-transition metals

The thermal decomposition (TG, DTG and DTA) of the complexes of biguanide with the following metals was studied: V, Cr, Mn, Co, Ni, Cu and Zn. Structural water, when present, is first eliminated at ～100–150°C; this is followed by a main decomposition state at ～300–350°C. Pyrolytic residues are analysed and characterised by their x-ray powder diffraction patterns and are found to be the oxides V₂O₅, Cr₂O₃, Mn₃O₄, Co₃O₄, NiO, CuO and ZnO, respectively. The decomposition curves of the free ligand (biguanide) and biguanide sulphate are also given. The decomposition characteristics are discussed.     

Kinetics and mechanism of the oxidation of amino acids by peroxomonosulphate oxidation of β-phenylalanine,isoleucine and threonine

The kinetics of oxidation of amino acids (AA) by peroxomonosulphate (PMS) in the presence and absence of formaldehyde were studied. Analysis of the results shows that the rate can be represented as, at constant [H⁺]and in the absence of formaldehyde

In the presence of formaldehyde and at constant [H⁺]the rate law is

Perusal of the kinetic results show that the formaldehyde catalysed reaction occurs $\text{\$}\text{?}$10⁵ times faster than uncatalysed and this is attributed to the formation of Schiff base. Mechanism of the reaction is discussed in terms of kinetic results.     

Vinylalumination for the synthesis of functionalized allyl alcohols, vinylepoxides, and alpha-alkylidene-beta-hydroxy-gamma-lactones

A modified hydroalumination protocol for the preparation of [alpha-(ethoxycarbonyl)vinyl]diisobutylaluminum and its beta-methyl or -phenyl analogues was developed. These vinylaluminum reagents react with aldehydes and ketones to provide the corresponding functionalized allyl alcohols in good to excellent yields. Perfluoroalkyl and -aryl carbonyl compounds, alpha-keto esters, alpha-acyl cyanides, and alpha-acetylenic ketones provide the corresponding alpha-hydroxyalkenes in high yields. The allyl alcohol product ratios from the vinylalumination of unsymmetrical alpha-diketones with [alpha-(ethoxycarbonyl)vinyl]diisobutylaluminum and its beta-methyl or -phenyl analogues depend on the steric and electronic environments of the ketones as well as the reagents. The products from the vinylalumination of alpha-bromoaldehydes and -ketones were cyclized with K2CO3 or KF under nonaqueous conditions to provide functionalized vinylepoxides in high yields. Vinylaluminations of keto-protected pyruvaldehyde provided the products, which were converted to alpha-alkylidene-beta-hydroxy-gamma-lactones.     

13C-13C rotational resonance width distance measurements in uniformly 13C-labeled peptides

The rotational resonance width (R2W) experiment is a constant-time version of the rotational resonance (R2) experiment, in which the magnetization exchange is measured as a function of sample spinning frequency rather than the mixing time. The significant advantage of this experiment over conventional R2 is that both the dipolar coupling and the relaxation parameters can be independently and unambiguously extracted from the magnetization exchange profile. In this paper, we combine R2W with two-dimensional 13C-13C chemical shift correlation spectroscopy and demonstrate the utility of this technique for the site-specific measurement of multiple 13C-13C distances in uniformly labeled solids. The dipolar truncation effects, usually associated with distance measurements in uniformly labeled solids, are considerably attenuated in R2W experiments. Thus, R2W experiments are applicable to uniformly labeled biological systems. To validate this statement, multiple 13C-13C distances (in the range of 3-6 A) were determined in N-acetyl-[U-13C,15N]l-Val-l-Leu with an average precision of +/-0.5 A. Furthermore, the distance constraints extracted using a two-spin model agree well with the X-ray crystallographic data.     

Capillary electrophoresis and fluorescence studies on molecular beacon-based variable length oligonucleotide target discrimination

Molecular beacons (MBs) are oligonucleotide probes having a compact hairpin structure, with a fluorophore attached to one end and a quencher molecule attached to the other end. In its native state, the fluorophore is quenched by virtue of its proximity to the quencher molecule. Upon hybridization with its complementary oligonucleotide target, fluorescence is elicited due to a conformational change that results in separation of the fluorophore and quencher molecule. The present study describes the hybridization interaction of an MB to various complementary target sequences. The effects of temperature and length of complementary target sequences on hybridization were investigated using capillary electrophoresis and solution-based fluorescence techniques. Hybridization efficiency was dependent on the ability of the target sequences to destabilize the stem region by binding directly to the stem region. Optimal hybridization occurred between 40 and 50 degrees C for all targets tested, with the true target forming a more stable hybrid complex.     

Polymethylated DOTA ligands. 2. Synthesis of rigidified lanthanide chelates and studies on the effect of alkyl substitution on conformational mobility and relaxivity

M4DOTA, [(2S,5S,8S,11S)-4,7,10-tris-carboxymethyl-2,5,8,11-tetramethyl-1,4,7,10-tetraazacyclododecan-1-yl]acetic acid (2e), and M4DOTMA, (R)-2-[(2S,5S,8S,11S)-4,7,10-tris-((R)-1-carboxyethyl)-2,5,8,11-tetramethyl-1,4,7,10-tetraazacyclododecan-1-yl]propionic acid (3e), are derivatives of ligand DOTA (1e) that form sterically crowded lanthanide chelates. M4DOTMA forms highly symmetric and totally rigid single Y(3+) and Yb(3+) species in which the ring substituents occupy corner positions in a square antiprismatic arrangement as shown by molecular mechanics calculations and by a quantitative interpretation of the relative magnitudes of the paramagnetic (1)H NMR shifts of dipolar origin. The NMR spectrum of YbM4DOTMA(-) displays two intense methyl peaks outside the 0-10 ppm range whose shift difference is strongly temperature dependent. YbM4DOTMA(-) (3d) could be a useful probe in magnetic resonance thermometric imaging. With only four methyl substituents on the tetraaza ring, M4DOTA forms three Yb(3+) species in solution. The methyl substituents prevent the inversion of configuration of the ethylenic groups but not of the acetate arms. Although the methyl groups are likely to preferably occupy ring corner positions, the dipolar equations do not allow one to distinguish with certainty between the two available corner (equatorial) orientations. Reliably applying the dipolar equations is less obvious than usually assumed. A single methyl substituent as in ligand MDOTA (5e) suffices to rigidify the tetraaza cycle but not the acetate arms. Racemic YbMDOTA(-) (5d) is present in solution as four totally asymmetric topomers with the methyl groups occupying either one of the two equatorial positions. A complete assignment of the solution structures on the basis of the dipolar equations is again uncertain. The nuclear magnetic relaxation dispersion curves of the Gd(3+) chelates of all the methylated DOTA ligands including DOTMA, (R)-2-[4,7,10-tris-((R)-carboxyethyl)- 1,4,7,10- tetraazacyclododecan-1-yl]propionic acid, are very similar, and intermolecular conformational processes appear to have no influence on the relaxivity of these small complexes for which the relaxation T(1) is mainly determined by the rotational correlation time (tau(r)). The hydration number of the Tb(3+) chelates measured by fluorescence decreases from DOTMA to M4DOTMA presumably because steric crowding leads to an increase of the metal-water distance.     

The Critical Importance of Water in the Asymmetric Allylboration of N-Trimethylsilylbenzaldimines with B-Allyldiisopinocampheylborane

The free aldimine is probably the intermediate in the asymmetric allylboration of N-trimethylsilylaldimines in the presence of water (see scheme), which is critical for the reaction. The aldimine is rapidly captured by the allylborating agent. Ipc₂BAll=B-allyldiisopinocampheylborane.     

Thermal conductivity of ZnSe by molecular dynamics simulation

Molecular dynamics simulation using the new fit of the empirical Tersoff Potential is applied to study the thermal properties of ZnSe at and below the room temperature. The resulting diffusivity and thermal conductivity are compared with the photoacoustic measurements on the grown ZnSe crystal. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)