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991.
Branko Kozuilić Ivančica Leuštek Branka Pavlović Pavao Mildner Slobodan Barbarić 《Applied biochemistry and biotechnology》1987,15(3):265-278
Each of the three high-mannose type glycoproteins studied, acid phosphatase, invertase, and glucose oxidase, could be specifically
cross-linked through its carbohydrate chains. The procedure involves periodate oxidation of carbohydrate residues followed
by reaction of the generated aldehyde groups with adipic acid dihydrazide as a cross-linker. The amount and size as well as
solubility of the formed polymers could be efficiently controlled by varying the reaction conditions, i.e., the oxidation
degree and the concentrations of glycoproteins, cross-linker, and hydrogen ions during the cross-linking reaction. It was
found that the quantity and size of polymers increased with oxidation degree and protein concentration and by lowering the
pH. When the protein concentration was above and pH below certain values, depending on the glycoenzyme, insoluble polymers
formed. The soluble cross-linked polymers retained a high level of original activity, and the minor decrease in specific activity
noticed was shown to occur during the periodate oxidation step. The cross-linked glycoenzymes are much more resistant to denaturation
by high temperature and by changes in pH, demonstrating the usefulness of this method in preparation of the stabilized glycoprotein
derivatives. 相似文献
992.
Spanik I Krupcik J Skacáni I Sandra P Armstrong DW 《Journal of chromatography. A》2005,1071(1-2):59-66
In this work, the separation of enantiomers of N-TFA-O-alkyl amino acids on the 2,3-di-O-pentyl-6-O-acyl alpha-, beta- and gamma-cyclodextrin stationary phases has been studied. The influence of structure differences in the alkyl substituents bonded to the stereogenic carbon atom (R1), as well as in the ester group (R2) of the selected amino acid derivatives, and the selectivity of modified alpha-, beta- and gamma-cyclodextrin phases in gas chromatographic separation of derivatized amino acid enantiomers was studied in detail. A model set of N-TFA-alkyl esters of four amino acids was separated on five columns. The separation of enantiomers was evaluated in terms of the interactions of the alkyl substituents bonded to the stereogenic carbon (R1) and/or the ester group (R2) of the N-TFA-O-alkyl amino acid derivatives as well as the nature of the 3-O-acyl group in the 2,6-di-O-pentyl-3-O-acyl alpha-, beta- and gamma-cyclodextrins. It was shown that the variation in the enantiomeric separation with temperature and the retention order of enantiomers on a given cyclodextrin capillary column depends both on the nature of the bonded R1 and R2 alkyl groups. It was found that the temperature dependencies of selectivity factors, ln alpha on 1/T, were mostly non-linear. The thermodynamic data [delta(deltaS) and [delta(deltaH)] which characterize the chiral recognition were used to gain more insight into the mechanistic aspects of enantio separation of the N-TFA-O-alkyl amino acid derivatives on 2,6-di-O-pentyl-3-O-acyl-alpha-, beta- and gamma-cyclodextrins. 相似文献
993.
Ivan V. Ogorodnyk Igor V. Zatovsky Vyacheslav N. Baumer 《Journal of solid state chemistry》2006,179(12):3681-3687
The single crystals of caesium magnesium titanium (IV) tri-oxo-tetrakis-diphosphate bis-monophosphate, Cs3.70Mg0.60Ti2.78(TiO)3(P2O7)4(PO4)2, crystallize in sp. gr. P-1 (No. 2) with cell parameters a=6.3245(4), b=9.5470(4), c=15.1892(9) Å, α=72.760(4), β=85.689(5), γ=73.717(4), z=1. The titled compound possesses a three-dimensional tunnel structure built by the corner-sharing of distorted [TiO6] octahedra, [Ti2O11] bioctahedra, [PO4] monophosphate and [P2O7] pyrophosphate groups. The Cs+ cations are located in the tunnels. The partial substitution of Ti positions with Mg atoms is observed. The negative charge of the framework is balanced by Cs cations and Mg atoms leading to pronounced concurrency and orientation disorder in the [P2O7] groups, which coordinate both. 相似文献
994.
Activation barriers of the title reactions are calculated using a DZ + P basis set with diffuse p functions on negative ions. In the correlation effects, which are large and negative, the importance of single and triple excitations is stressed. The barrier for the reaction of H? with CH3F is 12 kJ/mole, in good agreement with experiment, while the barrier for the reaction of F? with CH3F is predicted to be 14 kJ/mole. 相似文献
995.
Bruce N. Storhoff Daniel P. Harper Ivan H. Saval Jonathan H. Worstell 《Journal of organometallic chemistry》1981,205(2):161-166
Both (C6H5)2P(CH2)3CN and (C6H5)2P(CH2)4CN have been obtained from the reaction of Br(CH2)nCN (n = 3, 4) with (C6H5)2POCH3 followed by reduction with (C6H5)2SiH2. These phosphine-nitrile ligands form L2PdCl2 complexes which are shown by IR measurements to have trans geometries with the phosphine portions of the ligands coordinated. Reactions of o-BrC6H4CN with CH3(CH2)3Li followed by R2PCl (R = C6H5 or (CH3)2N) have been used to provide good yields of the corresponding R2P-o-C6H4CN products. 相似文献
996.
Haeffner F Jacobson MA Keresztes I Williard PG 《Journal of the American Chemical Society》2004,126(51):17032-17039
A density functional theory (B3LYP/6-31+G) study was undertaken in an effort to learn more about the mechanisms controlling the regioselective deprotonations of the synthetically versatile N-lithio-N-(tert-butyl)allylamide 1 and N-lithio-N-(trimethylsilyl)allylamine 2 compounds. The calculations suggest that deprotonation of 1 occurs exclusively at the allylic position. This agrees with experimental results. The calculations also suggest that deprotonation of allylamine 2 exclusively at the cis-vinylic position is due to kinetic control. 相似文献
997.
Jan Mrázek Vlastimil Matejec Ivan Kasik Milos Hayer Daniela Berková 《Journal of Sol-Gel Science and Technology》2004,31(1-3):175-178
This paper presents results on using the sol-gel process for the preparation of microstructure fibers (MSFs). In this preparation the sol-gel method is used for the formation of bonds between silica tubes and silica rods in the input stacks for MSF drawing and for filling-in the interstices between these parts. Gels applied on the stacks were prepared from sols composed of tetraethoxysilane, phosphorous oxychloride, boron tribromide, ethanol, HCl and water. Pastes prepared from the sols and solid fabricated through heat treatment of the sols at 1000°C were also used. The sols were applied on the stacks by the dip-coating method and the pastes were applied manually. The formed xerogels were heat-treated at temperatures of up to 1000°C. The MSFs of the grapefruit type were drawn from the stacks at a temperature of about 1900°C. Waveguiding properties of the fibers were examined under their excitation by an inclined collimated beam. It has been found that the sol-gel method is suitable particularly for filling-in the interstices at the end of the stack, which enables pressure control inside holes of the stack during MSF drawing. 相似文献
998.
Two alternative approaches to the synthesis of novel 6-(fluoromethyl)purine bases and nucleosides are described either by direct deoxyfluorination or by multistep functional group transformations starting from 6-(hydroxymethyl)purines. 6-(fluoromethyl)purine ribonucleoside displayed significant cytostatic effects. 相似文献
999.
Gilles Caporossi Ivan Gutman Pierre Hansen Ljiljana Pavlovi 《Computational Biology and Chemistry》2003,27(1):85-90
Let G be a graph and dv the degree (=number of first neighbors) of its vertex v. The connectivity index of G is χ=∑ (dudv)−1/2, with the summation ranging over all pairs of adjacent vertices of G. In a previous paper (Comput. Chem. 23 (1999) 469), by applying a heuristic combinatorial optimization algorithm, the structure of chemical trees possessing extremal (maximum and minimum) values of χ was determined. It was demonstrated that the path Pn is the n-vertex tree with maximum χ-value. We now offer an alternative approach to finding (molecular) graphs with maximum χ, from which the extremality of Pn follows as a special case. By eliminating a flaw in the earlier proof, we demonstrate that there exist cases when χ does not provide a correct measure of branching: we find pairs of trees T, T′, such that T is more branched than T′, but χ(T)>χ(T′). The smallest such examples are trees with 36 vertices in which one vertex has degree 31. 相似文献
1000.
G. Ivan Aurelia Meghea Maria Giurginca Nicoleta Iftimie 《Polymer Degradation and Stability》2003,80(3):397-402
New elastomers with high content of 3,4 isoprene units have been developed during the last decades in an attempt to ensure superior performances of the final products and the present study is devoted to the investigation of some peculiarities of their ageing behaviour. On thermo-oxidative degradation, 3,4 isoprene units are less affected in comparison to cis-1, 4 and trans-1, 4 isoprene units. The degradation process consists mainly in splitting of the main chains at temperatures of 80–100 °C while at higher temperatures (120–130 °C) branching becomes a significant modification and this reaction is enhanced for the polymers containing preponderantly 3,4 units. Such behaviour leads to the fact that the processability of polymers containing high amounts of 1,2 and 1,3 isoprene units is less affected by thermo-oxidative degradation in comparison with cis-1, 4-polyisoprene, which could be explained by the fact that large amounts of double bonds are not present in the backbone of macromolecules but in the pendent groups. 相似文献