Stöber silica particles as basis for redox modifications: particle shape, size, polydispersity, and porosity

The synthesis of Stöber silica particles as basis for redox modifications is optimized for desired properties, in particular diameter in a wide sub-micrometer range, spherical shape, monodispersity, the absence of porosity, and aggregation free isolability for characterization and later covalent modification. The materials are characterized by SEM, DLS, nitrogen sorption isotherms, helium as well as Gay-Lussac (water) pycnometry, and DRIFT spectroscopy. Particles with diameters between approximately 50 and 800 nm are obtained by varying the concentrations of the reagents and reactants, the type of solvent as well as the temperature. The use of high water concentrations and post-synthetic calcination at 600 °C results in silica particles that can be considered as nonporous with respect to the size of the active molecules to be immobilized. The effect of reaction temperature on size distribution is identified. Low polydispersity is achieved by performing the reaction in a temperature range in which a change in temperature has only a weak or no effect on the final particle diameter. Upon optimization of the sol–gel process, the shape of the particles is still spherical. The agreement between experimental and geometric data is within the expected precision of the characterization techniques.     

Equilibrium and NMR relaxometric studies on the s-triazine-based heptadentate ligand PTDITA showing high selectivity for Gd3+ ions

A complete potentiometric and NMR relaxometric solution study on the heptadentate 2,2',2″,2'″-[(6-piperidinyl-1,3,5-triazine-2,4-diyl)dihydrazin-2-yl-1-ylidene]tetraacetic acid (PTDITA) ligand has been carried out. This ligand is based on the 1,3,5-triazine ring with two hydrazine-N,N-diacetate groups in positions 2 and 4 and a piperidine moiety in position 6. The introduction of the triazine ring into the ligand backbone is expected to modify its flexibility and then to affect the stability of the corresponding complexes with transition-metal and lanthanide ions. Thermodynamic stabilities have been determined by pH potentiometry, UV spectrophotometry, and (1)H NMR spectroscopy for formation of the complexes with Mg(2+), Ca(2+), Cu(2+), Zn(2+), La(3+), Gd(3+), and Lu(3+) ions. PTDITA shows a good binding affinity for Gd(3+) (logK = 18.49, pGd = 18.6) and an optimal selectivity for Gd(3+) over the endogenous Ca(2+), Zn(2+), and Cu(2+) (K(sel) = 6.78 × 10(7)), which is 3 orders of magnitude higher that that reported for Gd(DTPA) (K(sel) = 2.85 × 10(4)). This is mainly due to the lower stability of the Cu(II)- and Zn(II)(PTDITA) complexes compared to the corresponding DTPA complexes, which suggests an important role of the triazine ring on the selectivity for the Gd(3+) ion. The relaxometric properties of Gd(PTDITA) have been investigated in aqueous solution by measuring the (1)H relaxivity as a function of the pH, temperature, and magnetic field strength (nuclear magnetic relaxation dispersion profile). Variable-temperature (17)O NMR data have provided direct information on the kinetic parameters for exchange of the coordinated water molecules. A simultaneous fit of the data suggests that the high relaxivity value (r(1) = 10.2 mM(-1) s(-1)) is a result of the presence of two inner-sphere water molecules along with the occurrence of relatively slow rotation and electronic relaxation. The water residence lifetime, (298)τ(M) = 299 ns, is quite comparable to that of clinically approved magnetic resonance imaging contrast agents. The displacement of the inner-sphere water molecules by bidentate endogeneous anions (citrate, phosphate, and carbonate) has also been evaluated by (1)H relaxometry. In general, the binding interaction is markedly weak, and only in the case of citrate, a ca. 35% decrease in relaxivity was observed in the presence of 60 equiv of the anion. Phosphate and carbonate also interact with the paramagnetic ion, likely as monodentate ligands, but formation of the ternary complex is accompanied by a modest increase of r(1) due to the contribution of second-sphere water molecules.     

[6,6]‐Open and [6,6]‐Closed Isomers of C70(CF2): Synthesis,Electrochemical and Quantum Chemical Investigation

Novel difluoromethylenated [70]fullerene derivatives, C₇₀(CF₂)_n (n=1–3), were obtained by the reaction of C₇₀ with sodium difluorochloroacetate. Two major products, isomeric C₇₀(CF₂) mono‐adducts with [6,6]‐open and [6,6]‐closed configurations, were isolated and their homofullerene and methanofullerene structures were reliably determined by a variety of methods that included X‐ray analysis and high‐level spectroscopic techniques. The [6,6]‐open isomer of C₇₀(CF₂) constitutes the first homofullerene example of a non‐hetero [70]fullerene derivative in which functionalisation involves the most reactive bond in the polar region of the cage. Voltammetric estimation of the electron affinity of the C₇₀(CF₂) isomers showed that it is substantially higher for the [6,6]‐open isomer (the 70‐electron π‐conjugated system is retained) than the [6,6]‐closed form, the latter being similar to the electron affinity of pristine C₇₀. In situ ESR spectroelectrochemical investigation of the C₇₀(CF₂) radical anions and DFT calculations of the hyperfine coupling constants provide evidence for the first example of an inter‐conversion between the [6,6]‐closed and [6,6]‐open forms of a cage‐modified fullerene driven by an electrochemical one‐electron transfer. Thus, [6,6]‐closed C₇₀(CF₂) constitutes an interesting example of a redox‐switchable fullerene derivative.     

Structural variability of endotoxins from R-type isogenic mutants of Shigella sonnei

The structural variations in the rough-type endotoxins [lipopolysaccharides (LPSs)] of Shigella sonnei mutant strains (S. sonnei phase II-4303, R41, 562H and 4350) were investigated by Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and tandem MS. A series of S. sonnei mutants had previously been the subject of analytical studies on the biosynthesis of heptose components in the core oligosaccharide region of LPSs. This study gives a complete overview on the structures of the full core and lipid A of S. sonnei mutant strains by MS. We found that the LPSs of the isogenic rough mutants were formed in a step-like manner containing 0:1:2:3 heptose in the deep core region of 4350, 562H, R41 and 4303, respectively, and the longest LPS from the mutant S. sonnei 4303 contained also five hexoses. The structural variations in the lipid A moiety and in the oligosaccharide part of the intact LPS were followed by MALDI-TOF-MS/MS. For the dissolution and the ionization of the samples, 2,5-dihydroxybenzoic acid in citric acid solution was applied as matrix. The detailed evaluation of the mass spectra indicates heterogeneity in the lipid part due to the differences in the phosphate and fatty acid composition.     

Binding of alkaloids into the S1 specificity pocket of α-chymotrypsin: evidence from induced circular dichroism spectra

Non-covalent binding of planar aromatic molecules into the S1 specificity pocket of the serine protease α-chymotrypsin (αCHT) can be detected by measuring induced circular dichroism (CD) spectroscopic signals. Utilizing this phenomenon, αCHT association of proflavine (PRF), the well known serine protease inhibitor has been investigated together with plant-derived compounds including isoquinoline, pyridocarbazole and indoloquinoline alkaloids, of which αCHT binding has never been reported. Non-degenerate exciton coupling between π-π* transitions of the ligand molecules and two tryptophan residues (Trp172 and Trp215) near to the binding site is proposed to be responsible for the induced CD activity. The association constants calculated from CD titration data indicated strong αCHT association of sanguninarine, ellipticine, desmethyl-isocryptolepine and isoneocryptolepine (K(a) ≈ 10(5) M(-1)) while berberine, coptisine and chelerythrine bind to the enzyme with lower, PRF-like affinity (K(a) ≈ 10(4) M(-1)). PRF-trypsin and ellipticine-trypsin binding interactions have also been demonstrated. The binding of the alkaloids into the S1 pocket of αCHT has been confirmed by CD competition experiments. Molecular docking calculations showed the inclusion of PRF as well as the alkaloid molecules in the S1 cavity where they are stabilized by hydrophobic and H-bonding interactions. These novel nonpeptidic scaffolds can be used for developing selective inhibitors of serine proteases having chymotrypsin-like folds. Furthermore, the results provide a novel, CD spectroscopic based approach for probing the ligand binding of αCHT and related proteases.     

Synthesis of 1,2,7,7a-tetrahydro-1aH-cyclopropa[b]quinoline-1a-carboxylic acid derivatives, doubly constrained ACC derivatives, by a remarkable cyclopropanation process

Reaction of N-benzoyl-1,2,3,4-tetrahydroquinoline-2-carboxylic acid with acetic anhydride resulted in 1H,3H,5H-oxazolo[3,4-a]quinolin-3-one derivative 13. Different cyclopropanation processes were applied to 13, but only diazomethane in the presence of water furnished the hitherto unknown methyl 1,2,7,7a-tetrahydro-1aH-cyclopropa[b]quinoline-1a-carboxylate 14, which can be considered as a doubly constrained 1-aminocyclopropane-1-carboxylic acid system. The mechanism of the cyclopropanation was studied in detail. The new ACC ester 14 was transformed into fused tetracyclic hydantoin derivatives, which comprised a new type of heterocyclic system.     

Toward a Constitutive Law of Cartilage: A Polymer Physics Perspective

To describe load bearing and lubrication of cartilage requires treating its collagen network and proteoglycan (PG) phases separately in a constitutive law of the tissue. We propose a framework for developing such an empirical constitutive law that treats the cartilage extracellular matrix (ECM) as a composite medium, with a PG phase that exerts a swelling pressure, and a collagen network phase that restrains it. We compare and contrast this model to a biomechanical constitutive law that aggregates the collagen and PG phases into a single “solid-like” elastic tissue matrix, and show that aggregation obscures essential differences in the physical-chemical properties of the collagen and PG constituents as well as their distinct biological roles within cartilage's ECM. We also relate moduli in the aggregate constitutive model to quantities measured in an osmotic stress titration experiment.     

Influence of the aluminum alkyl co-catalyst type in Ziegler-Natta ethene polymerization on the formation of active sites, polymerization rate, and molecular weight

The influence of the concentration of the co-catalysts triethylaluminium (TEAL), tri-iso-butylaluminium (TIBAL), tri-n-octylaluminium on the polymerization rate for standard Ziegler-Natta catalyst systems was studied. By comparing the influence of monomeric TIBAL with TEAL co-catalyst on the polymerization activity, the effect of TEAL dimerization was described. The use of the Eley-Ridealadsorption model instead of Langmuir-Hinselwood model is proposed for the absorption of monomeric aluminiumalkyl species and for the formation of active centers C*. It is further proposed that steric hindrance from different co-catalysts, which results in a higher molecular weight (MW) of polymers, is caused by active centers with reduced space for chain transfer reactions.     

Spectrochemical and Thermal Behaviour of the 5-chloro-2-nitrobenzoates of Rare Earth Elements

The conditions of the formation of yttrium and lanthanide 5-chloro-2-nitrobenzoates were studied and their quantitative composition and solubilities in water at 298 K were determined. They are anhydrous or hydrated complexes and their solubilities are of the order of 10-3 mol dm-3. The IR and X-ray spectra for the complexes were recorded. All complexes are crystalline compounds. Their thermal decomposition was studied. It was found that on heating above 523 K the complexes decompose explosively. Therefore their thermal decomposition was carried out in the temperature range 273-523 K. Hydrated complexes lose crystallization water molecules in one step. From the results it appeared that during dehydration process no transformation of nitro group to nitrito took place. Some of physico-chemical properties of rare earth element 5-chloro-2-nitrobenzoates were compared with 2-nitro- and 3-chlorobenzoates of those elements. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of cephalosporins carrying isoxazolyl acetamido and related side chains

Starting from 3,5-dimethylisoxazole the carboxylic acids I and V, the amino acids VIII (L-) and IX (D-), and the ureido acids X (L-) and XI (D-) were prepared, which were used for the synthesis of the new cephalosporins XVIIb, XXa-c (L-), and XXIb (D-). Thein vitro antibacterial activity of these semi-synthetic antibiotics was studied. The resorption of XVIIb was investigated in mice.Research Group for Antibiotics of the Hungarian Academy of Sciences, H-4010 Debrecen, P.O. Box 70, Hungary. Published in Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1524–1535, November, 1998.