A new tripodal receptor for molecular recognition of monosaccharides. A paradigm for assessing glycoside binding affinities and selectivities by 1H NMR spectroscopy

A new tripodal receptor for the recognition of monosaccharides is described. The prototypical host 1 features a 1,3,5-substituted 2,4,6-triethylbenzene scaffold bearing three convergent H-bonding units. The binding ability of the t-octyl derivative 1a toward a set of octylglycosides of biologically relevant monosaccharides, including Glc, Gal, Man, and GlcNAc, was investigated by 1H NMR in CDCl3. A protocol for the correct evaluation of binding affinities was established, which can be generally applied for the recognition of monosaccharides by 1H NMR spectroscopy. A three-constant equilibrium model, including 1:1 and 2:1 host-guest association and dimerization of the receptor, was ascertained for the interaction of 1a with all the investigated glycosides. An affinity index, which we defined median binding concentration BC50 in analogy to the IC50 parameter, intended to address the general issue of comparing dimensionally heterogeneous binding data, and a limiting BC0(50)quantity describing intrinsic binding affinities were developed for evaluating the results. BC0(50) values for 1a range from 1 to 6 mM, indicating an intrinsic binding affinity in the millimolar range and a selectivity factor of 5 toward the investigated glycosides. The treatment has been extended to include any generic host-guest system involved in single or multiple binding equilibria.     

Reaction of methyl 4-nitrobenzenesulfonate with Br- in water-formamide tetradecyltrimethylammonium bromide micellar solutions

Summary The reaction methyl 4-nitrobenzenesulfonate + Br^- was studied in water-formamide tetradecyltrimethylammonium bromide micellar solutions. A pseudophase kinetic model was used to quantitatively rationalize the kinetic micellar effects observed     

Adsorption isotherms of a molecular imprinted polymer prepared in the presence of a polymerisable template: Indirect evidence of the formation of template clusters in the binding site

The current opinion about molecular imprinted polymers (MIPs) is that their molecular recognition properties are due to the presence of nanocavities formed during a polymerization process developed in the presence of a template molecule. According to this principle, the shape of these nanocavities is complementary to that of the template and non-covalent interactions are established between the binding site and a single template molecule. Nevertheless, there are some experimental indications that the real molecular recognition mechanism involves clusters of template molecules being packed into the binding site. Recently, it has been proposed that template molecules covalently linked to the binding site can act as nucleation points, enhancing the formation of these molecular clusters.We have tested this hypothesis by studying the adsorption isotherms of polymers prepared by imprinting them with 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). Three different polymers were considered: P0, prepared without the template, P1, whose template was represented by 2,4,5-T molecules, and P2, whose template was 1/3 constituted by the polymerisable 2-(2,4,5-trichlorophenoxyacetoxy)-ethylmethacrylate (2,4,5-TEMA) and 2/3 by 2,4,5-T. The polymers were prepared by thermoinduced polymerization of template mixtures, 4-vinylpyridine and ethylene dimethacrylate. The crushed polymers were packed into HPLC columns and frontal chromatographic runs were performed by eluting the columns with a mobile phase containing variable amounts of 2,4,5-T.The experimental adsorption isotherms were fitted by using several isotherm models, and the Freundlich-Langmuir model was found to give the best fitting in terms of F-test. All the models considered showed a significant difference between the affinity constant values measured for the polymer P1 and P2, with a higher value for the polymer P2 (for Freundlich-Langmuir model: polymer P1, k=(2.00±0.43)×10⁴ M⁻¹; polymer P2, k=(1.93±0.0535)×10⁵ M⁻¹; ratio P2/P1, 9.65±2.09). Such experimental results support the hypothesis that a polymer prepared with a limited amount of template covalently attached to the binding site shows an increased affinity for the template itself.     

Novel Thiazole-Containing Complexing Agents and Luminescence of Their Europum(III) and Terbium(III) Chelates

The synthesis of novel thiazole-containing complexing agents and their luminescence properties with Eu^III and Tb^III ions are reported. One of these terpyridine analogues was also tested as an Eu^III labelling reagent, and its luminescence properties as an antibody conjugate were studied.     

Dependence of equilibrium constants ofL-valine on ionic strength according to Guggenheim,Scatchard and Pitzer models

Summary A systematic study of the dependence of acid-base stoichiometric constants on the ionic strength has been carried out for the aminoacidL-valine in ClK and BrK solutions. The observed dependence has been interpreted by using Guggenheim, Scatchard and Pitzer models for the activity coefficients of the species involved in the equilibria.

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Die Abhängigkeit von Gleichgewichtskonstanten desL-Valin von der Ionenstärke entsprechend den Modellen von Guggenheim, Scatchard und Pitzer

Zusammenfassung Es wurde anL-Valin in KCl- und KBr-Lösungen eine systematische Studie der Abhängigkeit der Säure-Base-Stöchiometrie von der Ionenstärke durchgeführt. Die beobachteten Abhängigkeiten wurden aufgrund der nach den Modellen von Guggenheim, Scatchard und Pitzer erhaltenen Aktivitätskoeffizienten der an den Gleichgewichten beteiligten Spezies interpretiert.

     

Multivariate optimization of an axially-viewed inductively coupled plasma multichannel-based emission spectrometer for the analysis of environmental samples

An experimental design procedure was applied to optimize the operating conditions of an axially-viewed inductively coupled plasma emission spectrometer instruments equipped with echelle optics with cross dispersion and charge transfer device. The multivariate effect of carrier gas flow rate and r.f. power on several analytical figures was investigated and discussed. Both ultrasonic and pneumatic nebulization were used. For the final choice of the optimum, different criteria were taken into account, mainly plasma robustness, instrumental precision, analyte and background net emission, detection limits and signal-to-background ratios. It was found that the use of moderate power (1100W) and mean carrier gas flow rate (0.75 L/min) allows to obtain sufficient plasma robustness, satisfactory precision, and excellent signal-to-background ratios and limits of detection, favorable for ultratrace element determinations in environmental matrices.     

Immobilization of endo-1,4-β-xylanase on polysulfone acrylate membranes: Synthesis and characterization

We report on a new enzyme/support system to immobilize proteins such as enzymes through a covalent bond on polysulfone membranes. In the present case the enzyme endo-1,4-β-xylanase (E.C.3.2.1.8) is attached to polysulfone previously derivatized by introducing an acrylate group. Membranes are properly prepared from this polysulfone acrylate. Afterwards the enzyme is immobilized though the amino groups of side chains of the amino acids of the enzyme and the acrylate group of the derivatized polysulfone. Such immobilization of the enzyme is confirmed by microelemental analysis as well as by amino acid analysis by HPLC. Moreover, the enzymatic activity of the membranes was evaluated and compared with that corresponding to the free enzyme. Certain physical parameters (asymmetry, irregularity, pore size and surface roughness) of the corresponding enzymatic membranes were obtained from SEM and AFM image interpretation.     

Protective effects of cyanidin-3-O-beta-glucopyranoside against UVA-induced oxidative stress in human keratinocytes

Ultraviolet-A (UVA) radiation causes significant oxidative stress because it leads to the generation of reactive oxygen species (ROS), leading to extensive cellular damage and eventual cell death either by apoptosis or necrosis. We evaluated the protective effects of cyanidin-3-O-beta-glucopyranoside (C-3-G) against UVA-induced apoptosis and DNA fragmentation in a human keratinocyte cell line (HaCaT). Treatment of HaCaT cells with C-3-G before UVA irradiation inhibited the formation of apoptotic cells (61%) and DNA fragmentation (54%). We also investigated antioxidant properties of C-3-G in HaCaT cells against ROS formation at apoptotic doses of UVA; C-3-G inhibited hydrogen peroxide (H2O2) release (an indicator of cellular ROS formation) after UVA irradiation. Further confirmation of the potential of C-3-G to counteract UVA-induced ROS formation comes from our demonstration of its ability to enhance the resistance of HaCaT cells to the apoptotic effects of both H2O2 and the superoxide anion (O2*-), two ROS involved in UVA-oxidative stress. Furthermore, in terms of Trolox Equivalent Antioxidant Activity, C-3-G treatment led to a greater increase in antioxidant activity in the membrane-enriched fraction than in the cytosol (55% vs 19%). The protective effects against UVA-induced ROS formation can be attributed to the higher membrane levels of C-3-G incorporation. These encouraging in vitro results support further research into C-3-G (and other anthocyanins) as novel agents for skin photoprotection.     

Selective extraction of astaxanthin from crustaceans by use of supercritical carbon dioxide

An on-line supercritical fluid extraction (SFE) system coupled to a continuous flow manifold including a UV detector was used as a screening system to extract astaxanthin from crayfish, which was found to be the major carotenoid present in the samples. This compound constitutes the principal additive used to dye salmon flesh. The flow manifold was used to confirm the presence of astaxanthin in the crustacean samples. Also, an HPLC/UV-vis method was used to ascertain that this compound was the major carotenoid extracted under the optimum SFE conditions employed. The influence of SFE operating variables such as pressure, temperature, equilibration time, extraction time, trap temperature, and volume of CO₂ modifier was examined in order to maximize the efficiency of analyte extraction. The use of supercritical CO₂ enables the expeditious, selective, quantitative extraction of astaxanthin from crustaceans.     

Synthesis of fused purinoquinazolines. Three new heterocyclic ring systems

The Ullmann reaction of 8-aminotheophylline or 8-aminocaffeine with 2-chlorobenzoic acid and of 8-bromotheophylline with ethyl-2-aminobenzoate afforded derivatives of three new heterocyclic systems: purino[7,8-α]quinazoline-5,9,11(6H,8H,10H)-trione, purino[8,9-b]quinazoline-2,4,11(1H,3H,5H)-trione and purino[8,7-b]quinazoline-2,4,6(1H,3H,11H)-trione, respectively.