Modelling and mutation studies on the histamine H1-receptor agonist binding site reveal different binding modes for H1-agonists: Asp116 (TM3) has a constitutive role in receptor stimulation

Summary A modelling study has been carried out, investigating the binding of histamine (Hist), 2-methylhistamine (2-MeHist) and 2-phenylhistamine (2-PhHist) at two postulated agonistic binding sites on transmembrane domain 5 (TM5) of the histamine H₁-receptor. For this purpose a conformational analysis study was performed on three particular residues of TM5, i.e., Lys²⁰⁰, Thr²⁰³ and Asn²⁰⁷, for which a functional role in binding has been proposed. The most favourable results were obtained for the interaction between Hist and the Lys²⁰⁰/Asn²⁰⁷ pair. Therefore, Lys²⁰⁰ was subsequently mutated and converted to an alanine, resulting in a 50-fold decrease of H₁-receptor stimulation by histamine. Altogether, the data suggest that the Lys²⁰⁰/Asn²⁰⁷ pair is important for activation of the H₁-receptor by histamine. In contrast, analogues of 2-PhHist seem to belong to a distinct subclass of histamine agonists and an alternative mode of binding is proposed in which the 2-phenyl ring binds to the same receptor location as one of the aromatic rings of classical histamine H₁-antagonists. Subsequently, the binding modes of the agonists Hist, 2-MeHist and 2-PhHist and the H₁-antagonist cyproheptadine were evaluated in three different seven--helical models of the H₁-receptor built in homology with bacteriorhodopsin, but using three different alignments. Our findings suggest that the position of the carboxylate group of Asp¹¹⁶ (TM3) within the receptor pocket depends on whether an agonist or an antagonist binds to the protein; a conformational change of this aspartate residue upon agonist binding is expected to play an essential role in receptor stimulation.Abbreviations 2-MeHist 2-methylhistamine - 2-PEA 2-pyridyl-ethylamine - 2-PhHist 2-phenylhistamine - CHO Chinese hamster ovary - E_int interaction energy - E_str strain energy - GES global energy structure - gpH₁R guinea pig H₁-receptor - GPCR G-protein coupled receptor - Hist histamine - N proximal nitrogen - N tele nitrogen - TM transmembrane domain - WT wild type     

Cyclen based bis-macrocyclic ligands as phosphates receptors. A potentiometric and NMR study

The host-guest interaction between orthophosphate, pyrophosphate and triphosphate anions and four cyclen based bis-macrocycles ligands possessing ortho-(BOC), meta-(BMC), para-xylenyl (BPC) or 2,6-pyridinyl (BPyC) linker was investigated by potentiometric measurements and NMR spectroscopy. Each ligand gave protonated species in aqueous solution that further formed ternary complexes after binding with anions; these complexes were analyzed as a result of hydrogen bond formation and Coulombic attraction between the organic host and the inorganic guest. The equilibrium constants for all the detected species are reported and the selectivity, illustrated with species distribution diagrams, is discussed. The results unambiguously showed the importance of the distance between the two cyclen cores and underlined, especially for the triphosphate species, the contribution of the nitrogen atom of the pyridinyl spacer as a supplementary anchoring point in acidic medium.     

Development of a reversed-phase thin-layer chromatographic method for artemisinin and its derivatives

In this study a clear separation between seven analogues of artemisinin on thin-layer chromatography (TLC) is presented. The developed TLC method is carried out on a RP-C18 thin-layer plate using acetonitrile-water (50:25 v/v) as the mobile phase. Spots are visualized by derivatization with an acidified 4-methoxybenzaldehyde reagent in methanol-water. This method allows the separation of a diverse group of compounds that have versatile hydrophilic/lipophilic characteristics; namely artemisinin, artesunate (AS), artelinic acid (AL), arteether (AE), both isomers of artemether (AM) (alpha and beta), dihydroartemisinin, and desoxyartemisinin. Separation of some degradation products and impurities, down to 2%, allows quality control and stability investigation of all actives in raw material and pharmaceutical formulations. The method is further developed via densitometric measurement for quantitative determination purposes for AL and AS. The derivatization technique is evaluated, showing good stability and reproducibility of the coloring process. Percent relative standard deviation values are less than 5% for replicates, and linearity is obtained in the range of 0.5 to 8 microg. A comparative study with high-performance liquid chromatography (HPLC) on a C18 column, applying the same mobile phase, proves the suitability of the TLC method, in which almost all presented analytes are separated from each other. In contrast, HPLC requires at least a 20-min analysis to chromatograph all of the compounds and only betaAM and AE are clearly separated from each other and from the other compounds.     

Water adsorption in disordered mesoporous silica (Vycor) at 300 K and 650 K: a Grand Canonical Monte Carlo simulation study of hysteresis

This numerical simulation paper focuses on the adsorption/desorption of water in disordered mesoporous silica glasses (Vycor-like). The numerical adsorbent was previously obtained by off lattice method, and was shown to reproduce quite well the micro- and mesotextural properties of real Vycor, as well as morphological (pore size distribution) and topological (pore interconnections) disorder. The water-water interactions are described by the SPC model while water-silica interactions are calculated in the framework of the PN-TrAZ model. The water adsorption/desorption isotherms and the configurational energies are calculated by the Grand Canonical Monte Carlo simulation method. The low pressure results compare well with experiments, showing the good transferability of the intermolecular potential. It is shown that if the hysteresis loop observed in the adsorption/desorption isotherm is considered as a true phase transition (which is actually still an open question in the case of disordered porous materials), then it is possible to calculate the grand potential by applying the thermodynamic integration scheme. The grand potential is shown to be multivalued for low (subcritical) temperature, and continuous for high (supercritical) temperature. A coexistence point is found within the hysteresis loop, actually close to the vertical desorption line. Below the equilibrium chemical potential, the gaslike branch is stable whereas the liquidlike branch is metastable. The situation is reversed above the coexistence point.     

Heterocyclic bioantioxidants. 2. Interaction of 3-nitro-4-substituted coumarins with mercaptans

3-Nitro-4-chlorocoumarin forms 3-nitro-4-substituted coumarins when it reacts with an equimolar quantity of benzyl mercaptan or thiosalicylic acid; with excess benzyl mercaptan, it forms 3,4-di-S-benzyl coumarin. 3-Nitro-4-methoxycoumarin under the same conditions, with an equimolar ratio of reagents, forms a mixture of monosubstituted and disubstituted products. A mechanism is proposed for these reactions.For Communication 1, see [1].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 11, pp. 1476–1479, November, 1991.     

A new,simple, and selective palladium-catalyzed cleavage of triethylsilyl ethers

[reaction: see text] A simple procedure for the cleavage of triethylsilyl (TES) ethers in the presence of 10 wt % Pd/C in methanol or 95% ethanol is reported. This method allows selective removal of alkyl TES ethers in the presence of aromatic TES ethers or tert-butyldimethylsilyl (TBS) protecting groups.     

Sodium diffusion through amorphous silica surfaces: a molecular dynamics study

We have studied the diffusion inside the silica network of sodium atoms initially located outside the surfaces of an amorphous silica film. We have focused our attention on structural and dynamical quantities, and we have found that the local environment of the sodium atoms is close to the local environment of the sodium atoms inside bulk sodo-silicate glasses obtained by quench. This is in agreement with recent experimental results.     

Chemical modification of the surface of a sulfonated membrane by formation of a sulfonamide bond

This paper describes a novel approach for the surface modification of a cation-exchange membrane, bearing sulfonate groups, by a cationic layer. The modification procedure involved the chlorosulfonation of the sulfonate groups of the base membrane with thionyl chloride, followed by a reaction with a diamine to yield a sulfonamide bond and a terminal amine. The latter could be quaternized by reaction with methyl iodide or protonated by soaking in acidic media. The membranes were characterized in detail by attenuated total reflectance Fourier transform infrared and X-ray photoelectron spectroscopies as well as elemental analysis to confirm that the above reactions occurred. The selectivity of these membranes toward the electrochemically assisted transport of protons versus Zn2+ metallic cations was determined during an electrodialysis in a two-compartment electrochemical cell. The data indicate a significant decrease of the transport of the metallic cations following modification of the membrane with the cationic layer. The later allows for the transport of protons from the catholyte to the anolyte compartment with much improved selectivity since the divalent cations are excluded from the membrane due to the electrostatic barrier of the cationic layer.     

Gravity-induced convective flow in microfluidic systems: electrochemical characterization and application to enzyme-linked immunosorbent assay tests

A way of using gravity flow to induce a linear convection within a microfluidic system is presented. It is shown and mathematically supported that tilting a 1 cm long covered microchannel is enough to generate flow rates up to 1000 nL.min(-1), which represents a linear velocity of 2.4 mm.s(-1). This paper also presents a method to monitor the microfluidic events occurring in a covered microchannel when a difference of pressure is applied to force a solution to flow in said covered microchannel, thanks to electrodes inserted in the microfluidic device. Gravity-induced flow monitored electrochemically is applied to the performance of a parallel-microchannel enzyme-linked immunosorbent assay (ELISA) of the thyroid-stimulating hormone (TSH) with electrochemical detection. A simple method for generating and monitoring fluid flows is described, which can, for instance, be used for controlling parallel assays in microsystems.