Experiment and theory of low-pressure nitrogen adsorption in organic layers supported or grafted on inorganic adsorbents: toward a tool to characterize surfaces of hybrid organic/inorganic systems

We report experimental nitrogen adsorption isotherms of organics-coated silicas, which exhibit a low-pressure desorption branch that does not meet the adsorption branch upon emptying of the pores. To address the physical origin of such a hysteresis loop, we propose an equilibrium thermodynamic model that enables one to explain this phenomenon. The present model assumes that, upon adsorption, a small amount of nitrogen molecules penetrate within the organic layer and reach adsorption sites that are located on the inorganic surface, between the grafted or adsorbed organic molecules. The number of accessible adsorption sites thus varies with the increasing gas pressure, and then we assume that it stays constant upon desorption. Comparison with experimental data shows that our model captures the features of nitrogen adsorption on such hybrid organic/inorganic materials. In particular, in addition to predicting the shape of the adsorption isotherm, the model is able to estimate, with a reasonable number of adjustable parameters, the height of the low-pressure hysteresis loop and to assess in a qualitative fashion the local density of the organic chains at the surface of the material.     

Analytical development for analysis of pharmaceuticals in water samples by SPE and GC–MS

An analytical procedure involving solid-phase extraction (SPE) and gas chromatography–mass spectrometry (GC–MS) has been developed for determination of pharmaceutical compounds (aspirin, caffeine, carbamazepine, diclofenac, ketoprofen, naproxen, ibuprofen, clofibrate, clofibric acid, and gemfibrozil) in a variety of aqueous samples (wastewater and surface water). After filtration, samples were extracted and concentrated using C₁₈ or HLB cartridges, depending on the type of compound. Sample storage conditions were checked and optimized to ensure preservation of the pharmaceutical substance, taking into consideration environmental sampling conditions. For most of the pharmaceuticals monitored, recovery was in the range 53 to 99% and the variability was below 15% for the complete procedure, with limits of detection ranging from 0.4 to 2.5 ng L⁻¹, depending on the compound. The methods were successfully applied to monitoring of pharmaceutical contamination of the Seine estuary. Concentrations varied from several dozens of nanograms per liter for surface waters to several hundreds of nanograms per liter for wastewaters.     

Synthesis and characterization of active ester-functionalized polypyrrole-silica nanoparticles: application to the covalent attachment of proteins

Novel ester-functionalized polypyrrole-silica nanocomposite particles were prepared by oxidative copolymerization of pyrrole and N-succinimidyl ester pyrrole (50/50% initial concentrations), using FeCl3 in the presence of ultrafine silica nanoparticles (20 nm diameter). The N-succinimidyl ester pyrrole monomer was prepared in aqueous solution using 1-(2-carboxyethylpyrrole) and N-hydroxysuccinimide in the presence of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide. The resulting nanocomposites (N-succinimidyl ester polypyrrole-silica) are raspberry-shaped agglomerates of silica sol particles "glued" together by the insoluble poly(pyrrole-co-N-succinimidyl pyrrole). The N-succinimidyl ester polypyrrole-silica particles were characterized in terms of their size, density, copolymer content, and polydispersity. Scanning electron microscopy and disk centrifuge sedimentometry confirmed that the nanocomposite particles had narrow size distributions. X-ray photoelectron spectroscopy analysis indicated a silica-rich surface and a high surface concentration of N-succinimidyl ester groups. These nanoparticles exhibited good long-term dispersion stability. The chemical stability of the ester functions in aqueous media after several weeks of storage was monitored by FTIR spectroscopy. The functionalized nanocomposites were tested as bioadsorbents of human serum albumin (HSA). The very high amount of immobilized HSA determined by UV-visible spectroscopy is believed to be due to covalent binding. Incubation of the HSA-grafted nanocomposite with anti-HSA resulted in immediate flocculation, an indication that they are alternative candidates for visual diagnostic assays.     

Hexaphyrin(1.0.1.0.0.0). A new colorimetric actinide sensor

Hexaphyrin(1.0.1.0.0.0) (isoamethyrin) undergoes a significant color change in the presence of UO₂²⁺, PuO₂²⁺, and NpO₂²⁺. The complexation of the first of these dioxo actinide cations was studied in semi-quantitative fashion in 1:1 MeOH-CH₂Cl₂. Under these conditions, the detection limit for UO₂²⁺ was found to be ca. 5.8 ppm by naked eye monitoring and <28 ppb by UV-vis spectroscopy. Isoamethyrin does not undergo a color change in the presence of most transition metals or when exposed to Gd(III). Isoamethyrin thus constitutes an attractive alternative to 2,2′(1,8-dihydroxy-3,6-disulfonaphthylene-2,7-bisazo)-bisbenzenarsonic acid (AzIII) and 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol (BrPADAP), systems currently used as actinide cation sensors.     

Phosphinoborane and Sulfidoborohydride as Chelating Ligands in Polyhydride Ruthenium Complexes: Agostic σ‐Borane versus Dihydroborate Coordination

A question of coordination mode : Two new borane compounds are prepared. They act as bifunctional ligands as illustrated by their reaction with ruthenium polyhydrides which leads to the formation of two complexes (see scheme) displaying either a δ‐agostic interaction of a η²‐B? H bond involving a trivalent boron atom or a dihydroborate ligation.

     

“Half‐Bonds” in an Unusual Coordinated S42− Rectangle

Don′t be square! A rare S₄^2? rectangle bridging two M₂Cp₂(μ₂‐CH₂)₂ (M=Rh, Ir) fragments is found to contain two “half‐bonds” with S? S distances of 2.70 or 2.90 Å. Computational studies explore the connection between these “half‐bonds” and a Jahn–Teller distortion, as well as possible intermediates that form M₄S₄²⁺ clusters having the S₄^2? rectangle rotated by 90°.

     

Cover Picture: Photoprogrammable Organic Light‐Emitting Diodes (Angew. Chem. Int. Ed. 22/2009)

Programmable organic light‐emitting diodes: UV irradiation converts an oxetane‐functionalized dithienylethene derivative from its colorless open form into a dark blue closed form, while visible light reverses the reaction. In the Communication on page 4038 ff., K. Meerholz and co‐workers describe a reversibly switchable organic light‐emitting diode based on this principle.

     

Oligonucleotide Analogues with Integrated Bases and Backbone. Part 22

The tritylated and silylated self‐complementary A*[s]U*[s]A*[s]U* and U*[s]A*[s]U*[s]A* tetramers 18 and 24 , linked by thiomethylene groups (abbreviated as [s]) between a nucleobase and C(5′) of the neighbouring nucleoside unit were prepared by a linear synthesis based on S‐alkylation of 5′‐thionucleosides by 6‐(chloromethyl)uridines, 7 or 10 , or 8‐(chloromethyl)adenosines, 12 or 15 . The tetramers 18 and 24 were detritylated to the monoalcohols 19 and 25 , and these were desilylated to the diols 20 and 26 , respectively. The association of the tetramers 18 – 21 and 24 – 26 in CDCl₃ or in CDCl₃/(D₆)DMSO 95 : 5 was investigated by the concentration dependence of the chemical shifts for H? N(3) or H₂N? C(6). The formation of cyclic duplexes connected by four base pairs is favoured by the presence of one and especially of two OH groups. The diol 20 with the AUAU sequence prefers reverse‐Hoogsteen, and diol 26 with the UAUA sequence Watson–Crick base pairing. The structure of the cyclic duplex of 26 in CDCl₃ at 2° was derived by a combination of AMBER* modeling and simulated annealing with NMR‐derived distance and torsion‐angle restraints resulting in a Watson–Crick base‐paired right‐handed antiparallel helix showing large roll angles, especially between the centre base pairs, leading to a bent helix axis.     

Factors that regulate the conformation of m-benziporphodimethene complexes: agostic metal-arene interaction, hydrogen bonding, and η2,π coordination

Group 12 and silver(I) tetramethyl‐m‐benziporphodimethene (TMBPDM) complexes with phenyl, methylbenzoate, or nitrophenyl groups as meso substituents were synthesized and fully characterized. The dimeric silver(I) complex displays an unusual η²,π coordination from the β‐pyrrolic C?C bond to the silver ion. All of the complexes displayed a close contact between the metal ion and the inner C(22)? H(22) on the m‐phenylene ring. The downfield chemical shifts of H(22) and large coupling constants between Cd^II and H(22) strongly support the presence of an agostic interaction between the metal ion and inner C(22)–H(22). Crystal structures revealed that the syn form is the predominant conformation for TMBPDM complexes. This is distinctively different from the exclusive anti conformation observed in m‐benziporphyrin and tetraphenyl‐m‐benziporphodimethene (TPBPDM) complexes. Evidently, intramolecular hydrogen‐bonding interactions between axial chloride and methyl groups stabilize syn conformations. Unlike the merely syn conformation observed in the solid‐state structures of TMBPDM complexes that contain an axial chloride, in solution these complexes display highly solvent‐ and temperature‐dependent syn/anti ratio changes. The observation of dynamic ¹H NMR spectroscopic scrambling between syn and anti conformations from the titration of chloride ion into the solution of the TMBPDM complex suggests that axial ligand exchange is a likely pathway for the conversion between syn and anti forms. Theoretical calculations revealed that intermolecular hydrogen‐bonding interactions between the axial chloride and CHCl₃ stabilizes the anti conformation, which explains the increased ratio for the anti form when dichloromethane or chloroform was used as the solvent.     

The [NiFe]-hydrogenase of the cyanobacterium Synechocystis sp. PCC 6803 works bidirectionally with a bias to H2 production

Protein film electrochemistry (PFE) was utilized to characterize the catalytic activity and oxidative inactivation of a bidirectional [NiFe]-hydrogenase (HoxEFUYH) from the cyanobacterium Synechocystis sp. PCC 6803. PFE provides precise control of the redox potential of the adsorbed enzyme so that its activity can be monitored under changing experimental conditions as current. The properties of HoxEFUYH are different from those of both the standard uptake and the "oxygen-tolerant" [NiFe]-hydrogenases. First, HoxEFUYH is biased toward proton reduction as opposed to hydrogen oxidation. Second, despite being expressed under aerobic conditions in vivo, HoxEFUYH is clearly not oxygen-tolerant. Aerobic inactivation of catalytic hydrogen oxidation by HoxEFUYH is total and nearly instantaneous, producing two inactive states. However, unlike the Ni-A and Ni-B inactive states of standard [NiFe]-hydrogenases, both of these states are quickly (<90 s) reactivated by removal of oxygen and exposure to reducing conditions. Third, proton reduction continues at 25-50% of the maximal rate in the presence of 1% oxygen. Whereas most previously characterized [NiFe]-hydrogenases seem to be preferential hydrogen oxidizing catalysts, the cyanobacterial enzyme works effectively in both directions. This unusual catalytic bias as well as the ability to be quickly reactivated may be essential to fulfilling the physiological role in cyanobacteria, organisms expected to experience swings in cellular reduction potential as they switch between aerobic conditions in the light and dark anaerobic conditions. Our results suggest that the uptake [NiFe]-hydrogenases alone are not representative of the catalytic diversity of [NiFe]-hydrogenases, and the bidirectional heteromultimeric enzymes may serve as valuable models to understand the diverse mechanisms of tuning the reactivity of the hydrogen activating site.