Recent advances with π-conjugated salen systems

Schiff base ligands have long been successfully employed as ligands in combination with various metals to give catalysts capable of realizing a variety of synthetic transformations. One of the most widely used Schiff base ligands, the "salen" ligand, has been extensively researched. Recently, there has been increased interest in π-conjugated salen systems, known as "salphen" ligands, as a result of the differences in reactivity of the complexes in catalytic applications compared with the salen analogues. Complexes of salphen ligands display interesting photophysical and supramolecular properties which are not always observed with salen systems as a result of their π-conjugation. This tutorial review therefore describes the most significant advances recently made with salphen and related π-conjugated ligand systems.     

Versatile Switching in Substrate Topicity: Supramolecular Chirality Induction in Di‐ and Trinuclear Host Complexes

Supramolecular chirality effects have been achieved both for ditopic and monotopic substrates by using a programmable bis‐salphen scaffold that incorporates either two or three Zn nuclei. The dinuclear host shows preferential chirogenesis in the presence of ditopic systems, whereas effective chirality transfer to the trinuclear complex is realized through monotopic binding. The mode of binding in the trinuclear host has been investigated through X‐ray crystallography, CD measurements, UV/Vis spectroscopy, and DFT analysis. The bis‐salphen scaffold holds promise for the development of substrate‐specific host systems useful for determination of the absolute configuration of various types of organic molecules.     

An efficient synthesis of tetrasubstituted cyclohexyl-1,2-diamines

The synthesis of symmetrically tetrasubstituted diamines derived from the (R,R)-cyclohexyl-1,2-diamine is reported. We comment on the efficiency of this sequential substitution approach as a method for the synthesis of tetrasubstituted diamines and discuss the acceptable substitution pattern for this methodology.     

Surface functionalization of silicon oxide at room temperature and atmospheric pressure

A novel method to derivatize silicon surfaces with 3-mercaptopropylsilane molecules has been developed and optimized. This method is based on an argon flow that increases the evaporation rate of the silane molecules by lowering the partial pressure of the silane molecules in gas phase above the liquid silane, at room temperature. X-ray photoelectron spectroscopy studies of the surfaces showed a dense monolayer coverage as well as hydrolysis of the silane methoxy groups. Atomic force microscopy was used to investigate the roughness of the surfaces after each step of the derivatization process. Since the final surface has a measured surface roughness of 0.19 nm, this method will be especially useful for further synthetic routes and advanced single molecule detection studies of interactions on surfaces as well as improvement of existing conventional techniques for surface derivatization and analysis.     

Electrocatalysis of a Europium-Dependent Bacterial Methanol Dehydrogenase with Its Physiological Electron-Acceptor Cytochrome cGJ

We report the first electrochemical study of a lanthanoid-dependent methanol dehydrogenase (Eu-MDH) from the acidophilic verrucomicrobial methanotroph Methylacidiphilum fumariolicum SolV with its own physiological cytochrome c_GJ electron acceptor. Eu-MDH harbours a redox active 2,7,9-tricarboxypyrroloquinoline quinone (PQQ) cofactor which is non-covalently bound but coordinates trivalent lanthanoid elements including Eu³⁺. Eu-MDH and the cytochrome were co-adsorbed with the biopolymer chitosan and cast onto a mercaptoundecanol (MU) monolayer modified Au working electrode. Cyclic voltammetry of cytochrome c_GJ reveals a well-defined quasi-reversible Fe^III/II redox couple at +255 mV vs. NHE at pH 7.5 and this response is pH independent. The reversible one-electron response of the cytochrome c_GJ transforms into a sigmoidal catalytic wave in the presence of Eu-MDH and its substrates (methanol or formaldehyde). The catalytic current was pH-dependent and pH 7.3 was found to be optimal. Kinetic parameters (pH dependence, activation energy) obtained by electrochemistry show the same trends as those obtained from an artificial phenazine ethosulfate/dichlorophenol indophenol assay.     

5‐(Pyrrolidin‐2‐yl)tetrazole‐Catalyzed Aldol and Mannich Reactions: Acceleration and Lower Catalyst Loading in a Continuous‐Flow Reactor

Continuous organocatalysis : Fast aldol and Mannich reactions require less catalyst when conducted in a microreactor. A proline tetrazole derivative (5–10 mol %) catalyzes asymmetric aldol reactions between various aromatic aldehydes and ketones in microreactor at 60 °C with reaction times ranging from 10 to 30 min.

     

Piezoresistive cantilever based nanoflow and viscosity sensor for microchannels

Microfluidic channels are microreactors with a wide range of applications, including molecular separations based upon micro/nanoscale physicochemical properties, targeting and delivery of small amount of fluids and molecules, and patterned/directed growth. Their successful applications would require a detailed understanding of phenomena associated with the microscale flow of liquids through these channels, including velocity, viscosity and miscibility. Here we demonstrate a highly sensitive piezoresistive cantilever to measure flow properties in microfluidic channels. By milling down the legs of the piezoresistive cantilevers, we have achieved significantly higher mechanical sensitivity and a smaller spring constant, as determined by AFM. These cantilevers were used in microchannels to measure the viscosity and flow rate of ethylene glycol mixtures in water over a range of concentrations, as well as of low viscosity biologically relevant buffers with different serum levels. The sensor can be used alone or can be integrated in AFM systems for multidimensional study in micro and nanochannels.     

Determination of plant sterol oxidation products in plant sterol enriched spreads, fat blends, and plant sterol concentrates

Plant sterols (PS) are very stable molecules but may undergo oxidation due to the presence of a double bond in the ring structure. In order to assess whether this occurs during heating and storage, an analytical procedure was developed for the determination of concentration levels and identity of PS oxidation products in functional food ingredients and products. The method is based on cold saponification, solvent extraction of unsaponifiables, isolation of sterol oxidation products by means of liquid chromatography, and final analysis by gas chromatography (GC) with flame ionization detection. Identification of the key PS oxidation products was performed by means of GC-mass spectrometry (GC-MS). Isotope dilution MS was used to verify the absence of the formation of potential artifacts by the method. The method described is applicable to spreads (containing 20-65% water), oils, sterol esters, pure sterols, and fat extracts from food. The between-day reproducibility of the total content of sterol oxidation products in control samples sample was 8%, and of individual sterol oxidation products, 6-15%. The recovery of sterol oxidation products was 91%. The limit of detection was 0.1 mg/kg.     

De Novo Left‐Handed Synthetic Peptidomimetic Foldamers

The development of peptidomimetic helical foldamers with a wide repertoire of functions is of significant interest. Herein, we report the X‐ray crystal structures of a series of homogeneous l ‐sulfono‐γ‐AA foldamers and elucidate their folding conformation at the atomic level. Single‐crystal X‐ray crystallography revealed that this class of oligomers fold into unprecedented dragon‐boat‐shaped and unexpected left‐handed helices, which are stabilized by the 14‐hydrogen‐bonding pattern present in all sequences. These l ‐sulfono‐γ‐AApeptides have a helical pitch of 5.1 Å and exactly four side chains per turn, and the side chains lie perfectly on top of each other along the helical axis. 2D NMR spectroscopy, computational simulations, and CD studies support the folding conformation in solution. Our results provide a structural basis at the atomic level for the design of novel biomimetics with a precise arrangement of functional groups in three dimensions.