Synthesis of a new and versatile macrocyclic NADH model

A new macrocyclic NADH model 1 has been designed and synthesised. The new model consists of the same subunits as previously reported models. However, the present model is designed as such that the pyridine nitrogen of the nicotinamide units are not incorporated in the macrocyclic framework. Thus, properties such as solubility can easily be varied by alkylation with an appropriate agent. The macrocyclic framework was prepared in 7 steps. Methylation of the pyridine nitrogens followed by reduction gave the desired model. This model compound was found to reduce methyl benzoylformate stereoselectively in good yield with 48% enantiomeric excess.     

Detection of adsorption of Ru(II) and Os(II) polypyridyl complexes on gold and silver nanoparticles by single-photon counting emission measurements

The present study describes a new application of ruthenium(II) tris(bipyridine) (Ru(bpy)3(2+)) and osmium(II) tris(bipyridine) (Os(bpy)3(2+)) as phosphorescent labels for the quantification of surface binding of molecules to gold and silver nanoparticles. The fraction of Ru(bpy)3(2+) and Os(bpy)3(2+) that is in solution can be distinguished from the surface-bound fraction by the relative lifetimes and integrated emission yields as determined by time-correlated single-photon counting (TCSPC) spectroscopy. Complementary steady-state measurements were carried out to confirm surface attachment of the phosphorescent label molecules. Although the emission of solutions of Ru(bpy)3(2+) and Os(bpy)3(2+) is quenched proportional to the concentration of 10 nm Au or 20 nm Ag nanoparticles, the quenching is static and not diffusional quenching observed in Stern-Volmer plots. The results demonstrate that time-resolved spectroscopy provides a rapid method for the measurement of surface binding of labeled molecules on metallic nanoparticles. While steady-state measurements require the preparation of a series of samples with varying quencher concentrations and a reference, the method described herein requires a single sample plus reference. The mechanism for phosphorescence quenching on Au and Ag nanoparticles is discussed in terms of energy and electron transfer theories.     

Preparation and crystal structure of Nb6.74 Ta5.26S4 – a new compound in the ternary system Ta?Nb?S

Nb_6.74Ta_5.26S₄ has been prepared by high temperature techniques. The crystal structure has been determined from single crystal X-ray diffraction data (R/R_w = 0.0588/0.0655). The compound crystallizes in the orthorhombic space group Pnma with unit cell dimensions a = 959.11 (26) pm, b = 336.37 (10) pm, and c = 3282.51 (74) pm. The orthorhombic cell contains four formula units. Its structure is similar to that of Nb-rich sulfides, rather than to that of Ta-rich sulfides. The metal coordinations are capped distorted cubic prisms and pentagonal prisms while the coordinations of sulfur are capped trigonal prisms.     

Covalent attachment of a nickel nitrilotriacetic acid group to a germanium attenuated total reflectance element

The surface of a germanium internal reflectance element (IRE) was modified to bind 6X-histidine (his)-tagged biomolecules. The step-by-step surface modification was monitored via single-pass attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FT-IR). Initially an adlayer of 7-octenyltrimethoxysilane (7-OTMS) was formed on the Ge crystal through the surface hydroxyl groups, which were produced via ozonolysis of the Ge surface. The vinyl moiety of 7-OTMS was oxidized to a carboxylic acid, which was activated by 1,1'-carbonydiimidazole (CDI) to produce a labile imidazole. The labile imidazole that resulted from the CDI coupling was then displaced by the primary amine of nitrilotriacetic acid (NTA). Nickel sulfate was added to the system, and it coordinated with the three carbonyl groups and the nitrogen on NTA, thus leaving the ability of Ni to coordinate with two adjacent histidine residues. Binding of his-tagged biotin to nickel nitrilotriacetic acid (Ni-NTA) was observed by ATR-FT-IR spectroscopy. The surface modification method presented in this paper had minimal nonspecific binding, the Ni-NTA surface was reusable if stored properly, and complete removal of the organic surface was achievable.     

Electrochemically assisted Fenton reaction: reaction of hydroxyl radicals with xenobiotics followed by on-line analysis with high-performance liquid chromatography/tandem mass spectrometry

Oxygen radicals are generated in vivo by various processes, often as toxic intermediates in different metabolic transformations, and have been shown to play an important role for a large number of diseases. In this article we introduce an electrochemical flow-through system that allows generation of hydroxyl radicals for reaction with xenobiotics and subsequent detection of the oxidation products on-line with high-performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS). The system is based on the Fenton reaction and is predominantly aimed at the generation of hydroxyl radicals; however, by minor variations to the system, a broad range of other radicals can be produced. Optimization of the system was performed with the radical scavenger 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). Under the same physical conditions, one injection through the electrochemical cell gave a higher yield of the oxidation product N-hydroxy-5,5-dimethylpyrrolidin-2-one than what was attained after 60 min with a chemical Fenton system catalyzed by ascorbic acid. Since the iron is added as Fe(3+), the initial mixture is 'inactive' until it reaches the electrochemical cell. This makes it very suitable for on-line analysis of the generated compounds, since the whole reaction mixture, including substrate, can be kept in a vial in an autosampler. The system described provides a useful tool for investigation of new radical scavengers and antioxidants. Since the hydroxyl radical adds readily to unsaturated pi-systems, the technique is also suitable for on-line generation and characterization of potential drug metabolites resulting from hydroxylation of double bonds and aromatic systems.     

Synthesis of homoceramides, novel ceramide analogues, and their lack of effect on the growth of hippocampal neurons

The synthesis of a new series of D-erythro-homoceramide analogues is described. Several synthetic approaches were investigated. Homoceramides can be successfully synthesized from L-homoserine as chiral building block and a protected Weinreb-amide as a key intermediate. The synthesis of short-chain analogues with a heptyl side chain, as well as with a phenyl residue in the sphingoid part (instead of the naturally occurring tridecyl side chain), was effected. The homoceramides 15-17 and 24 were investigated for their potential to reverse the inhibitory effect of fumonisin B(1) on axonal growth. Unfortunately, none of the tested compounds showed any biological activity due to their lack of metabolism to glucosylhomoceramide.     

Defect ordering in early transition-metal sulfides with the NaCl-type structure

The defect rock-salt type materials Zr_1?xS, Sc_1?xS, and Lu_1?xS have been shown to exhibit a complex variety of ordered superstructures. These superstructures are reviewed within the context of Landau's theory of symmetry and phase transitions, and it is concluded that there is a significant similarity in the transitions when they are considered from the point of view of reciprocal space.     

Synthesis of a versatile (S)-3-(hydroxymethyl)butane-1,2,4-triol building block and its application for the stereoselective synthesis of N-homoceramides

[structures: see text] A versatile (S)-3-(hydroxymethyl)butane-1,2,4-triol building block has been synthesized starting from D-isoascorbic acid, a common food preservative. The key transformation in this approach was the introduction of branching through a high yield and fully regioselective epoxide opening. This flexible synthon has been elaborated to a new class of (dihydro-)N-homo(phyto)ceramides.