The MPEG effect: improving asymmetric processes by simple additives

Small amounts of simple methoxy poly(ethylene glycol)s (MPEGs) have a beneficial effect on catalyzed asymmetric aryl and alkyl transfer reactions onto aldehydes. The enantiomeric excesses of the products are improved, and this "MPEG effect" allows a reduction of the catalyst loading by a factor of 10.     

Olefins as steering ligands for homogeneously catalyzed hydrogenations

Iridium(I) complexes containing a (5H-dibenzo[a,d]cyclohepten-5-yl)-phosphane (tropp(R); R = phosphorus-bound substituent = Ph, Cyc) as a rigid, concave-shaped, mixed phosphane olefin ligand were prepared and tested as catalyst precursors in the hydrogenation of imines. With the complex [Ir(tropp(Cyc))(cod)]OTf, turnover frequencies (TOFs) of >6000 h(-1) were reached in the hydrogenation of N-phenyl-benzylidenamine, PhN==CHPh. Lower activities (TOF>80 h(-1)) are observed with N-phenyl-(1-phenylethylidene)amine, PhN==CMePh. Chiral tropp-type ligands were prepared in few simple steps. Monosubstitution of the olefinic unit in the dibenzo[a,d]cycloheptenyl moiety with (R)- or (S)-mentholate gave mixtures of diastereomers that could be separated and isolated in enantiomerically pure form. Iridium(I) complexes with these ligands are rare examples of side-on bonded enolether complexes. In catalytic imine hydrogenations, complete conversion (>98 %) was reached in all cases (conditions: p[H(2)] = 50 bar, T = 50 degrees C, t = 2 h, substrate/catalyst 100:1). The best enantiomeric excess (ee = 86 % S isomer) was reached with PhN==CMePh as substrate and the R,R form of the (10-menthyloxy-5H-dibenzo[a,d]cyclohepten-5-yl)diphenylphosphane ligand. The iridium(I) complex containing the same phosphane gave a 60 % ee (S isomer) with the enamide N-(1-phenylvinyl)acetamide as substrate (conditions: p[H(2)] = 4 bar, T = 50 degrees C, t = 18 h, substrate/catalyst = 50:1). These reactions constitute the first examples in which chiral olefins have been used as steering ligands in catalytic enantioselective hydrogenations.     

Aggregation of bead-monolayers in flat microfluidic chambers - simulation by the model of porous media

In this paper, we for the first time simulate the process of hydrodynamic bead aggregation in a flat micro-fluidic chamber by a porous-media model in an iterative routine. This allows us to optimize the chamber design of our recently developed experimental method to form periodical monolayers from the flow of bead suspension. Periodical monolayers are advantageous for parallel assay formats since they enhance the mechanical rigidity of the aggregated pattern. This is important to avoid a spatial rearrangement along various steps of a read-out procedure which would impair the correlation between measurements. Furthermore, the monolayer formation guarantees the individual optical accessibility of all probe beads. By modelling the monolayers with porous media, we can drastically reduce the degrees of freedom in a two-phase, multi-particle problem. This way, we are able to compute stationary hydrodynamic flow patterns in the chamber. In order to simulate the complete filling process from these stationary solutions, we developed an iterative master routine which takes the transient aggregation pattern as the initial condition, then evaluates the placement of the newly introduced beads, and finally converts the points of aggregation into porous media.     

Chemical activity of the nitrogenase FeMo cofactor with a central nitrogen ligand: density functional study

We investigate the chemical consequences of a central ligand in the nitrogenase FeMo cofactor using density functional calculations. Several studies have shown that the central ligand most probably is a nitrogen atom, but the consequences for the chemical reactivity of the cofactor are unknown. We investigate several possible routes for insertion of the central nitrogen ligand and conclude that all routes involve barriers and intermediate states, which are inaccessible at ambient conditions. On this basis we suggest that the central nitrogen ligand is present at all times during the reaction. Furthermore, we investigate how the FeMoco with the central ligand can interact with N(2) and reduce it.     

Copper(II)-quenched oligonucleotide probes for fluorescent DNA sensing

A copper(II)-quenched molecular beacon was prepared by attaching fluorescein to the 3'-end and a copper(II) complex to the 5'-end of DNA. In the presence of complementary DNA, copper(II) and dye are spatially separated in the duplex and fluorescence increases up to 15-fold, with excellent discrimination of single base mismatches.     

Complexation of polyacrylates by Ca2+ ions. Time-resolved studies using attenuated total reflectance Fourier transform infrared dialysis spectroscopy

The attenuated total reflectance Fourier transform infrared dialysis technique is introduced for the time-resolved investigation of the binding processes of Ca2+ to polyacrylates dissolved in water. We observed transient formation of intermediates in water with various types of coordination of the carboxylate group to Ca2+ throughout the complexation steps. Time-resolved changes in the spectra were analyzed with principal component analysis, from which the spectral species were obtained as well as their formation kinetics. We propose a model for the mechanisms of Ca2+ coordination to polyacrylates. The polymer chain length plays an important role in Ca2+ binding.     

Colon cancer risk factors from nutrition

Nutrition and health are closely connected and malnutrition can seriously endanger health. The consequences are higher risks of developing diseases. Of these, cancers are of special importance. The most frequent nutrition-associated type of cancer is colon cancer. This review summarises the predisposing factors for the development of colon cancer, and molecular mechanisms responsible for sporadic colon cancer. Moreover, it is pointed out that individual genetic predisposition such as polymorphisms of biotransformation systems might affect susceptibility to cancer risk factors. Selected findings of epidemiological research and nutritional habits, foods, and metabolites that could increase cancer risk are discussed. Furthermore, toxicological assessment of food-related risk factors, e.g. heterocyclic amines, and potential protective effects of food ingredients, e.g. the induction of phase II enzymes or removal of already degenerated cells from tissue by apoptosis, inhibition of proliferation, and cell differentiation are discussed. Risks are not necessarily reduced by avoiding certain substances but by healthy and well-balanced nutrition. Knowledge of individual susceptibility will also make it possible to recognise risks and design more specific nutritional recommendations.