Derivatizing agents from phosphorus trichloride and terpenyl tartrates for determining the enantiomeric purity of alcohols

Heterocyclic phosphorous acid chlorides, prepared from C₂-symmetric menthyl, borneyl, or fenchyl tartrates and phosphorus trichloride, are inexpensive derivatizing agents for determining the enantiomeric purity of alcohols via phosphorus NMR. The most versatile agent identified from this study, a (1R,2S,5R)-menthyl (R,R)-tartrate-derived 2-chloro-1,3,2-dioxaphospholane, gives, upon esterification with chiral alcohols, diastereomeric phosphites showing phosphorus NMR-shift dispersions between 0.1 ppm and 1.5 ppm.     

Conserved Tyr223(5.58) plays different roles in the activation and G-protein interaction of rhodopsin

Rhodopsin, a seven transmembrane helix (TM) receptor, binds its ligand 11-cis-retinal via a protonated Schiff base. Coupling to the G-protein transducin (G(t)) occurs after light-induced cis/trans-retinal isomerization, which leads through photoproducts into a sequence of metarhodopsin (Meta) states: Meta I ? Meta IIa ? Meta IIb ? Meta IIbH(+). The structural changes behind this three-step activation scheme are mediated by microswitch domains consisting of conserved amino acids. Here we focus on Tyr223(5.58) as part of the Y(5.58)X(7)K(R)(5.66) motif. Mutation to Ala, Phe, or Glu results in specific impairments of G(t)-activation measured by intrinsic G(t) fluorescence. UV-vis/FTIR spectroscopy of rhodopsin and its complex with a C-terminal G(t)α peptide allows the assignment of these deficiencies to specific steps in the activation path. Effects of mutation occur already in Meta I but do not directly influence deprotonation of the Schiff base during formation of Meta IIa. Absence of the whole phenol ring (Y223A) allows the activating motion of TM6 in Meta IIb but impairs the coupling to G(t). When only the hydroxyl group is lacking (Y223F), Meta IIb does not accumulate, but the activity toward G(t) remains substantial. From the FTIR features of Meta IIbH(+) we conclude that proton uptake to Glu134(3.49) is mandatory for Tyr223(5.58) to engage in the interaction with the key player Arg135(3.50) predicted by X-ray analysis. This polar interaction is partially recovered in Y223E, explaining its relatively high activity. Only the phenol side chain of tyrosine provides all characteristics for accumulation of the active state and G-protein activation.     

Odd-even effects in charge transport across self-assembled monolayers

This paper compares charge transport across self-assembled monolayers (SAMs) of n-alkanethiols containing odd and even numbers of methylenes. Ultraflat template-stripped silver (Ag(TS)) surfaces support the SAMs, while top electrodes of eutectic gallium-indium (EGaIn) contact the SAMs to form metal/SAM//oxide/EGaIn junctions. The EGaIn spontaneously reacts with ambient oxygen to form a thin (～1 nm) oxide layer. This oxide layer enables EGaIn to maintain a stable, conical shape (convenient for forming microcontacts to SAMs) while retaining the ability to deform and flow upon contacting a hard surface. Conical electrodes of EGaIn conform (at least partially) to SAMs and generate high yields of working junctions. Ga(2)O(3)/EGaIn top electrodes enable the collection of statistically significant numbers of data in convenient periods of time. The observed difference in charge transport between n-alkanethiols with odd and even numbers of methylenes--the "odd-even effect"--is statistically discernible using these junctions and demonstrates that this technique is sensitive to small differences in the structure and properties of the SAM. Alkanethiols with an even number of methylenes exhibit the expected exponential decrease in current density, J, with increasing chain length, as do alkanethiols with an odd number of methylenes. This trend disappears, however, when the two data sets are analyzed together: alkanethiols with an even number of methylenes typically show higher J than homologous alkanethiols with an odd number of methylenes. The precision of the present measurements and the statistical power of the present analysis are only sufficient to identify, with statistical confidence, the difference between an odd and even number of methylenes with respect to J, but not with respect to the tunneling decay constant, β, or the pre-exponential factor, J(0). This paper includes a discussion of the possible origins of the odd-even effect but does not endorse a single explanation.     

Mechanistic investigation of the Ru-catalyzed hydroamidation of terminal alkynes

The ruthenium-catalyzed hydroamidation of terminal alkynes has evolved to become a broadly applicable tool for the synthesis of enamides and enimides. Depending on the catalyst system employed, the reaction leads chemo-, regio-, and stereoselectively to a single diastereoisomer. Herein, we present a comprehensive mechanistic study of the ruthenium-catalyzed hydroamidation of terminal alkynes, which includes deuterium-labeling, in situ IR, in situ NMR, and in situ ESI-MS experiments complemented by computational studies. The results support the involvement of ruthenium-hydride and ruthenium-vinylidene species as the key intermediates. They are best explained by a reaction pathway that consists of an oxidative addition of the amide, followed by insertion of a π-coordinated alkyne into a ruthenium-hydride bond, rearrangement to a vinylidene species, nucleophilic attack of the amide, and finally reductive elimination of the product.     

Impact of molecular flexibility on binding strength and self-sorting of chiral π-surfaces

In this work, we have explored for the first time the influence of conformational flexibility of π-core on chiral self-sorting properties of perylene bisimides (PBIs) that are currently one of the most prominent classes of functional dyes. For this purpose, two series of chiral macrocyclic PBIs 3a-c and 4a-c comprising oligoethylene glycol bridges of different lengths at the 1,7 bay positions were synthesized and their atropo-enantiomers (P and M enantiomers) were resolved. Single crystal analysis of atropo-enantiomerically pure (P)-3a not only confirmed the structural integrity of the ethylene glycol bridged macrocycle but also illustrated the formation of π-stacked dimers with left-handed supramolecular helicity. Our detailed studies with the series of highly soluble chiral PBIs 4a-c by 1- and 2-D (1)H NMR techniques, and temperature- and concentration-dependent UV/vis absorption and circular dichroism (CD) spectroscopy revealed that in π-π-stacking dimerization of these PBIs chiral self-recognition (i.e., PP and MM homodimer formation) prevails over self-discrimination (i.e., PM heterodimer formation). Our studies clearly showed that with increasing conformational flexibility of PBI cores imparted by longer bridging units, the binding strength for the dimerization process increases, however, the efficiency for chiral self-recognition decreases. These results are rationalized in terms of an induced-fit mechanism facilitating more planarized π-scaffolds of PBIs containing longer bridging units upon π-π-stacking.     

General and selective iron-catalyzed transfer hydrogenation of nitroarenes without base

The first well-defined iron-based catalyst system for the reduction of nitroarenes to anilines has been developed applying formic acid as reducing agent. A broad range of substrates including other reducible functional groups were converted to the corresponding anilines in good to excellent yields at mild conditions. Notably, the process constitutes a rare example of base-free transfer hydrogenations.     

Formation of a mesoscopic skin barrier in mesoglobules of thermoresponsive polymers

With the combination of molecular scale information from electron paramagnetic resonance (EPR) spectroscopy and meso-/macroscopic information from various other characterization techniques, the formation of mesoglobules of thermoresponsive dendronized polymers is explained. Apparent differences in the EPR spectra in dependence of the heating rate, the chemical nature of the dendritic substructure of the polymer, and the concentration are interpreted to be caused by the formation of a dense polymeric layer at the periphery of the mesoglobule. This skin barrier is formed in a narrow temperature range of ~4 K above T(C) and prohibits the release of molecules that are incorporated in the polymer aggregate. In large mesoglobules, formed at low heating rates and at high polymer concentrations, a considerable amount of water is entrapped that microphase-separates from the collapsed polymer chains at high temperatures. This results in the aggregates possessing an aqueous core and a corona consisting of collapsed polymer chains. A fast heating rate, a low polymer concentration, and hydrophobic subunits in the dendritic polymer side chains make the entrapment of water less favorable and lead to a higher degree of vitrification. This may bear consequences for the design and use of thermoresponsive polymeric systems in the fast growing field of drug delivery.     

Effects of blue light irradiation on human dermal fibroblasts

Previous studies have reported that separately from UV-radiation also blue light influences cellular physiology in different cell types. However, little is known about the blue light action spectrum. The purpose of this study was to investigate effects of blue light at distinct wavelengths (410, 420, 453, 480 nm) emitted by well defined light-emitting-diodes on viability, proliferation and antioxidative capacity of human dermal fibroblasts. We found that irradiation with blue light (410, 420 nm) led to intracellular oxidative stress and toxic effects in a dose and wavelength dependent manner. No toxicity was observed using light at 453 nm and 480 nm. Furthermore, blue light (410, 420, 453 nm) at low doses reduced the antioxidative capacity of fibroblasts. At non-toxic doses, irradiations at 410, 420 and 453 nm reduced proliferation indicating a higher susceptibility of proliferating fibroblasts to blue light. Our results show that blue light at different wavelengths may induce varying degrees of intracellular oxidative stress with different physiological outcome, which could contribute to premature skin photoaging. On the other hand, the use of blue light due to its antiproliferative and toxic properties may represent a new approach in treatment and prevention of keloids, hypertrophic scars and fibrotic skin diseases.     

Spectroscopic properties of lanthanoid benzene carboxylates in the solid state: Part 2. Polar substituted benzoates

Europium and terbium complexes of ortho, meta and para substituted benzoate ligands including nitrobenzoate (NBA), aminobenzoate (ABA), hydroxybenzoate (OHBA) and methoxybenzoate (MeOBA) have been synthesised by metathesis reactions, carried out in aqueous media. The complexes were characterised by elemental, compositional and structural investigations, including microanalysis, EDTA titrations, differential thermal analysis, infra red spectroscopy, X-ray powder diffraction and single crystal structural analyses. Besides this, strong emphasis was on the determination of the optoelectronic properties of the compounds in the solid state. In this regard, reflectance, excitation and emission spectra were recorded. From these, the emission and excitation efficiencies were determined. The relative intensities as well as the splitting patterns of the ⁵D₀ → ⁷F_J transitions in the europium emission spectra are discussed.     

On the equilibrium between monomeric and dimeric iminochlorophosphanes R–N=P–Cl

The monomer/dimer equilibrium has been studied for a series of alkyl and aryl substituted and functionalized iminochlorophosphane species of the type R–N=P–Cl. In agreement with experiment, theoretical data always favor the dimer when the group R is small, while bulky groups such as Mes* destabilize the dimer by a considerable steric repulsion. This effect is superimposed by electronic effects. Para-substitution in the aryl systems either favors the monomer (energy gain ca. 15–30 kJ/mol) when a π-electron donating group such as p-NMe₂ is used or favors the dimer when a π-electron withdrawing group such as p-NO₂ is used (energy gain ca. 1–13 kJ/mol).