Experimental evidence of chiral crown ether complexation with aromatic amino acids

The complexation of L ‐ and D ‐enantiomers of phenylglycine, phenylalanine, and tryptophan with D ‐mannonaphto‐crown‐6‐ether in methanol solution was studied by NMR and isothermal titration calorimetry (ITC) at 298.15 K. The total heat effects attributed to the binding phenomena were measured in the range of 1.8 to 7.7 mJ, and the complexation was found stereo‐specific. The binding topologies were estimated basing on ¹H 2D‐ROESY experiments. The analysis of Job plots obtained from ¹H NMR‐monitored titrations proved the coexistence of 1:1 and 1:2 (crown ether:amino acids) complexes, which thermodynamic parameters, K_s, ΔG, ΔH°, and TΔS were determined with the aid of ITC. The 1:1 complexes were found enthalpically stabilized, generally by electrostatic interactions between the charged NH group of amino acid and crown ether macrocyclic moiety, while the binding of the second amino acid molecule was driven entropically due to solvatophobic effect. Strong enthalpy–entropy compensation points towards the uniform binding mode of all complexes studied. The mode of complex formation was found solvent dependent. For phenylalanine guest studied in various solvent systems, in contrast to the aqueous media, the noticeable chiral recognition is observed in methanol solution, and the complex stoichiometry (1:2 ether:Phe) differs from the 2:1 one, determined previously for the same host‐guest system in water (J. Thermal. Anal. Cal. 2006; 83: 575–578). Copyright © 2007 John Wiley & Sons, Ltd.     

Artificial Neural Networks for Guest Chirality Classification through Supramolecular Interactions

A novel strategy for classification of guest chirality based on the combination of artificial neural networks and anion‐receptor chemistry is reported. The receptor reported herein forms supramolecular complexes with a variety of biologically important carboxylates, in which the chemical shift changes during addition of anions result in complex guest‐stereochemistry‐dependent patterns as followed by ¹H NMR spectroscopy. The neural network had learnt these patterns from a training set of 12 anions, and successfully identified the “unknown” chirality of 14 guests present in the test set. Additionally, principal component analysis could discriminate most of the guests studied (26) and allowed for identification of the receptor protons, which are responsible for information transfer of guest chirality.     

Secants of Lagrangian Grassmannians

We study the dimensions of secant varieties of Grassmannian of Lagrangian subspaces in a symplectic vector space. We calculate these dimensions for third and fourth secant varieties. Our result is obtained by providing a normal form for four general points on such a Grassmannian and by explicitly calculating the tangent spaces at these four points.     

A case for large Auger recombination cross sections associated with deep centers in semiconductors

It is argued that the recent quantitative results concerning localized defects in semiconductors (e.g. GaAs) are consistent with the possibility of large Auger-type cross sections associated with recombination at these centers.It is proposed that many of the capture cross sections reported to be in the range 10^?13–10^?16cm², which exhibit only weak temperature dependence, and which do not depend on carrier concentration, might be explained by this mechanism.     

4-isocyano-2,2,6,6-tetramethylpiperidin- 1-oxyl: a valuable precursor for the synthesis of new nitroxides

To enrich the limited set of isonitriles typically employed, 4-isocyano-2,2,6,6-teramethylpiperidin-1-oxyl (1) is proposed as an isonitrile bearing a nitroxyl moiety. Isocyanide 1 was used in some reactions characteristic of isonitriles. Isoselenocyanate, amides (products of Passerini and Ugi reactions), and tetrazole derivative were obtained. The EPR spectra of the urea derivative 5b and a product of an Ugi reaction 7 (both dinitroxides) were analyzed. [structure: see text]     

Restrictions on symplectic fibrations

Moduli spaces of stable pseudoholomorphic curves can be defined parametrically, i.e., over total spaces of symplectic fibrations. This imposes several restrictions on the spectral sequence of a symplectic fibration. We prove, among others, that under certain assumptions the spectral sequence collapses at E₂. In the appendix, we prove nontriviality of certain Gromov-Witten invariant for blow-ups. As an application we obtain that any Hamiltonian fibration with the blow-up of  along four dimensional submanifold as a fibre c-splits. That is its spectral sequence collapses.     

Improved direct measurement of the parity-violation parameter Ab using a mass tag and momentum-weighted track charge

We present an improved direct measurement of the parity-violation parameter A(b) in the Z boson-b-quark coupling using a self-calibrating track-charge technique applied to a sample enriched in Z-->bb events via the topological reconstruction of the B hadron mass. Manipulation of the Stanford Linear Collider electron-beam polarization permits the measurement of A(b) to be made independently of other Z-pole coupling parameters. From the 1996-1998 sample of 400,000 hadronic Z decays, produced with an average beam polarization of 73.4%, we find A(b)=0.906+/-0.022(stat)+/-0.023(syst).