The effect of hydrogen bond donors in asymmetric organocatalytic conjugate additions

A series of primary amine organocatalysts with various hydrogen bond donors were prepared and examined in the conjugate addition of isobutyraldehyde and acetone to trans-β-nitrostyrene and (E)-methyl 2-oxo-4-phenylbut-3-enoate. The effect of N–H acidity and hydrogen-bonding modes of the catalysts on the catalytic activity and enantioselectivity was studied. The experimental results did not support a general correlation of N–H acidity and hydrogen-bonding modes with catalytic activity and enantioselectivity. The catalysts with double hydrogen-bonding interactions provided better catalytic activities and enantioselectivities than the catalysts with single hydrogen-bonding interactions for the reaction of trans-β-nitrostyrene. The catalyst with the most acidic N–H bond showed the best catalytic activity and enantioselectivity for the reaction of (E)-methyl 2-oxo-4-phenylbut-3-enoate. These results suggest that the effect of hydrogen bond donors in organocatalytic reactions may be highly dependent on the substrates and the reaction conditions.     

Biomimetic modification of chitosan with covalently grafted lactose and blended heparin for improvement of in vitro cellular interaction

Lactose‐ and heparin‐modified chitosan films were prepared and their physical and biological properties were compared with chitosan, chitosan‐g‐heparin, and chitosan‐g‐lactose films. Atomic force microscopy (AFM) measurement showed that all these films in the dry state were rather flat with a roughness smaller than 20 nm. While the chitosan‐g‐lactose/heparin and chitosan‐g‐lactose films have the highest swelling and weight loss ratios, the chitosan and chitosan‐g‐heparin films have the lowest. The chitosan‐g‐lactose/heparin film showed stronger ability to induce chondrocyte attachment, proliferation, viability, and glycosaminoglycan (GAG) secretion than that of the chitosan, chitosan‐g‐heparin, and chitosan‐g‐lactose films. Chondrocyte aggregates and nodules were observed on the chitosan‐g‐lactose/heparin and chitosan‐g‐lactose films, which still preserved viable metabolic ability. These results show that the lactose‐modified and heparin‐incorporated chitosan film can enhance the cell–biomaterial interaction synchronously. The resulting chitosan‐g‐lactose/heparin material is more bioactive that might be applicable as promising scaffold for chondrogenesis. Copyright © 2007 John Wiley & Sons, Ltd.     

Electron temperature measurement in a surface-wave-produced argon plasma at intermediate pressures

The main objective of this work is to obtain the electron temperature in an argon surface-wave-produced plasma column at intermediate gas pressures. After proving that argon upper excited states remain in Excitation Saturation Balance, the value of electron temperature along the plasma column has been obtained using a modified Saha equation and a corrected Boltzmann-plot. Moreover, the electron energy distribution function has been verified to be nearly Maxwellian in a 0.8-2.8 torr intermediate pressure range. Received 24 July 2000 and Received in final form 19 January 2001     

Evolution of a Constitutional Dynamic Library Driven by Self‐Organisation of a Helically Folded Molecular Strand

Conversion of macrocyclic imine entities into helical strands was achieved through three‐ and four‐component exchange reactions within constitutionally dynamic libraries. The generation of sequences of the intrinsic helicity codon, based on the hydrazone–pyrimidine fragment obtained by condensation of pyrimidine dialdehyde A with pyrimidine bis‐hydrazine B , shifted the equilibrium between all the possible macrocycles and strands towards the full expression (>98%) of helical product [ A / B ]. Furthermore, it was shown that chain folding accelerated the dynamic exchange reactions among the library members. Lastly, in four‐component experiments (involving A , B , E and either C or D ), even though the macrocyclic entities ([ A / C ], [ B / E ]; [ A / D ], [ B / E ]) were the kinetically preferred products, over time dialdehyde A relinquished its initial diamine partners C or D to opt for bis‐hydrazine B , which allowed the preferential formation of the helically folded strand. The present results indicate that self‐organisation pressure was able to drive the dynamic system towards the selective generation of the strand undergoing helical folding.     

Doubly tethered tertiary amide linked and ionically bonded diproline chiral stationary phases

Oligoproline chiral stationary phase (CSP) is a new class peptide chiral stationary phase. Many proline chiral stationary phases with different proline chain lengths and linkers have been prepared and evaluated. However, the doubly tethered and ionic type linkers have not been adequately investigated. In this study, covalently and ionically bonded chiral stationary phases with doubly tethered linker were prepared and characterized. The new covalently bonded doubly tethered diproline CSP was applied successfully to resolve various enantiomers of acidic, basic, and neutral compounds with phenyl, naphthyl, anthryl, or similarly sized groups. The enantiorecognition performances of singly and doubly tethered diproline CSPs were comparable. Variation of the type and content of organic modifiers in hexane or heptane mobile phase showed that the branch alcohols such as 2‐propanol and 2‐butanol, 1,2‐dichloroethane, methyl tert‐butyl ether, and ethyl acetate in the mobile phase enhanced chiral separation. End‐capping on doubly tethered diproline CSP did not always improve the separation factor and resolution. Due to the rigid structure of the double tether, the enantioseparation ability of ionically bonded diproline CSP was well expressed to some analytes.     

Temperature effects on a doubly tethered diproline chiral stationary phase: Hold‐up volume,enantioselectivity and robustness

Effect of temperature on hold‐up volume, enantioselectivity and robustness of a novel doubly tethered diproline chiral stationary phase (CSP1) was studied. In‐column end‐capping of residual silanol was utilized as a tool to exhibit in situ change of CSP1. The hold‐up volume marker, 1,3,5‐tri‐tert‐butylbenzene, was observed to be weakly retained (<1 s ) on a 5 cm×4.6 mm chiral column, and its retention time was changed with the carrier solvent and column temperature. The apparent thermodynamic parameters of 1,3,5‐tri‐tert‐butylbenzene indicated an enthalpy‐driven retention process with the hexane/isopropanol mobile phase, while an entropy‐driven process with the hexane/methyl tert‐butyl ether mobile phase. The ΔΔH and ΔΔS values of chiral separation for the four probes including 1,1′‐bi‐2‐naphthol and warfarin were negative on CSP1. Nonlinear van't Hoff plots were observed for some analytes before and after the end‐capping treatment. Depending on compound, end‐capping strengthened or weakened the enantioseparation. Moreover, the enantioselectivity of CSP1 was shown to be robust by testing with heating–cooling cycles and step‐temperature programs.     

The computation of multiple roots of a polynomial

This paper considers structured matrix methods for the calculation of the theoretically exact roots of a polynomial whose coefficients are corrupted by noise, and whose exact form contains multiple roots. The addition of noise to the exact coefficients causes the multiple roots of the exact form of the polynomial to break up into simple roots, but the algorithms presented in this paper preserve the multiplicities of the roots. In particular, even though the given polynomial is corrupted by noise, and all computations are performed on these inexact coefficients, the algorithms ‘sew’ together the simple roots that originate from the same multiple root, thereby preserving the multiplicities of the roots of the theoretically exact form of the polynomial. The algorithms described in this paper do not require that the noise level imposed on the coefficients be known, and all parameters are calculated from the given inexact coefficients. Examples that demonstrate the theory are presented.     

Breakdown of the single-exchange approximation in third-order symmetry-adapted perturbation theory

We report third-order symmetry-adapted perturbation theory (SAPT) calculations for several dimers whose intermolecular interactions are dominated by induction. We demonstrate that the single-exchange approximation (SEA) employed to derive the third-order exchange-induction correction (E(exch-ind)((30))) fails to quench the attractive nature of the third-order induction (E(ind)((30))), leading to one-dimensional potential curves that become attractive rather than repulsive at short intermolecular separations. A scaling equation for (E(exch-ind)((30))), based on an exact formula for the first-order exchange correction, is introduced to approximate exchange effects beyond the SEA, and qualitatively correct potential energy curves that include third-order induction are thereby obtained. For induction-dominated systems, our results indicate that a "hybrid" SAPT approach, in which a dimer Hartree-Fock calculation is performed in order to obtain a correction for higher-order induction, is necessary not only to obtain quantitative binding energies but also to obtain qualitatively correct potential energy surfaces. These results underscore the need to develop higher-order exchange-induction formulas that go beyond the SEA.     

Simple conjugation and purification of quantum dot-antibody complexes using a thermally responsive elastin-protein L scaffold as immunofluoresecent agents

Fluorescent quantum dots (QDs), because of their tunable spectral properties, are ideal for simultaneous multiplexed detection in an antibody array format. Despite these advantages, their widespread usage is limited by the costly and tedious conjugation and separation protocol. Herein, we report a simple platform for the direct conjugation and separation of highly luminescent CdSe-ZnS QD-antibody complexes using a genetically engineered polyhistidine tagged elastin-protein L fusion (His-ELP-PL). The principle of immunoassay-ready conjugates was to take advantage of the direct conjugation of QDs via metal coordination with the His tag, the unique temperature-responsive property of ELP, and the high affinity of the antibody-binding protein L domain toward IgGs. Simple separation of the QD- His-ELP-PL-IgG complex was achieved by thermally triggered precipitation without any interference on the QD functionality. The utility of the biofunctionalized OD probes was demonstranted in an antibody array for the detection of carcinoembryonic antigen.     

Size-manipulable synthesis of single-crystalline BaMnO3 and BaTi1/2Mn1/2O3 nanorods/nanowires

We report a size-manipulable synthesis of single-crystalline nanorods/nanowires of barium manganite (BaMnO(3)) and barium titanium manganite (BaTi(1/2)Mn(1/2)O(3)) by using the composite-hydroxide-mediated approach. The synthesis cleanly yields nanorods with a hexagonal perovskite structure. Typical nanorods have widths ranging between 50 and 100 nm, and the lengths can be easily controlled by time and temperature or by adding a small amount of water during the synthesis process. Resistance measurement shows that a phase transition happened at 58 K on BaMnO(3). The photoluminescence spectrum of BaTi(1/2)Mn(1/2)O(3) presents two emission peaks at wavelengths of 465 and 593 nm, corresponding to blue and green fluorescence. The ability to synthesize nanorod manganites of a desired length should enable detailed investigations of the size-dependent evolution of magnetism, magnetoresistance, nanoscale phase separation, and realization of a nanodevice of magnetic sensors.