Synthesis and Self‐assembly of a Helical Polymer Grafted from a Foldable Polyurethane Scaffold

Herein a polyurethane graft poly‐l ‐glutamate amphiphilic copolymer was synthesized from a polyurethane (PU)‐based macro‐initiator (containing pendant primary amine groups) through the ring opening polymerization of N‐carboxy anhydride of γ‐benzyl‐l ‐glutamate ( BLG‐NCA ). On average, twenty two l ‐glutamic acids were grafted from each amino group which was pendant on the polyurethane chain with 10 repeating units. The grafted polymer ( PU‐PP‐1 ) exhibits self‐assembly to produce a hydrogel in a wide pH window ranging from pH 5.0 to 8.0 with a critical gelation concentration (CGC) of 5.0 wt % (w/v) at pH 7.4. Furthermore, circular dichroism study revealed the transition of the α‐helix to a random coil upon increasing the pH. Due to the protonation of side chains at pH 4.0, PU‐PP‐1 adopted an α‐helical conformation whereas at pH >8.0 the side‐chain carboxylic acid groups of the PLGAs were ionized, leading to the formation of an extended random coil conformation as a result of charge repulsion. Conformational switching was also supported by FTIR spectroscopy.     

Synthesis of the pentasaccharide repeating unit of the O-antigen of Escherichia coli O175 using one-pot glycosylations and its conformational analysis

Synthesis of a pentasaccharide repeating unit of the cell wall O-antigenic polysaccharide of enteroaggregative Escherichia coli O175 has been achieved using one-pot, three components [1+2+2] block glycosylations. The intermediate steps were good yielding and satisfactory stereoselectivity was observed in the glycosylations. The synthesized pentasaccharide was subjected to conformational analysis in aqueous environment using 2D ROESY NMR spectral analysis in conjugation with molecular dynamics (MD) simulation technique.     

The Role of Ligand‐Field States in the Ultrafast Photophysical Cycle of the Prototypical Iron(II) Spin‐Crossover Compound [Fe(ptz)6](BF4)2

Light‐induced excited spin‐state trapping (LIESST) in iron(II) spin‐crossover compounds, that is, the light‐induced population of the high‐spin (S=2) state below the thermal transition temperature, was discovered thirty years ago. For irradiation into metal–ligand charge transfer (MLCT) bands of the low‐spin (S=0) species the acknowledged sequence takes the system from the initially excited ¹MLCT to the high‐spin state via the ³MLCT state within ca. 150 fs, thereby bypassing low‐lying ligand‐field (LF) states. Nevertheless, these play a role, as borne out by the observation of LIESST and reverse‐LIESST on irradiation directly into the LF bands for systems with only high‐energy MLCT states. Herein we elucidate the ultrafast reverse‐LIESST pathway by identifying the lowest energy S=1 LF state as an intermediate state with a lifetime of 39 ps for the light‐induced high‐spin to low‐spin conversion on irradiation into the spin‐allowed LF transition of the high‐spin species in the NIR.     

Studies on the Suzuki reaction on methyl 1-(2-bromoaryl)-5-oxo-3-aryl/heteroaryl-pyrrolidin-2-carboxylate derivatives: synthesis of N-aryl modified monocyclic γ-lactam derivatives in search for newer antibacterial agents

A large number of novel N-aryl modified monocyclic γ-lactam derivatives have been prepared via Pd(0) catalyzed heteroarylation in the N-aryl part of (±) cis and (±) trans γ-lactam carboxylate derivatives 3(a–f) and 6(a–f), respectively, with furan-2-boronic acid and thiophen-2-boronic acid. (±) cis Methyl 1-(2-bromoaryl)-5-oxo-3-aryl/heteroarylpyrrolidin-2-carboxylate derivatives 3(a–f), were prepared in good yields from 1(a–f) via hydrolysis, stereoselective decarboxylation, followed by esterification. Corresponding trans isomers 6(a–f) were prepared by the standard method already reported by us.     

Kinetics of thermal degradation and estimation of lifetime for polypropylene particles: Effects of particle size

The thermal stability and degradation behavior of polypropylene (PP) particles having diameter varying from few micrometers to nanometers were studied by thermogravimetric analysis (TGA). The PP particles of average diameter ∼20 μm, ∼10 μm, ∼5 μm, ∼1 μm and <500 nm were studied over a range of temperature from 25 to 600 °C in N₂ atmosphere and heating rates of 5, 10 and 15 °C/min. Thermal stability of PP particles initially decreases and then increases as particle size further decreases to nanometer scale. The five single heating rate techniques such as Friedman, Freeman-Carroll, Chang, Coats-Redfern and second Kissinger; and three multiple heating rate techniques such as the first Kissinger, Kim-Park and Flynn-Wall were used to compute the kinetic parameters of degradation reaction, e.g., activation energy (E_a), order of reaction (n) and frequency factor [ln(Z)]. The lifetime of macro-, micro- and nanosized PP particles was also estimated by a method proposed by Toop. It was found that the activation energy and lifetime of nanosized PP particles are moderately high compared to the microsized PP particles. Moreover, the decomposition temperature, order of reaction (n), frequency factor [ln(Z)] not only depend on the heating rate and calculation technique but also on the particle size of polymer. The results are compared with macrosized PP.     

The effect of solutal undercooling on double‐diffusive convection and macrosegregation during binary alloy solidification: a numerical investigation

In this paper, we present a macroscopic numerical model that is capable of capturing the interaction between the double‐diffusive convective field and a localized fluid flow on account of solutal undercooling during non‐equilibrium solidification of binary alloys. The model is essentially based on a fixed‐grid enthalpy based control volume approach. In the present model, microscopic features pertaining to non‐equilibrium effects on account of solutal undercooling are incorporated through the formulation of a modified partition‐coefficient. The effective partition‐coefficient is numerically modelled by means of a number of macroscopically observable parameters related to the solidifying domain. This feature has made the present treatment different from micro‐macro modelling of alloy solidification, which involves certain parameters that may not be macroscopically resolvable. Numerical simulations are performed for the case of two‐dimensional transient solidification of Pb–Sn alloys (both hypoeutectic and hypereutectic) in a rectangular cavity, employing the present model. The simulation results are also compared with the corresponding experimental results quoted in the literature, and the agreement is excellent. From the results, it can be concluded that non‐equilibrium effects on account of solutal undercooling result in a more enhanced macrosegregation. Copyright © 2002 John Wiley & Sons, Ltd.     

Visualization of convection loops due to Rayleigh–Benard convection during solidification

This paper describes the techniques adopted for visualization of Rayleigh–Benard convection during solidification of eutectic, hypo- and hyper-eutectic salt solutions. Neutrally buoyant hollow glass spheres were inducted in the body of the solidifying salt solution. The convection currents were captured by scattering of a laser beam by these beads. The apparatus has two components, namely: (i) measurement of interface/mushy region movement and temperature distribution and (ii) flow visualization with laser beam scattered by hollow glass spheres. The distinct nature of convection cells in the eutectic and hypo eutectic region is brought out.     

Plane wave analysis of acoustic wedges using the boundary-condition-transfer algorithm

The present work delineates the applicability of the boundary-condition-transfer algorithm for analyzing the performance of the acoustic wedge. The theoretical model, based on the plane wave acoustical coupling between two interacting ducts of variable cross-sectional area, outlines the simplicity as well as the usefulness of the current approach. The proposed model assumes bulk reaction and hence a wave propagation in the wedge material. An asymptotic solution using the Peano-Baker series of matrix calculus is derived with the aid of an inherently stable boundary condition transfer (BCT) algorithm. The proposed method is validated extensively against the available literature in order to establish the utility of this elemental approach over its much more complex and time consuming counterparts like FEM and BEM.     

Formanilide and carbanilide from aniline and carbon dioxide

Earlier syntheses of formamides from the catalytic hydrogenation of CO₂ in the presence of amines were only successful for the preparation of dialkylformamides. After an analysis of the reason for the failure of the reaction using aniline as a starting material, formanilide has been prepared, for the first time, from CO₂, H₂ and aniline with the use of 1,8-diazabicyclo[5.4.0]undec-7-ene. Omission of the H₂ reductant causes the selectivity to switch to the production of carbanilide (1,3-diphenylurea).     

Sobolev regularity of the second biharmonic problem on a rectangle

Summary It is proved that for any f &esin; L²(Ω) the weak solution of the second biharmonic problem on a rectangle satisfies u&esin; H⁴(Ω). The proof uses the decomposition of the problem into two Poisson equations and a general condition for H⁴-regularity via the eigenvalues and eigenfunctions of second order elliptic operators.