Aggregation and chimney formation during the solidification of ammonium chloride

Experiments study large-scale pattern formation during the growth of ammonium chloride (NH4Cl) from solution in a thin (Hele-Shaw) geometry. In particular a solid-liquid mixture ("mushy layer") forms in which growing solid NH4Cl crystals form a solid network interspersed with liquid. There are different ways that the mushy layer can be formed, however. If the cell is heated from below and cooled from above, thermal convection generates large-scale recirculating flows that carry seed crystals from the upper (cold) boundary to the (warmer) side and bottom boundaries. Ballistic deposition of these seed crystals leads to aggregation patterns with significant voids (filled with liquid) with a wide range of length scales. If the cell is cooled from below with a warm environment, the solid NH4Cl grows dendritically without deposition, resulting in a compact mushy layer. Plume convection within this mushy layer produces one or two well-defined "chimneys." If the environment is cool (comparable to the liquidus temperature of the solution), the mushy layer forms by a combination of dendritic growth and ballistic deposition, resulting in a more permeable mushy layer and enhanced chimney formation. The effects of ballistic deposition are enhanced if the cell is tipped, in which case the voids reappear. Plume convection and chimney formation are dramatically enhanced in this case. Additional experiments are done in which fluid flows in the system are enhanced artificially to verify that enhancements in chimney formation are due primarily to the aggregation process, and not to the increases in fluid flows due to thermal and compositional convection.     

Titanium reagents for the synthesis of 2-substituted benzo[b]thiophenes on the solid phase

Titanium(IV) benzylidenes (Schrock carbenes) bearing a masked sulfur nucleophile in the ortho position were generated from thioacetals with use of low-valent titanocene complex Cp(2)Ti[P(OEt)(3)](2) and alkylidenated Merrifield resin-bound esters to give enol ethers. Treatment of the resin-bound enol ethers with a 5:5:90 mixture of TFA, TFAA, and dichloromethane led to cleavage from resin, removal of the tert-butyldimethylsilyl (TBDMS) protecting group, and concomitant cyclization to complete the traceless solid-phase synthesis (SPS) of benzothiophenes. Switching the nature of the linker from acid-stable to acid-sensitive ensured good purity.     

Quadrupole moment measurement of the highly deformed πg 9/2⊗νh 11/2 band in 130Pr

The quadrupole moment for the πg _9/2⊗νh _11/2 band in the ^130>Pr nucleus has been measured using the Doppler-shift attenuation method. A centroid shift analysis was carried out and a value of Q₀=6.1±0.4 eb, corresponding to an axial prolate deformation of β₂=0.35(3), has been determined. This is the first direct experimental confirmation of the deformation-driving character of the πg _9/2 orbital in an odd-odd nucleus in the A≃135 superdeformed region. Received: 23 April 1998     

Ferroelectric liquid crystals induced by atropisomeric biphenyl dopants: the effect of chiral perturbations on achiral dopants

The addition of the achiral biphenyl dopant 2,2',6,6'-tetramethyl-4,4'-bis(4-n-nonyloxybenzoyloxy)biphenyl (3) or its dithionoester or dithioester analogue (4, 5) to a 4 mol % mixture of the atropisomeric biphenyl dopant (R)-2,2',6,6'-tetramethyl-3,3'-dinitro-4,4'-bis(4-n-nonyloxybenzoyloxy)biphenyl, (R)-1, in the phenylpyrimidine SmC host PhP1 produces a significant amplification of the spontaneous polarization induced by (R)-1. This amplification may be due to a chiral perturbation by (R)-1 which causes a shift in the equilibrium between enantiomeric conformations of the achiral dopant. The degree of polarization amplification afforded by the achiral dopant, as expressed by the polarization amplification factor PAF, varies with the nature of the linking group. This may be ascribed to different rotational distributions of the core transverse dipole moments relative to the polar axis of the SmC* phase and/or to differences in lateral bulk of the polar linking groups. The latter may affect the degree of chiral molecular recognition achieved by 3-5 in the binding site of the SmC* phase.     

Engineering chemokines to develop optimized HIV inhibitors

Since the discovery that to enter target cells HIV uses receptors for the class of proteins known as chemokines, attempts have been made to generate anti-HIV molecules based on the chemokine ligands. A significant level of knowledge of the structure-activity relationships of chemokines has been amassed since the beginning of the 1990s. This, together with work that has elucidated the mechanisms underlying the inhibitory activity of chemokines, has guided not only the rational design of anti-HIV chemokine analogues, but also strategies by which chemokine variants with potent anti-HIV activity can be isolated from large libraries by phage display. This review summarizes the current knowledge about the structure-activity relationships and receptor biology of chemokines that is relevant to the development of analogues with anti-HIV activity. We present specific examples of engineered chemokine analogues with potent anti-HIV activity and describe the challenges that will need to be faced if these molecules are to be further developed for clinical applications. Finally, we discuss how these challenges might be met through further engineering of the molecules.     

The Deterministic,Two-Product,Inventory System with Capacity Constraint

This paper considers the standard deterministic inventory system for two products with a capacity constraint and describes how to find the optimal policy amongst all policies which have fixed order quantities. This involves the idea of staggered initial orders and periodic policies, and includes the classical Lagrange multiplier technique and the equal order intervals method as special cases. It is shown that the usual Lagrange multiplier technique will never produce the optimal policy (in the class described above) except in the trivial case, when the capacity constraint is satisfied by the optimal unconstrained policy.