Polymerizable phospholipids: a novel class of biomaterials

Polymerizable phospholipids are organic monomers that are able to self-assemble into membranes. This is quite similar to the self assembly of naturally occurring lipid membranes found in biological systems. The polymerizable lipids can then be polymerized to stabilize these membranes. These stabilized structures have a number of different morphologies which offer a variety of technically attractive features. This article reviews the chemical and physical properties of these materials in terms of their technological applications.     

Effect of water, methanol, and acetonitrile on solvent relaxation and rotational relaxation of coumarin 153 in neat 1-hexyl-3-methylimidazolium hexafluorophosphate

The dynamics of solvent relaxation in ionic liquid (IL)-water, IL-methanol, and IL-acetonitrile mixtures have been investigated using steady state and picosecond time-resolved fluorescence spectroscopy. We have used Coumarin 153 (C-153) and 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF(6)]) as fluorescence probe and IL, respectively. The steady-state emission spectra showed that the gradual addition of cosolvents increases the polarity of the mixtures. In neat [hmim][PF(6)] and all IL-cosolvent mixtures, solvation occurs in two well-separated time regimes within the time resolution of our instrument. A substantial portion of the solvation has been missed due to the limited time resolution of our instrument. The gradual addition of cosolvents decreases the viscosity of the medium and consequently solvation time also decreases. The decrease in solvation time is more pronounced on addition of acetonitrile compared to water and methanol. The rotational relaxation time of the probe is also decreasing with gradual addition of the cosolvents. The decrease in viscosity of the solution is responsible for the decrease in the rotational relaxation time of the probe molecule.     

Insertion of a small peptide of six amino acids into the β7–β8 loop of the p51 subunit of HIV-1 reverse transcriptase perturbs the heterodimer and affects its activities

Background  

HIV-1 RT is a heterodimeric enzyme, comprising of the p66 and p51 subunits. Earlier, we have shown that the β7-β8 loop of p51 is a key structural element for RT dimerization (Pandey et al., Biochemistry 40: 9505, 2001). Deletion or alanine substitution of four amino acid residues of this loop in the p51 subunit severely impaired DNA binding and catalytic activities of the enzyme. To further examine the role of this loop in HIV-1 RT, we have increased its size such that the six amino acids loop sequences are repeated in tandem and examined its impact on the dimerization process and catalytic function of the enzyme.     

Liquid-liquid extraction of zinc(II) as its ethyl xanthate

A method for quantitative extraction of zinc with potassium ethyl xanthate into carbon tetrachloride is described. The optimum conditions are: pH 5-6 and Zn(II) to reagent ~1:8 mole ratio. The effects of other ions on the extraction of Zn(2+) have been investigated, and its separation from As(3+), Pb(2+), Cu(2+) and Fe(3+)is described. The possibility of repeated use of the solvent (still loaded with xanthate after the zinc has been stripped) for further extractions (after addition of a little extra xanthate) has been explored. Various stripping agents have been examined, and the optimum conditions found.     

Universal density functional approach to the calculation of correlation energies of atoms

A new local density functional approach for the calculation of correlation energies of many-electron atomic systems is proposed by using the exact results for the correlation energy of a two-electron system bound by a harmonic oscillator external potential. This is motivated by the fact that the correlation energy is a universal functional of the electron density, and the form of this functional is independent of the external potential. The calculated numerical results for the correlation energies show very good agreement with the standard values reported in the literature. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62: 461–465, 1997     

Scheduling crackdowns on illicit drug markets

This paper presents an analytical approach for scheduling crackdowns on street-corner drug markets. The crackdown scheduling problem is shown to be NP-complete. We then provide efficient optimal algorithms for several special cases and approximation algorithms for the general case. These results show that the optimal strategy is to give priority to markets that take longer to bring down and which require low levels of post-crackdown maintenance. The results are then extended to incorporate dealer displacement between drug markets.     

Study of large fragments of uranium projectiles of energy 0.927 GeV/n in Al target

Summary A stack consisting of CR-39(DOP) detector with 2.6 cm Al target was exposed to a²³⁸U beam of energy 0.927 GeV/n from LBL BEVALAC to study the large-fragmentation phenomena during U+Al interaction. The diameter distribution of the etch pits formed in the plastic detectors for the first three projectile fragments of chargeZ _F≥89 yields the partial cross-section data which is comparable to the data obtained from the cone length distribution. The present result on the partial cross-section is significantly higher than those expected from the abrasion-ablation model of Wilson, Townsed and Badavi (1987). The authors of this paper have agreed not to receive the proofs for correction.     

A note on random coin tossing

Harris and Keane [Probab. Theory Related Fields 109 (1997) 27-37] studied absolute continuity/singularity of two probabilities on the coin-tossing space, one representing independent tosses of a fair coin, while in the other a biased coin is tossed at renewal times of an independent renewal process and a fair coin is tossed at all other times. We extend their results by allowing possibly different biases at the different renewal times. We also investigate the contiguity and asymptotic separation properties in this kind of set-up and obtain some sufficient conditions.Keywords:renewal process, absolute continuity, singularity, contiguity, asymptotic separation, martingale convergence theorem     

Capacity and entry issues in online exchanges

With the advent of open standards and Internet technologies, the number of sellers who can participate in online exchanges is greatly increased. We model the competition between identical sellers vying for the same business, and find that there exists a mixed-strategy equilibrium in prices. The results help us understand the dynamics between a seller’s capacity and his motivation to participate in an auction.